Diplomarbeit

Mechanisms of nutrient dynamics in the canopy

of a tropical lowland rain forest

zur Erlangung des akademischen Grades

Magistra der Naturwissenschaften

an der

Fakultät für Lebenswissenschaften

der Universität Wien

eingereicht von

Nina Hinko

Wien, Oktober 2007

Index

1. General Introduction 1

Tropical rain forests 1

Nutrient cycling in tropical rain forests 1

Throughfall 4

Dry deposition 7

Canopy exchange processes 9

Study aims 14

References 15

2. Manuscript 20

Mechanisms of nutrient dynamics in the canopy of a tropical lowland rain forest

Abstract 20

Introduction 21

Materials and Methods 24

Results 31

Discussion 34

References 41

Figures and tables 45

3. Zusammenfassung 52

4. Appendix 55

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1. General Introduction

1.1. Tropical rain forests

Tropical rain forests are distributed between the tropics of Cancer (23° 26′ 22″ N latitude) and

Capricorn (23° 26′ 22″ S latitude) and occur in all three possible tropical land areas: Africa,

South-East Asia and Australia (paleotropics) and America (neotropics) where the Americas

possess the most extensive rain forests. They exhibit an exorbitant extreme species richness

and it is estimated that approximately two thirds (170.000) of the world’s plant species occur

in tropical forests (Whitmore 1998). Further tropical rain forests are characterized by high solar

radiation because of the low latitude, a relatively stable climate throughout the year with

continuously high temperatures and high relative humidity with dry seasons being short or

negligible. Tropical rain forests therefore exhibit the highest annual rates of net primary

production (NPP) of all terrestrial ecosystems (Montagnini 2005; Jordan 1971).

Nonetheless tropical ecosystems are highly complex systems and considered to be fragile or

ecologically sensitive. Therefore effects of global change, implying not only climate change but

land use change and change in atmospheric composition and deposition can be critical for

rainforest functioning. Despite tropical rainforests exhibiting the highest biodiversity and

biomass production, most tropical soils are supposed to be highly leached and weathered and

therefore to have an extremely low fertility. Hence the major part of the nutrient pool is stored

in above-ground biomass and nutrient cycling and conservation mechanisms play a crucial role

in maintaining ecosystem productivity in tropical rain forests (Whitmore 1998).

1.2. Nutrient cycling in tropical rain forests

In addition to external nutrient inputs to and outputs from ecosystems nutrient cycling i.e.

internal fluxes of nutrients within ecosystems are thought to be crucial for sustained high

primary production of tropical rain forests. Nutrient inputs are supplied through chemical

weathering of parent material, atmospheric deposition and biological fixation of atmospheric

nitrogen. Nutrient losses occur through erosion and leaching of soils into stream- and

groundwater, emission of gases, wind-driven relocation and disturbances by natural events

such as fire. Nutrient cycling within an ecosystem is comprised of uptake of nutrients by

vegetation and microorganisms, following incorporation into organic material and finally the

release of nutrients via litterfall and throughfall and their decomposition by microorganisms. In

natural ecosystems, for instance tropical rain forests, the amount of nutrients which cycle

internally exceeds those of inputs and outputs by far and tropical rain forests therefore can be

considered as rather closed systems. In consideration of human activities, nutrient inputs and

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outputs in ecosystems obtain an increasing significance because of additional entry of nutrients

due to fertilization, fossil fuel combustion and removal through harvest and fire (Chapin III

2002).

The productivity in terrestrial ecosystems is constrained by climate (rainfall, temperature,

seasonality, etc) and nutrient supplies mostly from the soil. Nutrients are made available for

uptake by plants through the processes of decomposition and mineralization of dead organic

matter which in turn comes from plants via litterfall, animals and microbes. Dissolved mineral

nutrients are absorbed by plants via roots and move upward in the xylem with the transpiration

water stream into leaves. There they are allocated to production of new tissue retranslocation,

or storage. Finally nutrients return to the soil via two main pathways: litterfall and throughfall

i.e. precipitation which is penetrating the canopy before it is reaching the ground. Additional

plants can trap dissolved organic nutrients e.g. nitrogen from decomposing material as soon as

they are released as well through direct transfer mediated mainly by mycorrhiza (Herrera

1978).

As mentioned above nutrient cycling is essential for continued plant growth in tropical forest

ecosystems and nutrient cycles therefore are generally found to be rather closed. Nevertheless

it must be pointed out that not all tropical forests grow on low fertility soils despite a great

proportion are oligotrophic ecosystems such as the Amazon Basin. In contrast to these nutrient

poor rain forests which receive nutrients solely from litter- and rainfall there also existing

nutrient rich tropical rain forest ecosystems especially in Central America where many soils

derived from volcanic rock. They consist of a substantial fraction of (mineral) nutrients in the

rooting zone and show nutrient cycles that are more open.

Factors that influence nutrient cycling in tropical rain forests are of biological (plant, bacterial

and fungal species), environmental (moisture and temperature) and chemical nature (nature of

soil nutrients, secondary plant components) (Attiwill 1993). Decomposition in tropical forests is

generally enhanced at high temperature, high relative moisture and availability of sufficient

oxygen which accelerate microbial activity. Hence tropical lowland rainforests exhibit a small

litter pool (Chapin III 2002; Whitmore 1998).

Litter consists of aboveground leaf litter and of belowground root litter. Soils in tropical rain

forests exhibit a remarkable amount of fine root biomass in the surface soil and litter layer

(Klinge 1973). Stark (1978) demonstrated with tracer-experiments that the fine root mat can

directly absorb dissolved nutrients released from decomposing litter and reaching the surface

soil via precipitation and throughfall. This nutrient conserving mechanism of the fine root mat

plus associated mycorrhizas enhances the uptake of nutrients, especially phosphorus (Herrera

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1978). Nearly all tree species worldwide have ecto- or arbuscular mycorrhiza. Consequently,

the root mat plays an important role in the retention and recycling of nutrients on the forest

floor.

In tropical lowland rainforests internal recycling sustains the availability of N and P for plants

though phosphorus return via litterfall is very small compared to nitrogen (Vitousek 1984).

According to Vitousek (1982) it is assumed that P is cycled very efficient and N seems to have

low within-stand efficiency despite its rapid circulation. This supports that P availability and not

that of N limits productivity of most tropical lowland forests given that the most growth limiting

nutrient defines the cycling rate of all other nutrients (Chapin III 2002). It is not proven that N

is not used efficiently due to the fact that in those forests cycling occurs really fast and a unit of

N could circulate several times per year (Vitousek 1984). However, plant available phosphorous

is generally present at very small concentrations and is retained within the plants. Intra-plant

nutrient conservation (Jordan 1980) occurs via reabsorption and translocation of mobile

nutrients from senescing tissue through the phloem to other plant parts, i.e. to young leaves

where mineral nutrients are required to build up organic matter or to storage organs.

Nevertheless some nutrients such as Calcium are immobile in the phloem and therefore cannot

be resorbed.

Transfers of nutrients to the forest floor occur through litterfall (LF) and through precipitation in

the form of throughfall (TF) and stemflow (SF). The contribution of TF and SF as pathways for

solute (nutrient) inputs to the forest floor is important particularly in forests with abundant

rainfall and infertile soils, although amounts of nutrients in precipitation are generallly lower

than those recycled by litterfall (Tobón 2004). Nevertheless throughfall fluxes can be

ecologically significant for certain elements. In the case of potassium McDowell (1998)

demonstrated that K+ flux in TF is twice as much as in LF for a tropical wet rain forest in Puerto

Rico. In contrast nitrogen as well as calcium and phosphorus inputs to the soil derive largely

from litterfall. Calcium content in litterfall usually exceeds that of living foliage because Ca is

immobilized in cell wall components such as pectates (Johnson 1992) and can not be removed

from senescent leaves before shedding. Cavelier (1997) compared annual rates of N, P and K in

throughfall with rates in litterfall from Veneklaas (1991) for a tropical montane cloud forest. N

inputs in net throughfall NTF were significantly lower than mean inputs in litterfall while P

inputs were similar in both LF and NTF. In contrast K inputs were significantly higher in NTF

than in LF. The considerable increases of K in TF can be attributed to its high mobility and

abundance in leaves (Tobón 2004).

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1.3. Throughfall

It is has been often demonstrated that precipitation is altered in its chemical composition

during interception through the forest canopy and that throughfall is predominantly enriched in

certain elements (Parker 1983; Lovett 1996; Veneklaas 1990; Whitmore 1998). Precipitation

which penetrates the forest canopy can reach the forest floor as throughfall (rainfall which

passed through the canopy) and as stemflow (rainfall which runs off the tree stem). Jordan

(1980) found that throughfall made up 80-90% of incident precipitation in a tropical rainforest

whereas stemflow accounted for only 1-2% of total solute fluxes. This agrees with results from

Chuyong (2004) who reported that throughfall and stemflow in a central African rain forest

accounted for 92.4-96.6% and 1.5-2.2%, respectively. Despite the very small contribution to

total nutrient fluxes stemflow is generally more enriched in nutrients than throughfall and

therefore displays a substantial yet localized nutrient input to forest floor (Parker 1983). The

difference (1.9-5.4%) between incident precipitation and TF (and SF) represents the

interception loss within the canopy. Throughfall, stemflow and interception measurements for a

tropical montane rain forest in Costa Rica resulted in 70, 2 and 28% of incident rainfall,

respectively (Hölscher 2004).

In consideration of ecosystem nutrient fluxes precipitation is a supplementary and small but

important nutrient input to forests by which nutrients from outside enter the ecosystem. For

certain elements throughfall is the major pathway for recycling and therefore the internal

nutrient dynamics of the forest (Parker 1983), particularly in wet tropical rainforests where

rainfall is high and nutrient cycling is known to be of considerable importance to meet the

nutrient demand by vegetation. Furthermore dissolved nutrients in throughfall are highly

available to organisms compared to those of litterfall, may be an important pulse at the

beginning of the rainy season and probably facilitate litter decomposition and mineralization

(Chuyong 2004) thereby reducing litter turnover times. Processes involved in chemical

alteration of precipitation are exchange processes between the canopy and the water on plant

surfaces such as leaching and uptake of dissolved material by canopy components, as well as

dissolution and wash off of dry deposited atmospheric materials such as aerosols, particulate

and gaseous substances (Lovett 1984; Hansen 1994; Parker 1983).

Parker (1983) reviewed that for K+, Na+, Mg2+ and S, the rates of NTF exceed those of litterfall

while N, P and Ca derive largely from litterfall. It is assumed that potassium derives mainly

from foliar or canopy leaching because of it is high mobility. Calcium and magnesium are also

found in higher concentrations in TF compared to BP. Calcium was shown to originate almost

equally from leaching of exchangeable pools in leaf apoplast and from wash off of particulate

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dry deposition (Forti 1992). In contrast Mg2+, Na+ and Cl- are introduced mainly by sea salt

particles. H+ concentration (acidic deposition) in rainwater decreases during passage through

the canopy which implies that uptake (ion-exchange) processes occur. The absorption from H+

triggers increased leaching of other soluble or ion exchangeable cations such as K+, Ca2+ and

Mg2+, and weak organic acids from foliage. Numerous studies demonstrated a decline of

inorganic nitrogen concentration in precipitation during canopy penetration, especially for NH4+,

suggesting retention of N in the canopy. Apart from throughfall enrichment of mineral nutrients

it is well known that TF is additional loaded with organic compounds such as DOC and DON.

The origin of these sources is found within the canopy through leaching from canopy litter,

herbivore activities and leaching from bryophytes and exchangeable foliar pools (Coxson

1992).

Throughfall is defined as:

TF = BP + DD + CE

where TF is throughfall, BP is incident (bulk) precipitation, DD is dry deposition and CE

describes canopy exchange processes with uptake and leaching of nutrients within the canopy.

The net throughfall flux can then be calculated as:

NTF = TF – BP = DD + CE

referring to the sum of dry deposition and canopy exchange representing net throughfall

(Lovett 1996). Positive rates of NTF indicate a net enrichment of elements in throughfall due to

dry deposition and canopy leaching whereas negative rates present a net decrease in nutrient

concentration in throughfall due to canopy uptake or interception loss.

The separation between these processes, dry deposition and canopy exchange, and

quantification of their contribution to throughfall still remains a challenge. Moreover, it has

been suggested that both processes are highly dynamic (Hansen 1994) regarding to time,

space and the influence by several factors. Present data are often difficult to compares because

sampling design and analytical methods are highly variable. Collection and measurement of

bulk precipitation includes among wet deposition (elements dissolved in precipitation) also a

fraction of dry deposition such as dust and particles which enter the sampling gauge. Moreover,

dry deposition occurs to canopy surfaces causing an overestimation of canopy leaching rates

(Parker 1983). An essential part of dry deposition – deposition of gaseous substances – is

therefore not accounted for and moreover is complicated to measure. Though studies exist for

temperate forests in tropical rain forests the knowledge is even scare. Temperate forests

exhibit a less complex forest and canopy structure and are often even-sized and even-aged

compared to tropical rain forests and therefore allow an easier access for investigations.

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The flux and chemistry of throughfall are affected by various factors and processes of

atmospheric, hydrological, chemical and biological nature, and as well human activities, which

themselves are highly temporally and spatially variable (Parker 1983; Potter 1991; Tobón

2004). Numerous studies reported that fluxes of dissolved nutrients are highly correlated with

the amount, duration and the intensity of precipitation penetrating the canopy (Reiners 1984;

Parker 1983; Lovett 1996; Tobón 2004; Potter 1991) that are much higher in tropical forests

than in temperate forests. A high intensity of rainfall leads to a decrease in ion concentration in

TF during the event while low intensities correspond to elevated solute concentrations.

