Tropical ecosystems

The delimitation of tropical regions: rainfall, evaporation, and temperature

Seasonal versus daily temperature changes: -latitudinal gradient- frost occurrence at sea level-altitudinal temperature gradient in the tropics-separation of tropical plant formations according to temperature and rainfall

Eco-climatic classificationsThe climate-diagramm (Gaussen-Walter)The Holdridge Life Zone conceptBailey’s humidity indexRainfall-evaporation comparisons

Distribution of humid and seasonal dry forests

General physiognomic changes along environmental gradientsRainfall (or water availability) 1) medium to high fertility; 2) low fertilityAltitudinal gradientsFlooding gradients

Forest-poor regions in medium to high rainfall areas: the savannasoperating factors- soil fertility, fire, flooding

Altitudinal temperature gradient (average year temperature ºC vs altitude m) in Venezuela measured in conventional meteorological stations (•,o,x) or using Boussingaults method (∆). In tropical climates (daily temperature range larger than average monthly temperature range) soil temperature below 30 cm depth, under shade, corresponds to average annual temperature

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Tropical humid forest (San Carlos de Río Negro, 65 m)

B.I. : 20.1; No. dry months: 0

Rain: 3521 mm ; Temp.: 26.1º

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Tropical moist forest ( Pto. Ayacucho, 73 m)

B.I. : 12.2; No. dry months: 4

Rain: 2144 mm ; Temp.: 27.1º

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Tropical dry forest (San Fernando, 47 m)

B.I. : 8.1; No. dry months: 6

Rain: 1447 mm ; Temp.: 27.1º

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Tropical semi-arid forest ( Coro, 16 m)

B.I. = 2.3; No. dry months 12

Rain: 427 mm ; Temp.: 27.7º

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Lower montane moist forest ( Colonia Tovar 1790 m)

B.I. : 10.7; No. dry months: 3

Rain: 1248 mm ; Temp. 16.8º

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Premontane moist forest ( Mérida 1479 m)

B.I. 13.0; No. dry months: 3

Rain: 1677 mm ; Temp.: 19.0º

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Premontane semi-arid forest (Barquisimeto 813 m)

B.I. = 3.3; No. dry months: 12

Rain: 517 mm ; Temp.: 23.8º

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Premontane dry forest ( Caracas 835 m)

B.I. : 6.1; No. dry months 6

Rain: 885 mm ; Temp.: 21.7º

Rainfall Gradient

Temperature Gradient

Differentiation of forest communities along rainfall and temperature gradients using Bailey’s humidity index

Holdridges model of Eco-Climatic classification of potential vegetation

Relationship between forest vegetation and rainfall (y-axis) and durastion of the dry period in months (x-axis) in India.I. Evergreen and II. Semi-evergreen tropical rain forest,III. Monsoon forest (A. wetter, B. drier), IV. Savanna (thorn forest), V. Desert(from Walter Die Vegetation der Erde 1973)

Seasonal Forest Formation Series

Dry Evergreen Formation Series

Decreasing Rainfall

Montane Forest Formation Series

SeasonalSwamp Formation Series

Increasing altitude

Critical structural and functional properties of tropical plant formations

DiversityFacts and figures (Gentry et al.)Explanations and theoriesMonodominant tropical forests

Fertility,Flooding, Salinity, SymbiosisMutualistic symbiosis

Mycorrhiza: ectomycorrhizas and VAMN2-fixing associations: Rhizobia,

Life-forms, biotypes, etc.The classical (and still working) Raunkiaer’s systemDiversity and abundance of life -forms in different tropical climatesLife-forms and ecosystem function in tropcial forests

Productivity of tropical forests and their potential as carbon sinksExtension of forest plant communities and Net Primary ProductivityPrecision and uncertainties:

operational models and underground productivityCarbon balance of the biosphere: Houghton’s and Field et al’s estimations

Changes in species diversity according to species intrinsic growth rate and frequency or intensity of disturbanceHuston 1994)

Hypothesis on determination of species diversity based on intrinsicGrowth rates and frequency or intensity of disturbance (Huston 1994)

Inverse relationship between soil fertility and species richness (Huston 1994: Biological Diversity. Cambridge University Press

Costa Rica Ghana

West Malesia Amazonas

Ectotrophic mycorrhiza----> highly specific ----> monodominant forests1) in Africa: Gilbertiodendron dewevrei and Brachystegia laurentii (Zaire), Cynometra alexandri (Uganda) and Tetraberlinia tubmaniana (Liberia).2) in tropical South America: Mora excelsa, M. gonggrijpi, and Eperua falcata (Trinidad and Guyana); Pentaclethra macroloba (Costa Rica).