Precipitation of low intensity prolongs the residence time for water within the canopy and hence

exchange processes have more time to take place (Lovett 1992; Hansen 1994). Throughfall

chemistry also strongly depends on the type of precipitation such as clouds, droplets, mist and

dew. Indirect horizontal precipitation such as clouds have elevated concentrations of elements

and can be important especially in tropical montane cloud forests where quantities of

throughfall can exceed those of bulk precipitation and therefore cause an extra input of water

(Cavelier 1997) and nutrients (Veneklaas 1990).

Factors such as altitude, forest type and canopy structure, stand age or successional state,

species composition, seasonality and nutrient supply are also important factors influencing

throughfall fluxes. The leaf area index (LAI) as an index for the canopy density also influences

the amount of throughfall or rather interception loss. During the drier season in a tropical

montane cloud forest the interception by the canopy was lower than during the rainy season

probably due to a decreased LAI because of less growth and an interception maximum at the

beginning of the wet season as a result of increased LAI from leaf flush (Cavelier 1997). Potter

(1991) reported also up to 12 times higher NTF during the growing season than during

dormant season in the Appalachian forests, indicating the considerable influence of foliage in

altering throughfall chemistry. The occurrence of epiphytes may increase also wet and dry

deposition through the enlargement of the surface area for impaction and sedimentation

(Reiners 1984).

Moreover Hansen (1994) reported that ion concentrations in TF during the initial part of a rain

event (the first 2 mm) were highest after long dry periods which indicated that inter-event

deposited (external source) and probably foliar material (internal source) are easily leached in

the early phase of wetting. In subsequent rain the ion concentrations generally decrease except

for NH4+ and K+ which showed a high variability. Atmospheric nutrient inputs (aerosols, dust or

gases) also vary with proximity to sources such as the ocean, volcanoes, industries and

agriculture (Attiwill 1993; Parker 1983). High concentrations of Na+, Cl- and Mg2+ are often

closely related to the sea nearby whereas enhanced N, P and S concentrations are linked to

deposition of air pollutants and hence derive from anthropogenic sources.

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Nevertheless, canopy exchange processes are also related to the chemical composition in

incident precipitation. If the concentration of a certain nutrient is high, net fluxes are more

likely to be negative i.e. a net uptake by the canopy while low concentrations in rain lead to a

positive net flux from the canopy to the forest soil i.e. a net loss of nutrients (Veneklaas 1990).

Highly leachable elements, e.g. potassium, do not follow this pattern.

As mentioned above the nature of the canopy such as epiphytic colonization and abundance of

lichens and bryophytes affect the interception and alteration of rainfall. The greater the canopy

roughness which means more species with different crown structures and more gaps in the

upper canopy, the higher dry deposition rates may occur and the more precipitation is

intercepted by the canopy (Lovett 1984; Veneklaas 1990). Forti (1991) found out that ion

concentrations of wet deposition samples above the canopy compared to those from clearing

are overestimated due to the fact that the roughness of the upper canopy layer generates

vortices and turbulent mass flow at the top of the canopy which resulted in enrichment of

aerosols above the canopy and an increase of ions in incident precipitation. In contrast, Hansen

(1996) reported that due to these turbulent air movements less incident precipitation was

collected and that ion concentrations above the canopy and from an open field were similar.

Dry deposition and canopy exchange as well are expected to change with canopy depth

(Hansen 1994; Parker 1983; Lovett 1992). DD is generally assumed to be highest in the upper

canopy due to their greater exposure to wind and turbulent air whereas in lower canopy layers

the incident rainfall is already enriched in nutrients through their wash-off or leaching from the

upper layers.

Evaporation of intercepted rainwater in inter-event periods can cause an increase of ion

concentration on foliage surfaces previously dissolved in rainwater. Additionally

evapotranspiration exceeds incident rainfall in dry seasons and may provoke an increase in

transpiration and thereby in nutrient release from leaves through their stomata (Parker 1983).

The main processes affecting the variability in throughfall chemistry, dry deposition and canopy

exchange (Tobón 2004; Ragsdale 1992; Lovett 1984), will be discussed in further detail below.

1.4. Dry deposition

Atmospheric deposition

Atmospheric deposition represents an external input of elements or nutrients to ecosystems

and includes wet deposition and dry deposition. Wet deposition comprises incident precipitation

i.e. water droplets in which aerosols, gases or ions are dissolved. Solutes can originate from

other locations nearby or distant (e.g. the ocean) or dissolution took place shortly before

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reaching the top of the canopy. Dry deposition combines all input processes between rain

events and is made up of physical processes such as sedimentation and impaction of particles

and aerosols and absorption of gaseous forms (Parker 1983). An additional crucial biotic

nutrient input within the canopy is the fixation of atmospheric nitrogen (N2) by free-living

cyanobacteria or symbiotic epiphyllous microorganisms (lichens) growing on leaf surfaces. A

considerable portion of this newly fixed N by epiphylls may be transferred to the host leaf

accounting for 10-25% of total leaf N content. Moreover N2 fixation rates are highly correlated

with forest and foliage age due to the time needed for colonization of epiphylls on leaf surfaces

(Bentley 1987; Bentley 1984).

Dry deposition

Dry deposited materials originate from natural sources such as soil processes through microbial

activity and anthropogenic sources: combustion of fossil fuel and coal, biomass burning,

industrial plants, agriculture and animal waste (Lovett 1992). Particulate dry deposits are

classified in dust, sand, volcanic ash, sea salts, air pollutants and fertilizer and related ions are

Ca2+, Mg2+, Na+, Cl-, NO3-, NH4

+, SO42-, PO4

3-. Human activities are also responsible for notable

emissions of gases and aerosols such as SO2, NOx (NO and NO2), NH3 and HNO3. Furthermore,

Veneklaas (1990) and Kellman (1982) reported sporadic nutrient inputs to tropical ecosystems

through proximate volcanic activity.

Despite N2 being the most abundant single element in the atmosphere with about 78% by

volume and its key role in plant nutrition, it is not available for higher plants since it occurs in

the non-reactive form N2. As mentioned above some bacteria or algae can fix N2 and

incorporate it into organic matter though its contribution to total N fluxes within a forest-

ecosystem is generally considered to be low. The less abundant but reactive N forms, mainly

oxides (NO, NO2, N2O, HNO3 vapour, NO3-) but also reduced forms (NH4

+ and NH3), are crucial

in atmospheric chemical reactions. Dry deposition of N primarily occurs through gaseous HNO3

and NH3 because they are more reactive and are easily absorbed by the plant surfaces (Lovett

1992; Hill 2005). Moreover, Balestrini (2001) found for alpine forests a close correlation

between nitrate/ sulphate and ammonium/ hydrogen deposition and presumed that these ions

were mostly deposited as ammonium sulphate and nitrate aerosols resulting from ammonia

and sulphuric and nitric acid reactions. At tropical latidues land use change e.g. conversion of

forest in agricultural lands and increasing biomass burning are assumed to be responsible for

increased N emissions to the atmosphere.

A large fraction (40-50%) of deposited inorganic N was captured by the canopy of temperate

forests because of uptake by plants and epiphytic lichens and of microbial immobilization

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(Garten 1998). This basically agrees with investigations from Clark (1998) who reported that

80% of NO3--N and 61% of NH4

+-N from atmospheric N deposition were retained by the canopy

in a tropical montane rain forest and in turn epiphytic bryophytes and its aggregations with

vascular epiphytes accounted for 80% of this retention. Experiments from Wilson (1998)

showed that uptake of wet deposited inorganic N made a small but sizeable contribution to the

total N demand of the vegetation. Moreover Garten (1998) suggested that the canopy uptake

of dry deposited N is a major contributor to N fluxes within the canopy. Hence throughfall

measurements underestimate the atmospheric deposition of N compounds to a large degree

(Hansen 1996).

The dry deposited material within or on the top of the canopy can enter the nutrient cycle of

the forest-ecosystem by several pathways: washing off of particulate substances and aerosols

from foliage surfaces by incident precipitation or absorption of gases through stomata and

cuticlefree spaces on tissue surfaces (ectodesmata) as well dissolution in the water film on the

leaf surface. The dissolved material then can either drip down as throughfall to the forest floor

or can be taken up by the leaves or associated epiphytes and epiphylls by ion exchange

processes. Dry deposition rates are mainly influenced by the duration of inter-event periods

and further by wind speed and turbulence, deposition velocity and the canopy structure such as

foliage characteristics, surface roughness and wetness (Lovett 1992).

1.5. Canopy exchange processes

In the last decades numerous studies paid attention to canopy exchange processes, focusing on

the extent to which they contribute to solute fluxes from the atmosphere to the soil surface

(Tobón 2004; Filoso 1999; Draaijers 1997; Hansen 1994; Lovett 1984; Lovett 1996; Lovett

1992). It has been proposed that under nutrient limited conditions the forest canopy exhibits

nutrient conservation mechanisms to secure nutrients for forest growth and production (Jordan

1980) and that epiphytes may play an important regulatory role in nutrient conservation in

tropical rain forests (Coxson 1995). Moreover, canopy effects on nutrient flux change during

forest succession because epiphyte loading increases with progressing succession. Various

methods were designed to obtain a deeper insight in canopy exchange processes and to

separate dry deposition from canopy exchange processes. The determination of their particular

contribution to throughfall fluxes has been attempted by several approaches such as labelling

of leachable nutrients with radioisotopes and recently with stable isotopes; collection of

throughfall and incident precipitation to estimate dry deposition (Hansen 1996; Lovett 1984;

Potter 1991) and mathematical modelling of canopy exchange budgets (Draaijers 1997).

However, controls of these processes such as wash-off of dry deposited solutes, leaching and

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uptake of solutes and their contribution to the nutrient cycle in tropical rain forests (Tobón

2004) are still not well known.

Canopy structure

The forest canopy represents the whole plant community plus dead organic matter above

ground and is highly stratified because of trees of many sizes and different ages. The canopy

can be divided into overstorey, made up by the crowns of dominant and fully illuminated trees,

and understorey which comprises all woody plants in the lowest, shaded layer. The outer

canopy is the canopy surface adjacent to the atmosphere (Parker 1995). Forest canopies in

tropical rain forests are unique and highly diverse subsystems and support a rich flora vascular

and cryptogamic plant species. Forest canopies exhibit a great spatial variability, e.g. through

huge trees (emergents) which widely exceed the average level of the canopy (Whitmore 1998)

and gaps through falling trees. Gaps allow the penetration of light to the forest floor, resulting

in marked differences in microclimate, structure, and nutrient and water availability.

Epiphytes

The plants within the canopy of tropical rain forests possess different life-forms including trees,

epiphytes, hemiepiphytes, woody vines and epiphylls. Tropical rain forest canopies therefore

exhibit an associated high diversity of animals mainly insects. Arboreal plants so-called

epiphytes live or grow upon or attached to trees (phorophytes) without parasitizing them.

Epiphytes do not have access to terrestrial soils and their roots serve as anchorage to support

them on the host tree, although they are nutritionally independent of their host tree. Epiphytes

are a substantial canopy component in tropical humid rain forests and, with the associated

dead canopy organic matter, they represent a remarkable part of above-ground biomass and

nutrient pools (Grubb 1977; Nadkarni 1992). The development and abundance of epiphytes is

strongly related to water supply and they therefore occur primarily in environments with

abundant rainfall and short or no dry period (Coxson 1995). The main part of epiphytic

vegetation within the canopy of a tropical rain forest is composed of non-vascular plants such

as mosses, liverworts and lichens. In tropical rain forests the phyllosphere (leave surface) is

also colonized by so-called epiphylls which consist of algae, bacteria, fungi, ferns and

bryophytes and is considered as an own microcosm. On the basis of model simulations

Hölscher (2004) suggested that of 28% interception loss of incident precipitation epiphytic

bryophytes contributed only 6%. Nevertheless, epiphytic bryophytes can contribute

significantly to the water storage capacity of forest canopies through withholding rainwater as

they can store several times more water than those of their own dry weight (Hölscher 2004;

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Nadkarni 1984). Non-vascular epiphytes and epiphylls are poikilohydric which means that they

equilibrate with environmental moisture therefore adequate water supply is pivotal to sustain

their metabolic activity since they are very sensitive to desiccation because of their lack of

roots, cuticula and epicuticular waxes. (Coxson 1991) reported that desiccated bryophytes are

highly susceptible to leaching during the initial rewetting phase. This resulted in a pulse release

of nutrients and organic solutes because of accumulation of sugars and polyols in intracellular

pools to stabilize membrane function during desiccation (Gupta 1977).

Epiphytes generally meet a large fraction of their water and nutrient demand from atmospheric

deposition. They sequester nutrients from allochthonous sources such as precipitation and dry

deposition and from N2-fixing microorganisms. Nutrients are stored intermittently and may be

released by drying and rewetting or displacement to the forest floor. Moreover, they can obtain

notable amounts of nutrients from wash off of dry deposited material and from autochthonous

sources such as leaching of foliar solutes of higher plants (Wania 2002). Vascular epiphytes

have different adaptations to capture nutrients: development of a root mat and morphological

and physiological adapted leaves to trap water and nutrients or plant detritus from

allochthonous and autochthonous sources. Therefore nutrient pools of epiphytes can be high

according to Nadkarni (1984). The nutrient capital retained by canopy epiphytes was similar to

that contained in canopy foliage in a tropical cloud forest. Apart from that a notable fraction of

dead organic material (mainly from epiphytes) decomposes in the canopy before reaching the

forest floor, resulting in a significant enrichment of mineral nutrients in throughfall and

stemflow (Veneklaas 1990; Nadkarni 1991). Nutrients from live or dead epiphytes are also

released into the nutrient cycles of forest ecosystems as epiphytic material falls in large clumps

to the forest floor through wind, disruption by animals, and the falling of supporting branches

and whole host trees (Coxson 1995; Nadkarni 1992). Moreover Wanek (2003; 2005)

demonstrated extensive bidirectional transfer of nutrients between epiphylls and their host

leaves and suggested that the phyllosphere plays a key role in controlling N fluxes, particularly

in the lower wet canopy of ravine forests.

Due to their dependence on water and nutrient supplies from the atmosphere epiphytes

indicate global change and climate more sensitively than co-occuring soil-rooted vegetation

(Benzing 1995). Especially nonvascular epiphytes such as lichens are sensitive to pollutant

loading through the atmosphere (Coxson 1995). Hence the notion that epiphytes enhance the

nutrient and water storage capacity in living aboveground vegetation, suggests that logging

and land use change may have strong effects on soil fertility and ecosystem function (Nadkarni

1984).

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Canopy uptake and leaching processes occur along the same pathways and are primarily based

on passive processes: (1) ion diffusion along concentration gradients, (2) ion exchange

between surface water films and plant tissues, and (3) the absorption of gases through

stomata or epidermal cells via ectosdesmata (leaf surfaces without cuticula) (Draaijers 1997;

Hambuckers 1993). However, active processes may also occur such as guttation and active

nutrient uptake by epiphytes and tree foliage, and may have been underestimated so far.

Canopy uptake

Uptake of nutrients can occur via the cuticula and via stomata. For instance the uptake of NH3

and gaseous HNO3 by leaf tissue occurs through stomata and cuticles (adsorption to

epicuticular waxes), or via dissolution in water films on leaf surfaces. The fate of adsorbed

gases and their major sink is not clear and difficult to trace. Gases can be retained by plants

permanently or be dissolved, leached or re-emitted to the atmosphere (Draaijers 1997).

Uptake rates differ strongly between higher plants and epiphytic or epiphyllous cryptogams

where absorption rates of bryophytes were more than 20-fold higher compared to foliage

(Wanek 2003). Jordan (1980) estimated that nutrient uptake by plants from atmospheric

sources (wet and dry deposition) represents a nutrient conserving mechanism to enable the

ecosystem to survive on low fertility soils. Other retention mechanisms prevent or minimize

nutrient loss from plants such as sclerophylly (Jordan 1981; Vitousek 1986) and retranslocation

of foliar nutrients from senescent leaves to other plant parts before abscission (Vitousek 1986).

Numerous studies demonstrated that concentrations of inorganic N decrease as incident rainfall

passes through the canopy. This suggests that N is taken up by canopy components (Parker

1983). Wilson (1992; 1998) reported that foliar uptake as well as release of inorganic N in

coniferous forests is a function of N concentration in the foliage and that needles prefer the

uptake of NH4+ compared to NO3

-. The authors explained the NH4+ preference on the basis that

the cuticle surfaces have a net negative charge which attracts cations. Moreover, they showed

that an increase in ammonium concentration in simulated rain solution increased influx of

ammonium to the canopy.

Canopy leaching

Leaching is of widespread occurrence in nature and represents a passive process where plants

loose nutrients from their tissue (mainly leaves) to rainwater. (Tukey 1962) defined leaching as

the loss of inorganic and organic metabolites from above-ground plant parts by the leaching

action of aqueous solutions including rain, mist and dew. Inorganic nutrients including all

essential minerals and organic metabolites such as sugars, amino acids and organic acids and

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were found to leach from foliage (Tukey 1966). The loss of nutrients occurs primarily through

passive processes such as diffusion and ion exchange between the water film on the plant

surface and a pool of exchangeable ions in the intercellular free space of foliage (Parker 1983;

Tukey 1970; Hambuckers 1993).

There are numerous factors, both internal and external to the plant, which influence the

leaching process: tissue age and type, foliage nutrient content and plant species. Furthermore

nutrients exhibit differences in their leachability depending on their function in metabolic

processes (Tukey 1970) and chemical characteristics (charge, size, valence, lipophily).

The susceptibility to loss increases with increasing maturity of plant tissue and reaches a

maximum at senescence. In addition Tukey (1970) reported that nutrients which are

accumulated in excess of requirements are leached more easily and rapidly after initial wetting.

Mature leaves possess larger pools of such exchangeable nutrients than young foliage because

the latter require and utilize these nutrients quickly for growth. Hence they are less susceptible

to leaching than older leaves. Moreover, characteristics of the tissue (leaf) surface affect

leaching as well. Sclerophylly as a morphological adaptation to reduce water loss via

transpiration contributes also to nutrient retention. For instance, the wettability of smooth and

waxy leaf surfaces is less than those of pubescent and unwaxed surfaces. Herbivore (insect)

activities within the canopy or any kind of injury of plant foliage can cause increased leaching

through damaged leaves. Moreover the efficiency of leaching is influenced by the amount,

intensity and chemical composition of precipitation. Foliage which is continuously wetted

through light rainfall is leached more easily than through short intensive rain events (Tukey

1970). In past 20-years it has been shown that deposition of air pollutants such as acid

deposition and ozone may also accelerate canopy leaching through cation exchange between

H+ and base cations and damage of cell membranes, respectively (Lovett 1992).

Foliar leaching is of major importance in K+- cycling due to the fact that throughfall often

exceeds litterfall as a K+- return pathway significantly. This is based on high apoplastic K+

concentrations in plant tissue, representing a large exchangeable pool (Sattelmacher 2001).

Ca2+ and Mg2+ are tightly bound in structural tissues or enzyme complexes and therefore leach

less extensively from the smaller exchangeable apoplastic pools in leaf foliage. The major

portion of calcium which is leached from plants derives from recently absorbed calcium in

exchangeable forms located outside the cell wall (Mecklenburg 1966). Calcium deficiency

through leaching has been shown to accelerate root uptake of calcium which then is

translocated into foliage and again is susceptible to leaching until it is immobilized through

incorporation into cell wall compounds.

- 14 -

1.6. Study aims

The present study aims at investigating the controls of throughfall chemistry such as dry

deposition, canopy leaching and canopy uptake and at determining to what extent the

respective canopy components contribute to canopy exchange processes. We hypothesized that

epiphytes, primarily epiphytic bryophytes, contribute most to the alteration in nutrient

throughfall chemistry. Since investigations of throughfall and bulk precipitation concentrations

alone do not provide further information on canopy exchange processes or the origin of

nutrients we performed tracer experiments to follow the pathway of macro-nutrients, especially

potassium, calcium and nitrogen. We assumed that nitrogen derives from dry deposition as well

as from canopy leaching and applied 15N stem injection in situ techniques and controlled

surface wash experiments to determine the origin of nitrogen. The study further intended to

quantify the gross and net fluxes of dissolved nitrogen, and its inorganic (NO3- and NH4

+) and

organic (DON) species within the canopy. To investigate gross fluxes we adopted a 15N-pool

dilution technique.

We further expected that the base cations, potassium and calcium, in throughfall originate from

autochthonous and allochthonous sources, respectively. Potassium, a highly mobile element

and important cytoplasmic solute is assumed to derive from canopy leaching whereas calcium

is supposed to be immobilized in structural plant tissues and therefore to originate mainly from

dry deposition and its wash off from foliage surfaces. To examine the sources of both base

cations to throughfall we used tracer injection experiments with Rb+ for K+ and Sr2+ for Ca2+,

respectively, as the tracers exhibit similar chemical characteristics as the tracees.

The experiments were conducted in a tropical wet lowland rain forest in the Piedras Blancas

National Park, Costa Rica.

- 15 -

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2. Manuscript

Mechanisms of nutrient dynamics in the canopy of a tropical lowland rain forest

ABSTRACT

Throughfall represents an important pathway for internal recycling and external input of

nutrients in tropical rain forests. The objectives of this study were (1) to differentiate

between the main processes, dry deposition and canopy exchange, that control throughfall

chemistry, (2) to assess gross canopy N fluxes, and (3) to investigate the effect of canopy

components on canopy N exchange fluxes by in situ tracer applications in a tropical wet

lowland rainforest, Costa Rica. The relative contribution of canopy exchange and dry

deposition to throughfall fluxes of K+, Ca2+, and N were investigated by application of Rb+,

Sr2+, and 15N as respective tracers via stem injection of subcanopy trees, sampling of

throughfall above and below the canopy and additional surface wash experiments of labelled

leaves. We further developed a static model and dynamic model to demonstrate the relative

contribution as unidirectional and bidirectional fluxes, respectively. Gross canopy N fluxes

(influx and efflux) and net flux of N forms ammonium, nitrate and DON within canopy-

related branches were assessed by adoption of the 15N-Pool dilution method. Throughfall

below subcanopy trees was enriched in all major inorganic solute nutrients (K+, Na+, NH4+,

Mg2+, Ca2+, SO42-, PO4

3-,NO3-, and Cl-) and organic compounds (DOC and DON). Significant

amounts of tracer were found in all trees and leachates with the exception of 15N for the

latter one. On the basis of a two-source mixing model foliage leaching of K+ and Ca2+

contributed to throughfall enrichment to 14-32% and 32%, respectively. Relative

contributions of dry deposition and canopy exchange to net throughfall fluxes calculated by

the dynamic model were 25-60% and 40-75%, respectively, for Potassium and 25% and

75%, respectively, for Calcium. Gross canopy NH4+ fluxes were significantly higher than net

NH4+ flux. Influxes of NO3

- and DON were significantly different from effluxes, resulting in

net uptake by the canopy. The highest quantity of incorporated 15N was found in epiphytic

bryophytes (16.4%) and highest 15N uptake rate was shown by the epiphyll-fraction. The

study demonstrates that tracer in situ applications allow to investigate the highly dynamic

and complex throughfall processes and to dissect the fractional contribution of dry

deposition and canopy exchange. We further show that small net fluxes are the result of

much larger gross influxes and effluxes of N compounds and canopy exchange is therefore

controlled by bidirectional exchange fluxes. The results further demonstrate that epiphytic

communities play a major role in solute fluxes in canopies and therefore in nutrient

dynamics of tropical rain forests.

- 21 -

Keywords: dry deposition, canopy exchange, net throughfall, 15N, rubidium, strontium, stem

injection, tropical lowland rainforest, canopy leaching, canopy uptake, 15N-Pool dilution,

epiphytic bryophytes

INTRODUCTION

The chemical composition of precipitation is generally altered during interception by the

forest canopy where resulting throughfall becomes enriched in major inorganic and organic

soluble compounds. In consideration of total nutrient fluxes throughfall provides a small but

important supplementary nutrient input from outside into forest ecosystems. However, it

represents a major pathway in internal nutrient recycling through transfer of soluble

nutrients from the canopy to the forest floor (Parker 1983; Potter et al. 1991; Cavelier et al.

1997; Filoso et al. 1999; Chuyong et al. 2004; Tobón 2004). The significance of this

throughfall pathway is highlighted in tropical rain forests with continuous high rainfall and

low nutrient availability where nutrient cycling is a key for sustaining the productivity of

forest ecosystems.

Two main processes control throughfall chemistry, i.e. (1) dry deposition and subsequent

wash-off of dry deposited aerosols, gaseous and particulate materials from canopy surfaces

(Lovett 1992) and (2) canopy exchange (uptake and leaching) processes of dissolved

substances between external surface solution and the apoplast (exchangeable pool) of plant

tissues (Parker 1983; Lovett and Lindberg 1984). The balance between these processes is

assumed to be highly dynamic. Canopy uptake and leaching preferably occurs through the

cuticle and is primarily based on passive processes along concentration gradients (diffusion

of ions or molecules, and ion exchanges) and to an unknown portion by active processes

that are energy-dependent and can operate against concentration gradients (Tukey 1966;

Tukey 1970; Hambuckers and Remacle 1993).

Moreover, throughfall fluxes, throughfall chemistry and controls thereof exhibit a high

temporal and spatial variability (Hansen 1996; Lovett et al. 1996) which is determined by

complex interactions of factors and processes of atmospheric, hydrological, chemical and

biological nature and human activities as well. The amount, duration and intensity of

incident rainfall (Parker 1983; Potter et al. 1991; Hansen et al. 1994), the length of

antecedent dry periods (Lovett et al. 1996; Tobón 2004), the proximity to external sources

(Veneklaas 1990), the forest structure, age and canopy composition (Nadkarni 1984;

Reiners and Olson 1984; Hölscher et al. 2003; Hölscher et al. 2004; Clark 2005), the

texture of interacting surfaces (Tukey 1970; Reiners and Olson 1984) and the nutrient

status of the forest (Parker 1983) have been demonstrated to affect throughfall chemistry

and fluxes.

In tropical rain forests, canopy dwelling plants (vascular epiphytes, bryophytes and lichens),

- 22 -

which make up a remarkable portion of the canopy, acquire notable amounts of their

nutrients from atmospheric sources and from internal sources such as foliar leachates of

host plants and litter (Wania et al. 2002; Wanek and Pörtl 2005). Further, they exhibit a

nutrient pool equivalent to that in canopy foliage considering their low contribution to total

forest biomass (Nadkarni 1984). This leads to the suggestion that epiphytes may have an

important impact on the nutrient dynamics in tropical forest ecosystems.