Vesicular arbuscular micorrhiza ----> promiscuous----> species rich forests

Central role of micorrhyzal mutualistic symbioses in tropical forest nutrition

Diversity of Life-Forms, according to Ellenberg (1979) in tropical forests: notice the larger diversity in dry forests

Common life-forms of tropical forests associated with forest structure and function

Ewel and Bigelow (1996) Denslow (1996)

CANOPY TREESDicot, long-lived trees Canopy and emergent trees

LegumesDicot, short-lived tress Palms

EmergentsRosette trees (palms)

UNDERSTORY TREESUnderstory trees Treelets

PioneerUnderstory

SHRUBS AND HERBSShrubs Herbs and shrubs

PioneerGiant-leaved herbs Understory

Large-leavedGraminoids Small-leaved

CLIMBING PLANTSVines Lianas and vines

LianasHemi-epiphytes Vines

EPIPHYTESEpiphytes Epiphytes and hemiepiphytes

Non-parasitic herbsParasitic and

hemiepiphytictrees and shrubs

Examples of linkages between plant life-forms and processes in tropical forests (Ewel and Bigelow Ecological Studies 122, 101-126.1996)

I

Life-form Role

Dicotyledonous trees, 1. Provide skeletal structure of entire forestlong-lived 2. Dominate primary productivity and material flows

3. Influence off-site climate and hydrology4. Provide shelter and roosts in hollow trunks

Dicotyledonous trees, 1.Reduce nutrient loss in early successionShort-lived 2. Reduce likelihood of site takeover by vines and shrubs

Rosette trees 1. Channel rainwater toward stem(e.g. palms) 2. Capture and aggregate litter

3. Concentrate Calcium4. Roots bore through soil pans, creating channels that

can be exploited by other plants5. Root foraging emphasizes scale

Understory trees 1. Scavenge sparse radiation in the understory (and have low nitrogen demand)2. Provide platforms (in humid microenvironment) for nitrogen-fixing epiphylls

Shrubs 1. Drive productivity of scansorial rodents and birds that feed on fleshy fruits2. Retard nutrient loss in early succession

Examples of linkages between plant life-forms and processes in tropical forests (Ewel and Bigelow Ecological Studies 122, 101-126.1996)

IILife-form Role

Giant-leaved herbs 1. Constitute large, homogeneous patches in otherwise heterogeneous understory2. Foster secondary productivity thropugh nectar and fruit production3. Provide roosting sites for bats and building sites for carton nests of social insectes

Vines 1. Provide trellises for movement of arboreal animals2. Act as weebing that ties trees together3. Buffer microclimatic changes by seasing forest edges

Graminoids 1. Constitute readily combustible dry-season fuel 2. Provide forage for grazers and food for seed-eating

birds, rodentes, ants and fungi

Hemiepiphytes 1. Increase mortality rates2. Provide slender vine trellises (aerial roots) in understory of closed canopy forest

Epiphytes 1. Augment leaf area (by colonizing opaque surfaces)2. Slow nitrogen through-flow3. Divert water from soil to the atmosphere4. Redistribute trhrough-fall and stem flow5. Provide unique habitats essential for reproduction of other species (e.g. insects)

Area and net primary production of organic matter (expressed as g C per unit area) of tropical forests and savannas estimated by direct measurements and using a process-

based ecosystem simulation model

Vegetation Units Area % NPP Total NPP % (x 106 km2) g C m-2 yr- 1 1015 g C yr-1

Whittaker and Likens (1973)World Total 149.0 58.8Tropical Rain forest 17.0 11.4 1100 18.7 31.8Tropical seasonal forest 7.5 5.0 800 6.0 10.2Savanna 15.0 10.1 450 6.8 11.6Total tropical 39.5 26.5 31.5 53.6

Melillo et al. (1993)World Total 127.3 53.2Tropical evergreen forest 17.4 13.7 1098 19.1 35.9Tropical deciduous forest 4.6 3.6 871 4.0 7.5Tropical Savanna 13.7 10.8 393 5.4 10.2Xeromorphic forests 6.8 5.3 461 3.1 5.8Total tropical 42.5 33.0 31.6 59.4

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y = -0.026x2 + 1.717x + 0.418

r2 = 0.984

data from Clark et al. Ecological Applications 11,371.2001

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3.3 (±0.2) = 5.5 (±0.5) + 2.0 (±0.8) - 2.0 (±0.8) - 2.2 (±1.3)

(1) The residual terrestrial sink is attributed to a combination of- CO2 fertilization- Nitrogen deposition- Interannual climatic variation

(2) Northern temperate forest show a residual terrestrial uptake of 0.6 ± 0.5 Pg yr -1

(3) Mechanism leading to terrestrial accumulation are not precisely known(4) Several of the potential mechanisms will be less effective in the future leading

to a sink reduction or to an aditional terrestrial source

Global Carbon Balance for 1990(Houghton 1999)

FossilFuel

Net emissionsdue to Land-use Change

OceansUptake

ResidualTerrestrial Sink

AtmosphericIncrease

Units: Pg yr-1

Decomposition

Possible mechanisms for the maintenance of a carbonsink in the biosphere (Field et al. 1992)

Increase atmospheric CO2

≈ 2 ppm yr-1Increased global temperature

0.5 ± 0.2 ºC in last 100 yr

Increased Carbon Storage1.6 ± 1.4 Pg yr-1

Growth Growth

N deposition≈ 25 Tg yr-1

Nutrient Availability

Tissue Nutrients

Decomposition

Photosynthesis Nutrient Use Efficiency Water Use Efficiency

Decomposition

Nutrient Availability

Nutrient Availability

Wood Production

Decomposition

Summary

Tropical climate regimes vary widely in water availability, from arid to perhumid rainfall regimes, and temperature, from lowland with averages above 25ºC, to high mountains with seasonal and daily frosts

Plant formations vary accordingly in structural development and ecophysiological tolerances

Soil fertility modulates vegetation structure and productivity Widespread occurrence of Savannas can be related to the

interactions between climate, soil fertility, and disturbance regimes represented by herbivory and fire

Climate change represented by atmospheric increase in CO2 concentrations and slowly increases in average temperature, may lead to increased productivity, organic matter turnover and carbon storage in symbiosis-dependent tropical forests