To correct nutrient fluxes for wet (bulk) deposition net throughfall (NTF) has been defined

on an basis (Lovett et al. 1996):

NTF = TF – WD = DD + CE

where TF is throughfall, WD the incident precipitation, DD dry deposition and CE describes

the canopy exchange. Positive rates of NTF indicate a net increase of soluble compounds or

elements in throughfall through wash-off of dry deposition and canopy leaching. Negative

rates of NTF imply a decrease of solute concentration in TF due to canopy uptake or

interception loss.

However, while wet deposition is relatively easy to measure, the processes

underlying NTF such as dry deposition and canopy exchange are difficult to quantify. In

recent years attempts (Potter et al. 1991; Hansen et al. 1994; Lovett et al. 1996; Clark et

al. 1998) were made to separate these processes and to quantify their fractional

contribution to NTF. This becomes the more relevant in view of global atmospheric changes

e.g. increasing deposition of N and P and its impact on terrestrial ecosystem functions since

wet and dry deposition constitutes an external atmospheric nutrient input to the ecosystem

(allochthonous source) while canopy exchange represents an internal transfer of nutrients

(autochthonous source) (Lovett et al. 1996).

Lovett and Lindberg (1984) put forward a multiple regression approach on the basis

of event throughfall and precipitation measurements to estimate the contribution of dry

deposition and canopy exchange in net throughfall. This approach is based on the

assumption that dry deposition is solely correlated to the length of the antecedent dry

period and canopy exchange exclusively occurs during rain event and therefore scales with

amount of event precipitation. This approach has been successfully applied to temperate

and boreal ecosystems but less so in tropical rainforests and doubts have been raised on

the validity of this approach. Dry deposition can be underestimated since material

accumulated between rain events is not washed off completely and therefore is not included

in the NTF or dry deposition will be overestimated as it includes secreted substances from

tissue surfaces and material accumulated in throughfall samplers during dry periods. The

canopy budget model was developed by Ulrich (1983) and applied by Draaijers and Erisman

(1995) where Na+ in throughfall was assumed to entirely derive from dry deposition. Since

the input of Na+ from sea salts exceeds by far any internal circulation and/ or internal

sources or sinks were thought to be negligible and Na+ was proposed as a conservative

tracer of dry deposited elements in the canopy. However, Wanek et al. (2007)

- 23 -

demonstrated a large contribution of canopy leaching to Na+ fluxes in net throughfall of

mangroves, and Na+ uptake has also been observed in the Esquinas forest, Costa Rica

(Hofhansl, 2007, pers. commun.). Therefore, the use of other, more adequate tracers, e.g.

stable and radioisotopes in situ may provide an opportunity to gain deeper knowledge into

the mechanisms of net throughfall chemistry.

Earlier tracer studies with the stable isotope 15N solely focused on nitrogen deposition and

foliar uptake of 15N in temperate and boreal forests after spraying tracer solutions on the

canopy or parts thereof (Bowden et al. 1989; Boyce et al. 1996; Garten et al. 1998; Wilson

and Tiley 1998) but they did not consider foliar leaching. The one exception was published

by Draaijers et al. (1997) where a 35S tracer experiment was conducted in a Douglas-fir

forest to estimate the amount of sulfate in TF originating from root-derived sulfate and

leached from foliage. They labelled the surrounding soils of trees with radioactive

ammonium 35S-sulfate and calculated the relative contribution of foliar leached sulfate to

sulfate in TF using 35S specific activity. However, no such tracer studies have been

undertaken to investigate the mechanisms of solute changes in throughfall in tropical

rainforests, where the complexity and heterogeneity of forest canopies by far exceeds that

of higher latitude forests.

The present study therefore aimed to differentiate between dry deposition and

canopy exchange of potassium, calcium and nitrogen, respectively, using tracer in situ

applications via stem injection. We used 15N as well-established tracer for 14N, Rb+ as tracer

for K+ and Sr2+ as tracer for Ca2+ fluxes since the tracer have been shown to behave

chemically and biologically similar as their respective investigated element. To derive the

relative contribution of DD and CE to net throughfall fluxes in a tropical lowland forest we

developed a static model and a dynamic model of canopy fluxes. While the former model is

based on net fluxes only (deposition, uptake, leaching) the latter model is based on bi-

directional exchange fluxes (influx, efflux) in the canopy that occur simultaneously and

therefore represent gross fluxes in contrast to net fluxes. Potassium and calcium were

chosen since throughfall in tropical rain forests is generally enriched in those major cations.

However, the major process for K+ enrichment is assumed to be foliar leaching while for

Ca2+ dry deposition is thought to contribute significantly (Lovett and Lindberg 1984;

Chuyong et al. 2004). This is based on their different chemical behaviour and biotic

compartmentation: K+ is highly mobile and occurs in high concentrations in exchangeable

pools of plant tissues (Sattelmacher 2001) whereas Ca2+ is predominantly incorporated in

stable pools of structural plant tissues such as cell walls (Mecklenburg et al. 1966). Nitrogen

shows a much more complex pattern of changes in throughfall, owing to more diverse forms

of N such as ammonium, nitrate, and low- and high-molecular weight organic compounds

and an additional input process, i.e. biological fixation of atmospheric N2. N has been shown

to be taken up mostly in inorganic forms and to be released in organic forms. We therefore

hypothesized that the major source of K+ and Ca2+ are leaching and dry deposition,

respectively, and that inorganic N will be taken up by the canopy.

- 24 -

For the first time a 15N Pool dilution method was adopted to assess the gross N fluxes within

the canopy. As canopy exchange processes are assumed to be highly dynamic, particularly

in the case of N, we hypothesized that high rates of gross N fluxes (influx to and N efflux

from the canopy) occur behind rather small net N fluxes. We further expected that

epiphytes, predominantly epiphytic bryophytes, are the most important canopy component

affecting canopy exchange processes, due to their lack of a cuticle and large abundance.

The experiments were conducted in a humid tropical lowland rain forest in the Piedras

Blancas National Park, Costa Rica.

MATERIALS AND METHODS

Study site

This study was performed in the Esquinas forest of the Piedras Blancas National Park, which

is located close to the Golfo Dulce on the southern Pacific coast of Costa Rica (8°42’46’’ N,

83°12’90’’ W). The site is classified as tropical wet lowland rainforest (Holdridge 1967) with

altitudes from 0 to 597 m a.s.l and possesses narrow ridges, steep slopes with dense

drainage networks and valleys interspersed with meandering rivers. Average annual bulk

precipitation from 1998 to 2006 was 5720 mm (recorded at the Tropical Field Station La

Gamba) with a short, weak dry season from January to March (monthly mean precipitation

approx. 180 mm), and high rainfall from September to November (monthly mean about 800

mm). Throughfall (2005 to 2006) constituted 90.6% and 87.4% of total rainfall in the

primary ravine forest and secondary ravine forest, respectively (Hofhansl, 2007, pers.

commun.). Mean annual temperature between 1997 and 2001 was 27.4°C (monthly

average from 23.2°C to 31.5°C) at the Tropical Field Station La Gamba, which is next to the

Esquinas Rainforest, and 25.2°C (monthly average from 22.3°C to 28°C) inside the forest

(Weissenhofer and Huber 2001). According to Pamperl (2001) the prevailing soils are, in the

order of occurrence, ulitsols, inceptisols and entisols.

The experiments were conducted in an undisturbed primary ravine forest and a secondary

ravine forest (age 25 yrs) the latter one has been used as a cacao plantation.

Tracer techniques

Preliminary tests were carried out in February 2005; the main experiments were conducted

in September and October 2005. For the tracer experiments two different labelling

techniques were developed and performed: (1) stem injection of 15N, Rb+ and Sr2+ for

subcanopy trees (5-12 m height) along with a controlled leaf surface wash experiment and

(2) application of 15N-enriched simulated rain events (spraying on the canopy) to study

gross and net nitrogen fluxes on selected host tree branches that are densely covered with

vascular and cryptogamic epiphytes.

- 25 -

(1) Stem injection of 15N, Rb+ and Sr2+

Plant material and experimental setup

For the stem injection approach appropriate understorey tree species with easily reachable

and adequately developed crown, low or no epiphytic colonization and a sufficient number of

plans were selected at the two forest sites, namely Psychotria elata (Sw) Hammel

(Rubiaceae) in the primary ravine forest and Siparuna thecaphora (Poepp. & Endl.) A. DC.

(Monimiaceae) in the secondary ravine forest. Four trees of S. thecaphora were treated with 15N- and Rb+/Sr2+-solution and each four trees of P. elata were labelled either with 15N- or

Rb+/Sr2+-solution.

To inject the tracer solutions into the xylem channels stems were drilled with a borer (inner

diameter = 4 mm) to about 3 mm depth and a short metal tube was fitted into the hole.

The tube was connected by a flexible PVC tube to a 20 ml syringe reservoir (plastic) which

was attached above the hole. The reservoir and connecting tubes were filled with 20 mL

labelling solution (bubble free) and the reservoir was covered with parafilm and fixed to the

plant stem with tape. The 15N-labelling solution was made of Na15NO3, 15NH4Cl and

15NH415NO3 (equimolar) with a total N concentration of 60 mM and 98 at % 15N and the

Rb+/Sr2+-solution consisted of RbCl (100 mM) and SrCl2 (50 mM). Rb+ has been shown to

behave (bio-) chemically similar to K+ (Nyholm and Tyler 2000) and Rb-radioisotopes have

e.g. been used to study K+ uptake kinetics and fluxes in roots. Ca2+ and Sr2+ have been

applied to infer ecosystem Ca2+ sources and dynamics. We therefore used rubidium as

tracer for potassium and strontium as tracer for calcium fluxes within the canopy of

understorey trees. During the treatment period (two weeks) the solution was replenished

and the amount taken up by the plant was recorded daily.

Sampling

Sampling of rain water above and below the treated plants started after two weeks of tracer

amendment and included 3 rain events (< 10 mm to minimize dilution of ion concentrations

in throughfall) for each plant in short intervals. Water samples (throughfall above the

canopy and below the canopy) were collected in four replicates (plants), i.e. four trees

labelled by stem injection, per rain event. Each throughfall sampler was constructed from

two gutters (0.75 m length) fixed above and beneath the canopy of the sample tree on a

bamboo-pillar. Both gutters were provided with an outlet where the incident rainfall was

channelled through flexible PVC tubes down into plastic flasks (1.5 L). The flasks were

emptied, cleaned carefully and washed with deionized water after each rain event during

the sampling period. Aliquots of bulk precipitation samples from the open field at the Field

Station La Gamba were also taken. The collected rain water (approx. 100 mL each) was

poured into PE flasks and HgCl2 was added (0.1 mM final concentration) to avoid microbial

growth and alteration of nutrient content in throughfall. The water samples were kept frozen

until transport to Vienna.

- 26 -

Leaf samples of each tree were collected before starting the labelling and after the labelling

period, between the two throughfall samplers after every collected rain event. One half of

fresh leaves was treated in a microwave oven to stop enzymatic activities and further dried

at 60 °C for 72 h. The other half of labelled leaf samples was used for controlled leaching

experiments. For this three to five leaves of each leaf sample per plant and event were put

into a plastic bag with zip and a defined amount (100 mL to 200 mL, depending on leaf

size) of deionized water (>18.2 MΩ cm-1) was added. After two hours the leaching

experiment was stopped and water with leachates was collected in scintillation flasks and

treated similar as throughfall samples. The fresh leached leaves were dried superficially,

scanned to determine leaf area and finally treated in the microwave oven and dried as

described above.

(2) Application of 15N-enriched simulated rain events

Plant material and experimental setup

For the 15N-enriched rainwater application branches (1.0-1.5 m length) with a typical load of

vascular epiphytes, epiphytic bryophytes and epiphylls on host leaves from the primary

ravine forest were selected. Two horizontal branches of Alchornea cf.costaricensis Pax & K.

Hoffm. (Euphorbiaceae) and three horizontal branches of Ficus tonduzii Standl. (Moraceae)

were cut and hung horizontally under a tin roof at the forest edge in the garden of the

Tropical Field Station La Gamba to avoid uncontrolled contact with incident rainfall. The cut

surface of the host branch was covered with water-soaked tissue in a plastic bag taped to

the branch to minimize water stress of host leaves.

Rainwater was collected on an event basis in a big PVC barrel (200 L) in the open field, each

before a simulated rain event as far as it was raining. In intervening periods with no rainfall

the barrel was covered by a plastic foil and put into the shadow to minimize chemical

alteration. The first simulated rain event applied was enriched with the stable isotope 15N in

equimolar amounts of NO3-, NH4

+ and DON (0.55 µM 15NH415NO3/

15N-L-glutamic acid

equivalent to 10 at% 15N). The following simulated rain events were conducted with

unlabelled rainwater. Under each branch a plastic cover was fixed to ensure complete

throughfall collection. The dripped off throughfall was channelled and filtered through small

mesh nylon into polyethylene canisters. Before starting the experiment the branches were

sprayed with unlabelled rainwater to test water retention and the amount of water required

to penetrate the whole branches and its canopy components. The amount was depending on

the load size of epiphytes (1.5-2.5 L). To provide enough time for solute exchange every

single simulated rain event was applied in two aliquots of 6 to 14 min duration with an

interval of about one hour in between. The branches were sprayed evenly with a defined

amount of rainwater using a pressure sprayer to simulate drizzle. Simulated rain events

were conducted twice a day in constant intervals over a period of four to five days. This was

- 27 -

the maximal possible time before the host leaves started to loose turgor. Separate

equipment was used for 15N labelled rain event and normal rain events, respectively, and

the equipment was washed and cleaned carefully after each rain event.

Sampling

To quantify the amount of rainwater applied the remaining volume within the pressure

sprayer was weighed. After a drip off time (two hours) the volume of throughfall collected in

the polyethylene canisters was measured. Aliquots (250 mL) of rainwater and throughfall

filtered through fine mesh nylon were transferred into PE bottles after each rain event,

HgCl2 was added (0.1 mM final concentration) and samples were kept frozen until transport.

At the end of the experiment the branches were divided into different canopy components:

vascular epiphytes, epiphytic bryophytes, epiphytic lichens, epiphylls, host leaves, canopy

soil, canopy litter, bark and wood. Unlabelled reference material of canopy components and

host leaves were sampled from other branches of the same trees at the original place. The

fractions were treated in a microwave to stop microbial or enzymatic activities and the

material further dried at 60 °C for 72 h. Dry weight of each fraction was determined and

samples were weighed and packed airtight until transport to Vienna.

15N-Pool-Dilution approach

Pool dilution techniques are commonly used to estimate gross N transformation rates in

soils (Luxhoi 2004). For instance to measure gross N mineralization the NH4+-pool is

labelled by addition of 15NH4+. N mineralization produces NH4

+ from an unlabelled organic N

pool, therefore adds 14NH4+ to the pool and ultimately leads to dilution of the 15N:14N signal

in the NH4+-pool. In contrast, processes that consume NH4

+ such as microbial uptake

(immobilization), plant uptake or nitrification, do not change the 15N:14N signature of this

pool and utilize NH4+ at the 15N:14N ratio present in the NH4

+-pool.

This method was transferred to gross canopy fluxes of N. The N fractions (NO3-, NH4

+ and

DON) of rainwater before canopy contact represent the pools which are labelled with 15N.

During penetration through and interception by the canopy the pools are altered. Inputs of 14N may occur through canopy leaching or N2 fixation processes and 15N:14N losses may

happen due to canopy uptake processes. Determination of isotope signatures and N

concentrations enabled the quantification of gross and net N canopy fluxes. Furthermore, 15N dilution is studied along a time axis of e.g. 4 to 24 hours. In the approach transferred to

canopy processes the time axis is represented by rainwater before and after canopy

exchange and rates are therefore presented on an event basis and not per hour or day.

- 28 -

Fig.1: Scheme of the 15N-Pool-Dilution approach for (A) determination of gross N mineralization rates and (B)

transferred to gross canopy fluxes of N; WD: wet deposition, TF: throughfall

Chemical analyses

Chemical analyses were carried out in Vienna, Department of Chemical Ecology and

Ecosystem Research, University of Vienna.

Stable isotope analyses

Dried organic materials such as leaves, bark and epiphytes of both experiments were

redried at 80°C for 24h and ground to a fine powder in a ball mill (Retsch MM2 and MM200).

Aliquots of 1-2 mg were weighed into Sn-capsules and 15N and total N content were

analysed by continuous-flow IRMS (isotope ratio mass spectrometer). The system was

made up of an elemental analyser (EA 1110, CE Instruments, Milan, Italy) which was

connected to the IRMS (DeltaPLUS, Finnigan MAT, Bremen, Germany). Reference gas (high

purity N2, AGA, Vienna, Austria) was calibrated to the atmospheric N2 standard (at-air)

using IAEA-NO3, IAEA-N-1, and -2 reference material (International Atomic Energy Agency,

Vienna, Austria).

Aliquots (50 mL) of rain samples (throughfall above and below the canopy, and leachates)

of the stem injection experiment were therefore evaporated to dryness with a rotary

evaporator (R-114/ R-124, Büchi, Switzerland). Samples were redissolved in 0.5 mL

distilled water, dried again using a SpeedVac concentrator (SC 110, Savant, NY) and

redissolved in 100 µL distilled water. Aliquots (50 µL) were dried in Sn-capsules and

analysed for 15N and total N content by IRMS.

Water samples of the simulated rain application tracer experiment

For stable isotope analyses rain and throughfall samples of the 15N-enriched rain event (i.e. 15N-Pool dilution approach) and the consecutive two unlabelled rain events were used. Rain

and throughfall samples of the 15N-enriched rain event and the first unlabelled rain event

Mineralization

Consumption

+ 15N addition

+ 14N

NH4+ pool

in soil

- 15N:14N

15N:14NH4+

(A)

Canopy Leaching

Canopy Uptake

+ 15N addition

+ 14N

RainwaterWD & TF

- 15N:14N

15N:14NH4+

15N:14NO3-

15N:14DON

(B)

- 29 -

were fractionated into the N forms ammonium, nitrate and dissolved organic nitrogen

(DON). Water samples of the second unlabelled rain event were treated as mentioned

above. Small aliquots of original samples were analysed for anions and cations by HPLC (DX

500, Dionex, Vienna, Austria) and total dissolved N and DOC concentration by NPOC/TDN-

analyzer (Shimadzu, Japan) as well.

Isolation of N fractions

According to Hertenberger and Wanek (2004) the microdiffusion approach to isolate

ammonium can be combined with common cation-exchange chromatography to separate

the organic nitrogen fraction from nitrate. Sample volumes were determined, pH was

adjusted to 5-6 with 0.1 N HCL to avoid volatile loss of NH3 or NO2 during evaporation, and

then samples were concentrated under reduced pressure to about 10 to 15 ml with a rotary

evaporator and weighed again. The concentrates were poured into Schott-flasks (50 mL)

and acid traps were added. Acid traps were made of quartz fibre filter discs containing 12 µL

2N H2SO4 which were enclosed in a strip of Teflon band that is gas permeable but

waterproof. After addition of MgO (100 mg) to increase pH >9.0, the bottles were

immediately closed and NH3 released into the head space was captured by the floating acid

traps. The flasks were placed on a shaker for 5-7 days (75 rpm, 37°C). Then acid traps, still

enclosed in Teflon band, were removed and dried over conc. H2SO4 under vacuum

(24-48 h). The dry filter discs were transferred into Sn-capsules shortly before IRMS-

analysis to avoid corrosion of the capsules. The remaining concentrates containing nitrate

and DON were filtered to remove MgO and pH was adjusted to 5-6. Samples were then

applied onto cation-exchange resin columns (2-3 mL DOWEX 50Wx8, 50-100 mesh,

H+-form, Fluka). The columns were washed two times with 10 mL distilled water to collect

nitrate in the flow-through and the eluate was neutralized with 0.1 N KOH. The nitrate

fractions were evaporated to dryness redissolved in 0.5 mL distilled water, again dried by

SpeedVac and redissolved in 70 µL distilled water. Samples were quantitatively transferred

and dried in Sn-capsules and analysed for 15N and total N content by IRMS. The adsorbed

DON fractions were eluted from the resin with 25 mL 1 N HCL and treated as the nitrate

fractions after addition of 100 µL 1 N KOH.

Ion, NPOC and TDN analyses

Hot-water extracts were prepared by extracting aliquots of 20 mg ground leaf material with

1 mL deionized water at 95°C for 60 min. Inorganic cations (K+, Rb+, NH4+, Na+, Ca2+, Sr2+,

Mg2+) and anions (Cl-, NO3-, SO4

2-, PO43-) in rain samples (bulk precipitation, throughfall

above and below the canopy, and leachates) and hot-water extracts of the stem injection

experiment were analysed by HPLC (high pressure liquid chromatography, DX 500, Dionex,

Vienna, Austria) and conductivity detection. Anions were separated on an anion exchange

column (AS11, 4x250 mm, Dionex) using a linear KOH gradient (2 to 40 mM in 6 min, total

run time 12 min). Cations were separated on a cation exchange column (CS16, 5x250 mm,

- 30 -

Dionex) by an isocratic method with methanesulfonic acid as eluent (28 mM, 45 min,

60 °C). Rain water samples of the simulated rain events were analysed for anions (Cl-, NO3-,

SO42-, PO4

3-) as mentioned above and but method to analyse cations (K+, NH4+, Na+, Ca2+,

Mg2+) were modified (30 mM methanesulfonic acid for 26 min and 40 °C).

NPOC (non purchable organic carbon) and TDN (total dissolved nitrogen) of rain samples

(bulk precipitation, throughfall above and below the canopy from both experiments) were

determined using a TOC-VCPH/CPN/ TNM-1 analyzer (Shimadzu, Japan). Inorganic carbon was

automatically removed during measurement through addition of 2 N HCl and purging with

synthetic air (CO2 free). DON was calculated by subtracting DIN (dissolved nitrate and

ammonium) from measured TN.

Calculations

Enrichment factor for the stem injection experiment

The enrichment factor (EF) for TFb (throughfall below the experimental plant) was

calculated as:

(i) EF = c(TFb)/ c(TFa)

where c is the concentration of the respective solute in TFb and TFa (throughfall above the

experimental plant)

Two-source mixing model for the stem injection experiment

The fractional contribution of leaching from host tree foliage to throughfall below the canopy

(TFb) was estimated for potassium, calcium and nitrogen with a modified two-source mixing

model:

(ii) % PLEA = 100 x (RTFa - RTFb)/ (RTFa – RLEA)

where PLEA represents the relative fraction of potassium, calcium or nitrogen, leached from

foliage of labelled woody plants to throughfall below the canopy (TFb), R the ratios of

tracers e.g. Rb+:K+, Sr2+:Ca2+ or 15N:14N in TFa, TFb, and leachates from controlled

leaching-experiment (LEA). In consideration of the dynamic model CE (canopy exchange)

and DD (dry deposition) are estimated in absolute numbers:

(iii) CE = TFb x % PLEA = TFa x EF x % PLEA

For TFa an arbitrary concentration of 1 is assumed and CE represents the relative

contribution of canopy exchange to TFb. Consequential the relative contribution of DD to

TFb can be estimated as:

- 31 -

(iv) DD = TFb – (TFa + CE)

15N-Pool-Dilution equation

The equations to determine gross N mineralization or nitrification from Bengston (2006)

were adopted and modified to calculate gross N canopy fluxes:

ln ((fTF-k)/(fWD-k))ln (WTF/WWD)

(v) Efflux = x WWD - WTF

ln ((fTF-k)/(fWD-k))ln (WTF/WWD)

(1 + ) x WWD - WTF(vi) Influx =

where k represents the natural abundance of 15N in rainwater (at% 15Nunlabelled WD), (fWD-k)

the APE (atom percent excess) of 15N in applied rainwater (wet deposition, WD), (fTF-k) the

APE of 15N in throughfall (TF), WWD the initial N pool in wet deposition (in µg N), and WTF the

N pool in TF (in µg N) of the respective fractions i.e. ammonium, nitrate or DON. Therefore,

each flux was calculated separately for NH4+, NO3

- and DON.

Statistical analyses

Statistical analysis was performed with STATGRAPHICS PLUS 5.0. Software (Statistical

Graphics Corp., Rockville, MD, USA). Differences between three or more groups were tested

by one-way analysis of variance (ANOVA) (multiple range test, LSD test) (P <0.05).

RESULTS

Stem injection experiment

The stem injection experiment was performed to examine the potential contribution of

subcanopy trees to nutrient enrichments in throughfall through foliage leaching. In

preliminary tests in February 2007 we observed that stem injection was an effective

labelling method for the understorey trees P. elata (representative for secondary forest) and

S. thecaphora (representative for primary forest). Foliar, hot-water extractable

concentrations of rubidium and strontium increased 15-fold and 1.5-fold, respectively,

compared to unlabelled plants, and δ15N values ranged from 80 to 2000 (data not shown).

During the main experiment conducted in September and October 2007 a

considerable enrichment of rubidium, strontium, and 15N in leaves was reached by stem

injection (Fig.2, closed bars). Moreover controlled leaching experiments were carried out to

determine tracer leaching efficiency of labelled leaves. Similar ratios were detected for

- 32 -

Rb+:K+ and Sr2+:Ca2+ in leachates (LEA) compared to leaves (L) in S. thecaphora (Fig. 2D

and 2E). P. elata exhibited a markedly lower leaching efficiency for all tracers applied, as

also shown for 15N in S. thecaphora (Fig 2A, 2B, 2C, and 2F).

Ratios of tracers and their corresponding elements (Rb+:K+, Sr2+:Ca2+ and 15N:14N) in bulk

precipitation, collected in the open field (P), were slightly higher in comparison to those in

throughfall collected above the canopy of treated plants (TFa), though no significant

differences were observed.

Comparison of tracer:element ratios of TFa and TFb (throughfall below the canopy) showed

no statistically significant differences. Enrichment factors for solutes in throughfall (TFb) are

shown in Table 1. Concentration of all determined solutes were enriched in TFb with

greatest enrichment factors in PO43- (3.17) followed by K+ (2.52) and the other solutes

(1.18 - 1.96). Interestingly EF was lowest for all N solutes, Na+ and DOC (Table 1).

Based on concentration ratios of Rb+:K+, and Sr2+:Ca2+, respectively, and on a modified

two-source mixing model we estimated the fractional contribution of leaf leachates from P.

elata and S. thecaphora to throughfall (TFb) enrichment of K+ and Ca2+ (Fig. 2A, 2D, and

2E). The two sources of K+ (Rb+) and Ca2+ (Sr2+) to TFb are TFa and LEA. This approach

was only used where the two sources, TFa and LEA were significantly different i.e. Rb+:K+ in

P. elata and S. thecaphora and Sr2+:Ca2+ in S. thecaphora (Fig. 2A, 2D, and 2E). We

assumed that TFb was composed of TFa and LEA and from dry deposition (DD). The ratios

of Rb+:K+ and Sr2+:Ca2+, respectively, in LEA represented the maximum possible

contribution of foliage leaching of woody plants to TFb, while the respective ratios in TFa

represent the minimum contribution by LEA. The results showed that S. thecaphora

considerably contributed to throughfall enrichment of both potassium and calcium via

foliage leaching i.e. 14% and 32%, respectively. P. elata contributed 32% of K+ to TFb by

leaching.

Though 15N:14N ratios were significantly higher in bulk leaves than in all other fractions the

tracer (15N) did not show up in leachable pools at the time of experiment (Fig. 2C, 2F). The 15N:14N ratios therefore did not differ significantly between LEA, P, TFa, and TFb for both P.

elata and S. thecaphora and therefore hindered the application of the two source model to

evaluate N sources in throughfall (see also Sr2+:Ca2+ in P. elata, Fig, 2B).

Simulated rain experiment

Net fluxes

A total of five branches (A. cf. costaricensis plus F. tonduzzi) where treated with simulated

rain events twice a day over a period of 3 days where the first applied event was enriched

with the stable isotope 15N. Net fluxes were calculated as volume-weighted concentrations

of TF minus those of WD (wet deposition). Negative net fluxes indicate net leaching of

- 33 -

canopy components while positive net fluxes refer to net uptake. Time dependent course of

event based net fluxes of N fractions (NO3-, NH4

+, DON – dissolved organic nitrogen), base

cations (K+, Mg2+, Ca2+ and Na+), anions (SO42-, PO4

3-) and DOC (dissolved organic carbon)

showed a high variability throughout the experiment but no correlation among ions and

fractions were evident, respectively (data not shown). Thus we calculated the sum of net

fluxes of seven events conducted within three days (Fig. 3). To increase the comparability

between branches event based and integrated net fluxes were calculated on the basis of the

sum of dry weights of potentially canopy components contributing to net solute exchange

(bark, canopy litter, canopy soil, epiphylls, vascular epiphytes, leaves, lichens, and epiphytic

bryophytes) not including wood.

Net fluxes of DOC were more than ten-fold higher compared to all other solutes and showed

the highest increase in throughfall (102 ± 42 µmol g-1 DW 3 d-1) with a range from 262 to

29 µmol g-1 DW 3 d-1 and therefore was excluded from statistical analysis of the other

solutes.

Net fluxes of N fractions indicated a net uptake by the canopy and mean values differed not

significantly with 0.29 ± 0.28, 0.82 ± 0.15 and 0.33 ± 0.69 µmol g-1 DW 3 d-1 for nitrate,

ammonium and DON, respectively.

All base cations exhibited a net loss (leaching) from the canopy. Potassium concentration

significantly increased from precipitation to throughfall (-7.1 ± 2.1 µmol g-1 DW 3 d-1)

followed by calcium and magnesium (-2.02 ± 0.29, -0.79 ± 0.37 µmol g-1 DW 3 d-1,

respectively). Canopy exchange of sodium and chloride resulted in net uptake (1.22 ± 0.79

and 6.2 ± 1.2 µmol g-1 DW 3 d-1, respectively). In contrast anions such as sulfate and

phosphate showed a weak net canopy leaching (-0.21 ± 0.10 and -0.21 ± 0.06 µmol g-1 DW

3 d-1, respectively).

Gross N fluxes

A 15N-Pool dilution assay was performed to determine gross canopy N fluxes i.e. efflux and

influx of NH4+, NO3

-, and DON (Fig. 4) and their integral i.e. net fluxes were assessed as

well. Overall all three fractions showed significant higher influx than efflux which resulted in

net canopy uptake. Furthermore, influx of nitrate and DON did not differ significantly from

net fluxes with the exception of ammonium. Both rates of NH4+-influx to and -efflux from

the canopy were significantly higher than net flux and were of same magnitude (-0.160 ±

0.040 and 0.156 ± 0.048 µmol 15N g-1 DW, respectively). The resulting net ammonium

uptake rate was therefore almost zero (0.004 ± 0.008 µmol 15N g-1 DW). Net uptake rates

for NO3- and DON were 0.019 ± 0.011 and 0.081 ± 0.035 µmol 15N g-1 DW, respectively.

Gross fluxes of nitrate were markedly lower compared to those of ammonium and DON i.e.

6-fold lower for efflux and up to 15-fold lower for influx. Statistically differences of N-

fractions ammonium, nitrate and DON in gross canopy fluxes (influx, efflux) and net N

- 34 -

uptake are shown in Table 2. Efflux and net N uptake of NH4+ significantly differ from those

of NO3- and DON.

Canopy components

The mean total biomass of the five branches averaged 137 ± 17 g dry weight whereof wood

accounted for 64.4% but was excluded from further calculations through negligible

contribution to exchange processes of solutes: <0.1% of the initial 15N-amount was found in

wood. The partitioning of dry biomass into different canopy components is shown in Figure

5A. The greatest fraction was made up of leaves (L) and bryophytes (M) which exhibited

similar quantities with 19.3 ± 5.6 g and 17.2 ± 4.0 g, respectively. In addition they

significantly differed from all other fractions, which ranged from 0.5 ± 0.2 to 5.0 ± 3.6 g in

descending order: vascular epiphytes (Ev), epiphylls (Eph), canopy litter (CL), bark (Bk),

canopy soil (CS), and lichens (Lich).

The recovery of 15N, applied by the first simulated rain event, in throughfall and canopy

components, amounted to 69.8%. Throughfall, the first three events were measured for 15N, accounted for almost two thirds of the 15N detected (43.3%). About 96% of throughfall 15N was found in throughfall collected during the first 15N-enriched rain event. The

partitioning of 15N within the canopy is shown in Figure 5B in percent of initial 15N amount

applied in rainwater. The highest quantity of incorporated 15N was determined in epiphytic

bryophytes with 16.4% or 0.2 µmol 15N. All other canopy fractions did not differ significantly

although epiphylls and leaves exhibited somewhat higher 15N contents (3.3% and 3.2%,

respectively) than residual fractions whose proportions ranged from 1.3% to 0.4% in the

descending order: bark, canopy litter, canopy soil, vascular epiphytes and lichens. 15N uptake rates of canopy components were estimated (Fig. 5C) but in contrast to

distribution of 15N within the biomass, the epiphyll-fraction showed the highest uptake rates

(0.033 ± 0.013 µmol 15N g-1 DW after 3 days). Bark, canopy litter, vascular epiphytes,

lichens and mosses incorporated 15N from incident rainwater at a lower rate (0.016 ± 0.008

to 0.012 ± 0.005 µmol 15N g-1 DW) compared to epiphylls. Despite leaves made up the

highest proportion of dry mass their uptake rates were lowest (0.001 µmol 15N g-1 DW).

DISCUSSION

Models to demonstrate controls of throughfall chemistry

We developed two models to interpret alterations in throughfall chemistry and their control

mechanisms (Fig. 6). On the one hand we adopted a static model which is based on

unidirectional net fluxes (NTF) alone (Fig. 6, left plot) which can be either negative or

positive, or zero. A negative NTF of a solute corresponds to a decrease in its concentration

- 35 -

after penetration of the canopy (Fig. 6, A) and is generally caused through uptake

processes within the canopy. By contrast an increase in nutrient concentrations in

throughfall results in positive net throughfall fluxes and is caused by foliar leaching and/ or

dry deposition (Fig. 6, C). Apart from nutrient depletions or enrichments in throughfall

solute fluxes of incident rainfall (TFa) and throughfall (TFb) may be balanced and the

resulting net flux is zero (Fig. 6, B) although exchange processes take place within the

canopy. In this case, investigations of net nutrient fluxes are inadequate to explain the

nutrient dynamics within the canopy. Canopy exchange in situations of zero net fluxes can

only be demonstrated by tracer studies.

Thus, we additionally put forward a dynamic model (Fig. 6, right plot) which implies

alterations in throughfall chemistry and concentrations through bidirectional fluxes i.e.

foliar leaching and foliar uptake. For instance, the concentration of a solute may not

change during penetration of the canopy, therefore the net nutrient flux is zero. However,

the solute might be taken up by some canopy components and be released by others at

the same time. According to the extent of canopy exchange fluxes, the dynamic model is

illustrated by three different scenarios: scenario one (Fig. 6, sc1) indicates no contribution

of exchange processes to throughfall chemistry, in scenario two (Fig. 6, sc2) 50% of

solutes are exchanged and scenario three (Fig. 6, sc3) shows a complete exchange of a

nutrient in rainwater during canopy passage.

To investigate these exchange processes and given that net fluxes do not provide answers

to the origin of soluble nutrients in throughfall, we conducted tracer experiments for

selected nutrients, namely potassium, calcium, and nitrogen, to follow their pathways

within the canopy. Tracers used were Rb+, Sr2+, and 15N that behave (bio-) chemically

similar to the tracee. Stem injection tracer experiments along with controlled foliar

leaching experiments, may allow differentiating whether nutrient enrichment in throughfall

resulted from dry deposition or foliage exchange and to what extent (Draaijers et al.

1997).

We demonstrate four cases in the stem injection experiment (Fig. 7) where tracer:element

ratios of throughfall (TFb) are compared to the corresponding ratios of throughfall above

the labelled canopy (TFa) and foliar leachates (LEA). Case 1 does not allow any derivation

of the fractional contribution of dry deposition or foliage exchange to TFb enrichment since

no significant amounts of tracer were found in leachates. The remaining cases (Fig. 7, case

2-4) allow detecting potential foliage exchange effects on throughfall chemistry. In case 2

ratios of TFb and TFa do not differ although the tracer was found in LEA. This suggests that

foliar exchange does not contribute to TFb chemistry. Net positive fluxes of potassium,

calcium or nitrogen in throughfall therefore are controlled by dry deposition and for

nitrogen by N2-fixation. Further this case agrees with scenario one from the dynamic

model. In case 3, TFb is obviously tracer enriched (higher ratio) due to foliar exchange

- 36 -

contributing about half to TFa but the contribution of dry deposition can not be estimated

with this approach and fits with scenario 2 of the dynamic model. The model can be

expanded based on enrichment factors (EF) and tracer:element ratios to derive the fraction

of canopy exchange (CE) and dry deposition (DD) (see (i) and (ii)). Finally in case 4,

according to scenario three of the dynamic model, throughfall chemistry is replaced

completely by foliar uptake and -leaching.

Throughfall enrichment

Measurements of throughfall chemistry below the canopy of understorey trees (P. elata

and S. thecaphora) in a tropical lowland forest showed an increase of all examined solutes

and therefore positive net throughfall fluxes (Table 1). We hypothesized that TF

enrichment of potassium was controlled by foliar leaching, whereas calcium derived mainly

from dry deposition and nitrogen originated from both foliar leaching and dry deposition,

and from N2-fixation.

In three cases of the stem injection experiment (Fig. 2A, 2D and 2E) injected rubidium and

strontium was found in leachates therefore allowing to estimate the contribution of canopy

exchange (leaching) to TFb for potassium and calcium in the understorey tree S.

thecaphora (in the primary ravine forest) and for potassium in P. elata (in the secondary

forest). According to the static model the relative contribution of foliar leaching to

potassium and calcium in throughfall below S. thecaphora canopies was 14% and 32%,

respectively, and for potassium in throughfall under P. elata 32% (see equation (ii)).

Considering that net leaching of solutes to throughfall is the result of canopy exchange

processes their contribution was evaluated by the dynamic model (see equation (iii) and

(iv)). K+ in throughfall under P. elata seems to derive to almost equivalent parts from foliar

exchange (4/9) and dry deposition (5/9) whereas potassium and calcium concentrations in

throughfall under S. thecaphora were controlled by canopy exchange (3/4). In the case of

P. elata results of the tracer experiment are contradicting to our hypothesis as it is

generally agreed that potassium derives mainly from canopy leaching due to its extreme

mobility and high apoplastic K+ concentration in plant tissues (Parker 1983; Sattelmacher

2001). Comparative data for dry deposition fluxes of potassium were found for temperate

forests and tropical rain forests. For a mixed hardwood forest (Lindberg et al. 1986)

estimated that dry deposition contributed about 60% of potassium as coarse particles.

Lovett and Lindberg (1984) suggested that dry deposited K+ derived from suspended soil

or biological material. Moreover they observed highest K+-aerosol concentrations from

within the forest itself indicating an in-canopy source.

Calcium in throughfall is considered to almost equally originate from wash off of particulate

dry deposition (Lovett and Lindberg 1984; Lindberg et al. 1986; Forti and Neal 1992) and

foliar leaching (Tukey 1970; Parker 1983). However, we observed that a large portion

- 37 -

derives from foliage leaching. Calcium is generally tightly bound in structural plant tissue,

e.g. cell walls, and leaching of calcium mostly occurs from exchangeable pools in leaves,

where calcium has been recently accumulated. Moreover, mature tissues are more

susceptible to leaching than younger leaves since the latter accumulate calcium directly

within cell walls and have a smaller exchangeable pool for calcium (Mecklenburg et al.

1966). Though Hansen et al. (1994 and 1996) reported that dry deposition rather has an

impact on throughfall fluxes in the upper canopy layers, where trees are highly exposed to

turbulent air movements, our results indicate an effect of dry deposition on K+ and Ca2+ in

the understorey.

In the other three cases the stem injection experiment failed, including those for nitrogen

for both species (Fig. 2B, 2C and 2F). We therefore could not clarify whether increased

concentrations of nitrogen and calcium in TFb were caused by dry deposition, foliar

exchange or in the case of nitrogen by atmospheric nitrogen fixation as well. Although the

tracer injection was successful and tracer was allocated to foliage, this signal was not

found in foliar leachates. It is therefore likely that the injected tracers, 15N and strontium,

were incorporated and immobilized in stable pools within the plant, e.g. cell walls and

proteins, and therefore were removed from the exchangeable pool, the apoplast solution,

during the two weeks of injection.

Consequently this experiment showed that foliar leaching from understorey woody plants

contributes to throughfall enrichment of potassium and calcium by 40-75% in a lowland

tropical rain forest. Nevertheless, the experimental setup was not suitable to investigate N

exchange processes within the canopy and to determine whether canopy impacts are

controlled by unidirectional (static model) or bidirectional (exchange) fluxes (dynamic

model). Though we were able to estimate net canopy fluxes (DD, leaching) the underlying

gross fluxes i.e. canopy uptake and canopy leaching, could not be studied by this

approach. Therefore we adopted the 15N isotopic pool dilution approach, generally used to

determine gross N mineralization rates in soils (Bengston 2006), to detect gross canopy N

fluxes, or more precisely influxes to and effluxes from the canopy, for different N species,

namely ammonium, nitrate and DON (dissolved organic nitrogen). The terms efflux and

influx instead of uptake and leaching, respectively are more appropriate as we were not

able to determine dry deposition of nitrogen on canopy surfaces. In this approach

rainwater labelled with 15N the tracer is diluted by canopy leaching (efflux) and “consumed”

by canopy uptake processes (influx).

The 15N pool dilution experiment clearly demonstrated that high rates of gross N fluxes of

solutes occur across canopy components in tropical rainforests even though net fluxes are

minor or negligible (see Fig. 4, NH4+). Moreover, this also supports the dynamic model

therefore pointing to a greater importance of canopy exchange processes than evident

- 38 -

from net flux measurements alone. Canopies obviously are exchanging nutrients in a

bidirectional way (Wanek et al. 2003; Wanek and Pörtl 2005) and at higher rates than

previously anticipated which is analogy to foliar CO2- and H2O- exchanges that by far

exceed net CO2 assimilation and water losses (Heldt 1998; Helliker and Griffiths 2007).

Additionally the 15N-pool dilution approach revealed a net uptake of all examined N species,

ammonium, nitrate and DON (Fig. 3 and 4). This is in contrast to previous studies (Coxson

1991; Lovett and Lindberg 1993; Filoso et al. 1999; Wania et al. 2002) where throughfall

measurements in tropical forests showed that inorganic N, nitrate and ammonium, were

indeed taken up by the canopy, but DON leached from the canopy. Filoso et al. (1999)

suggested that large enrichments of DON in throughfall resulted from retention and

assimilation of deposited inorganic N on canopy surfaces, mainly by epiphytic algae, and

subsequent washing off or leaching as DON during later rain events. Further Coxson

(1991) reported that nutrients, mainly organic compounds, were released to a great extent

by epiphytic bryophytes. Due to the fact that bryophytes often provide a habitat for N2-

fixing bacteria and the occurrence of lichens with cyanobacterial symbionts (Bentley 1987)

release of N from atmospherically fixed nitrogen might provide another source of DON

within the canopy. On the basis of our data we cannot solve this discrepancy. DON such as

amino acids are readily taken up by plants as an N source which plants can immediately

utilize. Since in this experiment labelled DON was provided in the form of 15N-Glutamic

acid, canopy components such as cryptogamic epiphytes might utilize added DON in

rainwater. Although the N was supplied in a single pulse similar results were found for

three days of treatment with simulated rain events with ambient rainwater (Fig. 3) and for

a whole year period (Hofhansl, pers. commun.).

Even though net uptake of ammonium and nitrate did not differ (Table 2) gross influx of

ammonium (and DON) were about ten times higher than influx of nitrate. This may be due

to negatively charged sites in cell walls or cuticles e.g. carboxyl groups of pectins which

affect the passage of ions through the apoplast. The movement of ions in cell walls

(apoplast) is characterized by electrostatic interactions leading to preferred uptake of

cations (NH4+, positive charged amino acids) and repulsion of anions (NO3

-) (Clarkson

1993; Marschner 1995; Sattelmacher 2001).

At the first sight, the data with regard to net N fluxes from the second tracer experiment

disagree with results from the stem injection tracer experiment, since in the latter

experiment weak net leaching of ammonium, nitrate, and DON was found. However, it has

to be considered that the investigated canopies of the stem injection experiment only

consisted of foliage and small branches of woody plants and did not exhibit any visual

epiphytic or epiphyllous colonization whereas the branches of the second experiment were

richly covered with epiphytic bryophytes and vascular epiphytes and host tree leaves were

- 39 -

colonized by epiphylls. Thus, results of both experiments can not be directly compared

together but clearly demonstrate that epiphytic communities obviously play a major role in

solute fluxes in canopies and therefore in nutrient dynamics of tropical rain forests. This is

consistent with other studies (Nadkarni 1984; Chuyong et al. 2004) which observed higher

retention of N within the canopy with greater abundance of epiphytic bryophytes. Nadkarni

(1984) determined for a tropical cloud forest that the epiphyte biomass constituted less

than two percent of total forest dry mass but retained nutrients equivalent to 45% of that

in forest foliage and therefore might act as a “buffer” or an additional nutrient pool for

previous inaccessible sources and later release to other canopy components.

Net throughfall fluxes of DOC (dissolved organic carbon) were substantially higher in the

second experiment than in the first experiment. This might indicate that epiphytes also

have an important effect on DOC fluxes (Table 1 and Fig.3). According to other throughfall

studies in tropical wet forests (Coxson et al. 1992; Möller 2005; Schwendenmann 2005)

DOC generally shows the highest fluxes and concentrations in throughfall compared to

other solutes (Parker 1983) and implies leaching of organic compounds from both higher

and lower plant tissues such as sugars, polyols and organic acids (Filoso et al. 1999); pulse

release of these compounds were observed by Coxson et al. (1992) from desiccated

epiphytic bryophytes during initial rewetting events.

The 15N-pool dilution experiment also allowed evaluating net 15N of various canopy

components. Dry mass of the colonized branches mainly consisted of their leaves and

epiphytic bryophytes (mosses) (Fig. 5A). In fact, the main part of the epiphytic community

was made up of bryophytes which absorbed the greatest fraction of the applied 15N tracer

(16%, Fig. 5B) whereas host leaves did not retain more as other fractions (< 5%). Clark et

al. (1998) reported that epiphytic bryophytes and epiphytic assemblages accounted for

80% retention of atmospheric deposited inorganic N and Clark (2005) estimated that 33-

67% of wet deposited inorganic N was retained by these canopy components in a tropical

montane forest. 15N-uptake rates of canopy components were highest for epiphylls (mainly

comprised bryophytes), intermediate for other epiphytes (vascular epiphytes, lichens,

mosses), and lowest for tree leaves. Even bark, canopy soil and canopy litter showed net 15N retention. Wilson and Tiley (1998) determined in a Norway spruce forest that uptake of

N by bark was 2-10 times higher than in needles, due to more permeable barriers and

greater wettability. Bryophyte gametophytes are virtually “barrier-free”, due to the lack of

a cuticle and epicuticular waxes, which facilitates active and passive ion exchange

processes. Similar results were observed by Wanek and Pörtl (2005) in a 15N-pulse

labelling experiment where epiphyllous bryophytes quickly incorporated 15N. Nevertheless,

epiphytic and epiphyllous bryophytes exhibit rapid turnover times (Coxson 1991) and

therefore do not act as a long term storage for nutrients. Moreover, concerning their

poikilohydric character, desiccated bryophytes and lichens loose large quantities of

- 40 -

nutrients in the first minutes during rewetting (Gupta 1976; Coxson 1991). The lack of

epiphytes and epiphylls in canopies of the stem injection experiments might possible

explain why nitrogen was released in throughfall and emphasizes that different canopy

components may show very different characteristics with respect to solute exchange.

Wanek et al. (2003) reported that plant species may exhibit large differences in foliar

solute exchange with the environment as they showed extensive N exchange between

specific host plant species and epiphylls when plants were 15N labelled.

Net fluxes of other cations and anions of both experiments are consistently with other

throughfall studies (Forti and Neal 1992; Cavelier et al. 1997; Filoso et al. 1999; Chuyong

et al. 2004) with exception of chloride and sodium in the second experiment. Previously it

has been assumed that Na+ (and Cl-) behaves conservatively in tree canopies i.e. show not

net uptake or leaching and therefore can be used as a tracer for dry deposition (Hansen

1996; Draaijers et al. 1997). This has been the basis of the canopy budget model (Ulrich

1983) to estimate the relative contribution of dry deposition and canopy exchange to net

throughfall. Net uptake of Na+ on a short term and an annual basis (Fig. 3; and Hofhansl,

pers. commun) as well as significant leaching of Na+ in mangrove canopies cast significant

doubt on the applicability of the canopy budget model. However, the controls and

pathways of sodium and chloride remain obscure in this study.

In conclusion the study demonstrates that (1) in situ application of tracers allows to

investigate the fractional contribution of external sources (dry and wet deposition) and

internal sources (canopy exchange) of solutes in throughfall, (2) that canopy exchange is

controlled by bidirectional fluxes i.e. canopy efflux and canopy influx, and (3) epiphytic

communities play a major role in nutrient dynamics in canopies of tropical rain forests.

- 41 -

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FIGURES AND TABLES

Table 1 Enrichment factors (concentration ratios of TF beneath the canopy:TF above the

canopy) of throughfall (TF) of base cations, anions, DOC (dissolved organic carbon) and N

fractions (DON: dissolved organic nitrogen, TN: total nitrogen) of P. elata (n = 12) and

S. thecaphora (n = 12); stem injection experiment, tropical lowland rainforest, Piedras

Blancas National Park, Costa Rica. Values show means with SE (n = 24). Two-way ANOVA

for solute and plant species showed no significant species effect, therefore data of both

species were pooled. Relative contribution of foliage leaching to TFb-enrichment was

determined with mean values for each species, respectively. Different letters indicate

significant differences between fractions. Significances were tested by one-way ANOVA

(LSD-test, P < 0.05).

Enrichment factor

Species pooled P. elata S. thecaphora

Cations Cations Cations

Na+ 1,249 ± 0,094 a 1,485 ± 0,063 1,013 ± 0,151

K+ 2,524 ± 0,570 bc 3,702 ± 0,967 1,239 ± 0,239

Rb+ 1,955 ± 0,609 ab 1,405 ± 0,948 2,506 ± 0,772

Ca2+ 1,696 ± 0,261 ab 1,625 ± 0,115 1,767 ± 0,520

Sr2+ 1,463 ± 0,210 ab 1,577 ± 0,336 1,348 ± 0,261

Mg2+ 1,679 ± 0,299 ab 1,546 ± 0,205 1,812 ± 0,574

Anions Anions Anions

SO42- 1,836 ± 0,304 ab 2,830 ± 0,410 0,843 ± 0,196

PO43- 3,173 ± 0,910 c 4,528 ± 1,747 1,818 ± 0,277

Cl- 1,076 ± 0,034 a 1,033 ± 0,016 1,119 ± 0,065

DOC 1,237 ± 0,121 a 1,140 ± 0,087 1,335 ± 0,228

N fractions N fractions N fractions

NH4+ 1,356 ± 0,283 a 1,022 ± 0,083 1,689 ± 0,554

NO3- 1,225 ± 0,170 a 1,082 ± 0,071 1,381 ± 0,349

DON 1,180 ± 0,138 a 1,114 ± 0,172 1,267 ± 0,236

TN 1,159 ± 0,103 a 1,050 ± 0,127 1,268 ± 0,162

- 46 -

Psychotria elataR

b/K

0,00

0,01

0,02

0,03

0,04

Sr/

Ca

0,00

0,05

0,10

0,15

0,20

0,25

Siparuna thecaphora

Rb

/K

0,00

0,01

0,02

0,03

0,04

Sr/

Ca

0,00

0,05

0,10

0,15

0,20

0,25

P TF a TF b LEA L

15N

/1

4N

0,002

0,003

0,004

0,005

0,006

0,007

0,008

P TF a TF b LEA L

15N

/1

4N

0,002

0,003

0,004

0,005

0,006

0,007

0,008

aa

a

b

c

a aa

bb

a a aa

b

abc

a

ab

bc c

* a aa

b

* aab

a

b

A

B

C

D

E

F

Fig. 2 Foliar concentration ratios of tracers and corresponding elements: Rb+:K+(A, D), Sr2+:Ca2+

(B, E) and 15N:14N (C, F) of different rainwater fractions, leachates and leaves of P. elata and

S. thecaphora; stem injection experiment, tropical lowland rainforest, Piedras Blancas National Park,

Costa Rica. Values are means with SE (n=12). Different letters indicate significant differences between

fractions. Significances were tested by one-way ANOVA (LSD-test, P < 0.05). TFa: Throughfall above

the canopy (n = 12), TFb: Throughfall below the canopy, LEA: Leachates of controlled leaching

experiment, L: Leaves. * P: Precipitation (n = 1) was excluded from statistical analysis.

- 47 -

Net

flu

x (

µm

ol.

g-1

DW

.3d

-1)

-10

-8

-6

-4

-2

0

2

4

6

8

10

Net

flu

x D

OC

(µ

mo

l. g

-1 D

W. 3

d-1

)

-160

-120

-80

-40

0

40

80

120

160

NO3- NH4

+ DON Cl- SO42- PO4

3- Na+ K+ Mg2+ Ca2+ DOC

a

b

bc c d

bcbcbcbc c

*

Fig. 3 Sum of net fluxes of N fractions (DON: dissolved organic nitrogen), anions, cations and DOC

(dissolved organic carbon) of branches from A. cf. costaricensis and F. tonduzzi treated with

simulated rain events over a period of 3 days, tropical lowland rainforest, Piedras Blancas National

Park, Costa Rica. Values show means with SE (n = 5). Different letters indicate significant

differences between fractions. Significances were tested by one-way ANOVA (LSD-test, P < 0.05).

* DOC fraction was excluded from the statistical analysis shown.

- 48 -

Ammonium

-0,4

-0,2

0,0

0,2

0,4

Nitrate

Flu

xes

(µm

ol

15N

. g

-1 D

W e

ven

t-1)

-0,06

-0,04

-0,02

0,00

0,02

0,04

0,06

DON

EFFLUX INFLUX NET UPTAKE-0,6

-0,4

-0,2

0,0

0,2

0,4

0,6

a

c

b

a

bab

a

b

b

Fig 4 Gross canopy N fluxes (efflux and influx) and net N

uptake of ammonium, nitrate and DON (dissolved

organic nitrogen) of the canopy components of branches

of A. cf. costaricensis and F. tonduzzi, determined by 15N-pool dilution experiment; tropical lowland rainforest,

Piedras Blancas National Park, Costa Rica. Values show

means with SE (n = 5). Different letters indicate

significant differences between fractions. Significances

were tested by one-way ANOVA (LSD-test, P < 0.05).

- 49 -

Table 2 Statistically differences of N fractions ammonium, nitrate

and DON (dissolved organic nitrogen) in gross canopy fluxes

(efflux and influx) and net N uptake of the canopy components of

branches of A. cf. costaricensis and F. tonduzzi (n = 5),

determined by 15N-pool dilution experiment; tropical lowland

rainforest, Piedras Blancas National Park, Costa Rica. Different

letters indicate significant differences between fractions.

Significances were tested by one-way ANOVA (LSD-test,

P < 0.05).

N species EFFLUX INFLUX NET UPTAKE

NH4+ a n.s. a

NO3- b n.s. ab

DON ab n.s. b

- 50 -

Fig. 5 Dry weights (A), allocation

of 15N in percent of initial 15N

amount applied in rain water (B),

and uptake rate of 15N (C) of

different canopy components from

branches of A. cf. costaricensis

and F. tonduzzi, simulated rain

application experiment, tropical

lowland rainforest, Piedras Blancas

National Park, Costa Rica Values

show means with SE (n = 5).

Different letters indicate significant

differences between fractions.

Significances were tested by one-

way ANOVA (LSD-test, P < 0.05).

Bk bark

CL canopy litter

CS canopy soil

Eph epiphylls

Ev vascular epiphytes

L leaves

Lich lichens

M mosses (bryophytes)

Allo

cati

on

(%

15N

of

RW

)

0

5

10

15

20

25

DW

(g

)

0

5

10

15

20

25

30

A

B

Bk CL CS Eph Ev L Lich M

Up

take r

ate

(µ

mo

l 1

5N

. g

-1 D

W.

even

t-1)

0,00

0,01

0,02

0,03

0,04

0,05

C

a a a a

a

a

b

b

aa a

a

a

a

b

a

ab

a

ab ab

a

ab

b

ab

- 51 -

Fig. 6 Static and dynamic models to

demonstrate alteration in

throughfall (TF) chemistry; the

static model illustrates

unidirectional net fluxes which

might be negative (A), positive (C)

or reset to zero (B); the dynamic

model indicates changes in TF

composition through bidirectional

exchange processes (foliage uptake

and leaching) in varying extents of

contribution: 0% (open bar), 50%

(scratched bar) or 100% (closed

bar) but exclude information about

DD contribution.

Fig. 7 Throughfall enrichment and possible contribution of foliage leaching are demonstrated in/ with

four case studies where ratios of solute concentrations (tracer/ proper elements) are compared for

different aquatic fractions by the stem injection tracer experiment: throughfall above the canopy

(TFa), throughfall below the canopy (TFb), and leachates of controlled surface wash experiments

(LEA)

- 52 -

3. Zusammenfassung

Die chemische Zusammensetzung von Niederschlag ändert sich während der Durchdringung

des Kronenraumes von tropischen Regenwäldern. Generell kommt es zu einer

Nährstoffanreicherung bzw. Anreicherung an gelösten anorganischen und organischen

Substanzen im daraus resultierenden Kronentrauf. Die dabei entscheidenden regulierenden

Prozesse sind Trockendeposition und Austauschprozesse im Kronenraum. Trockendeposition

stellt, neben dem Niederschlag, einen externen atmosphärischen Nährstoffeintrag an

partikulärem und gasförmigen Substanzen, sowie Aerosolen in Waldökosysteme dar. Diese

werden an Oberflächen im Kronenraum deponiert und mit dem einfallenden Regenwasser

abgewaschen. Austauschprozesse im Kronenraum stellen einen internen Rezyklierungsweg

an Nährstoffen dar, die dadurch, neben Streufall und -Abbau, wieder pflanzenverfügbar

werden. Die Nährstoffdynamik des Kronentraufes ist insofern für tropische Waldökosysteme

von großer Bedeutung, da sie durch kontinuierlich hohe Niederschlagsmengen

gekennzeichnet sind und aufgrund geringer Nährstoffverfügbarkeit ihren Bedarf vorwiegend

durch Recycling decken. Austauschprozesse im Kronenraum inkludieren einerseits die

Aufnahme gelöster Substanzen aus dem Regenwasser und darin gelösten Verbindungen aus

Trockendeposition über Oberflächen pflanzlichen Gewebes (Cuticula, Borke) und

andererseits Auswaschung gelöster Nährstoffe aus labilen wässrigen Pools (Apoplast) des

Pflanzengewebes. Frühere Studien mit unterschiedlichen Versuchsansätzen zeigten die

Problematik, diese Prozesse zu trennen und zu quantifizieren, was auf ein komplexes und

dynamisches Interagieren von Trockendeposition und Kronenraum-Austauschprozessen

schließen lässt. Die Applikation von (konservativen) Tracern in situ, wie z.B. stabilen

Isotopen, stellt eine Möglichkeit dar, tiefere Erkenntnisse über Nährstoffflüsse im

Kronenraum zu erlangen.

Zielsetzung der vorliegenden Arbeit war es, Trockendeposition und Aufnahme und

Auswaschung von gelösten Nährstoffen im Kronenraum eines tropischen

Tieflandregenwaldes anhand verschiedener in situ Tracer-Experimenten zu unterscheiden.

Zum einen wurde der jeweilige relative Beitrag von Trockendeposition und Kronenaustausch

zu Kronentrauf-Nettoflüssen von Kalium, Calcium und Stickstoff via Stamminjektion der

entsprechenden Tracer (Rb+ für K+, Sr2+ für Ca2+ und 15N für 14N) in Bäume im Unterwuchs,

kontrollierten Auswaschungsversuchen und anschließender Auswertung über zwei

Modellansätze ermittelt. Rubidium und Strontium eignen sich gut als Tracer, da sie

chemisch und biologisch ähnliche Eigenschaften wie Kalium und Calcium besitzen. In einem

weiteren Experiment wurde erstmals die so genannte „15N Pool dilution“-Methode, welche

bisweilen zur Bestimmung von Brutto N-Mineralisierungsraten in Böden angewandt wurde,

adaptiert um Brutto N-Flüsse innerhalb des Kronenraumes zu untersuchen. Kronenraum-

Austauschprozesse, also Aufnahme (Influx) und Auswaschung (Efflux) von Nährstoffen im

Kronenraum, gelten als sehr dynamische Prozesse sind aber schwer zu quantifizieren,

- 53 -

besonders im Falle von N, der in anorganischer Form eher aufgenommen wird und in

organischer Form von Kronenraumkomponenten eher ausgewaschen wird. Während im

ersten Versuch durch Modellansätze der Netto-Austausch von N im Kronenraum und dessen

Anteil in Kronentrauf-Flüssen ermittelt wurde, diente der letztere Versuch dazu, die

tatsächlichen Aufnahme-Flüsse (Influx) und Abgabe-Flüsse (Efflux) von Stickstoff im

Kronenraum zu untersuchen. Hierzu wurde Regenwasser mit 15N (in Form von NH4+, NO3

+

und DON) angereichert und auf den Kronenraum repräsentierende Äste mit starkem

Bewuchs an Epiphyten, wie Bryophyten, Flechten und vaskulären Epiphyten, sowie

Epiphyllen appliziert, um gleichzeitig festzustellen, welche Kronenraumkomponente am

stärksten solche Austauschprozesse beeinflusst.

Die Applikation von Tracer via Stamminjektion und die Auswaschungsversuche verliefen für

Kalium und Calcium erfolgreich. Kalium- und Calcium-Flüsse weisen oft eine Netto-

Anreicherung im Kronentrauf auf, unterscheiden sich jedoch hinsichtlich ihrer Herkunft.

Obwohl die Anreicherung beider Kationen im Kronentrauf sowohl aus

Kronenaustauschprozessen als auch Trockendeposition resultieren können, weist Kalium

einen höheren Eintrag durch Auswaschungsprozesse im Kronenraum auf und Calcium wird

zu einem größeren Anteil als partikuläres Material von außen eingetragen. Kalium kommt in

pflanzlichem Gewebe in hohen Konzentrationen in wässrigen Lösungen vor und ist sehr

mobil, während Calcium vor allem in permanenten Strukturelementen in der Pflanze

gebunden ist. Mit den Tracer-Experimenten konnte nachgewiesen werden, dass Kalium wie

auch Calcium im Kronentrauf von Unterwuchspflanzen in einem tropischen

Tieflandregenwald zu einem beträchtlichen Teil aus Trockendeposition und Kronenraum-

Austauschprozessen stammten.

Die jeweiligen Beiträge von Kronenraum-Austauschprozessen und Trockendeposition im

Kronentrauf wurden einerseits anhand eines statischen Modells als unidirektionale Netto-

Zunahme oder –Abnahme demonstriert und andererseits mittels eines dynamischen

Modells, welches Konzentrationsänderungen im Kronentrauf anhand bidirektionaler

Austauschprozesse im Kronentrauf aufzeigt, ausgewertet. Die Brutto N-Flüsse (Influx and

Efflux) innerhalb der „Kronenraum“-Äste zeigten sehr hohe Influx- wie auch Efflux-Raten

von Ammonium, welche einen Netto NH4+-Fluss von nahezu Null ergaben. Brutto NO3

-- und

DON-Flüsse resultierten in einer Netto-Aufnahme, doch nichts desto trotz fanden auch

beträchtliche Effluxe statt. Der höchste Anteil an inkorporiertem 15N innerhalb der

verschiedenen Kronenraumkomponenten wurde in epiphythischen Bryophyten

nachgewiesen, während die höchsten 15N Aufnahmeraten die Epiphyllen-Fraktion, die fast

zur Gänze auch aus kryptogamen Pflanzen besteht, aufwies.

Aus den Ergebnissen dieser Arbeit lässt sich schlussfolgern, dass die Applikation von Tracern

in situ eine exzellente, wenn auch aufwendige und erweiterbare, Möglichkeit darstellt, um

hoch dynamische und komplexe Prozesse, wie Kronenraum-Austauschprozesse und

- 54 -

Trockendeposition, zu untersuchen. Weiters ermöglicht das dynamische Model die den

Kronentrauf kontrollierenden Prozesse besser abzubilden als das statische Modell, des

ausschließlich auf Nettoflüssen basiert. Die hohen Bruttoflüsse gelöster Substanzen im

Kronenraum zeigen, trotz geringer Nettoflüsse, dass im Kronenraum tropischer

Tieflandregenwälder sowohl Aufnahme als auch Abgabe von Nährstoffen zeitgleich

stattfinden können und zudem der Bewuchs im Kronenraum durch kryptogame Epiphyten

von großer Bedeutung in der Nährstoffdynamik tropischer Tieflandregenwälder sind.

- 55 -

4. Appendix:

Fig. 8 Concentrations of the base cations K+ and Ca2+, and nitrogen against concentrations of their

tracers Rb+, Sr2+ and 15N, respectively, and mean ratio values with SE (n = 12) in rainwater fractions

(TFa and TFb) of P. elata and S. thecaphora; stem injection experiment, tropical lowland rainforest,

Piedras Blancas National Park, Costa Rica.

- 56 -

Fig. 9 Correlations of concentrations of base cations, anions, DOC (dissolved organic carbon) and N

fractions (DON: dissolved organic nitrogen) between throughfall above the canopy (TFa) and

throughfall below the canopy (TFb) of P. elata (∆, n = 12) and S. thecaphora (o, n = 12) in logarithm-

diagrams; stem injection experiment, tropical lowland rainforest, Piedras Blancas National Park, Costa

Rica.