Carbon Cycling in Organic Material Originating from Monospecific Overstories in Hawaiian Lowland Wet Forests

Cole Stites-Clayton

May 2014

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Carbon Cycling in Organic Material Originating from Monospecific Overstories in Hawaiian Lowland Wet Forests

An Honors Thesis Submitted to

the Department of Biology in partial fulfillment of the Honors Program

STANFORD UNIVERSITY

by Cole Stites-Clayton

May 2014

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Acknowledgements This research was made possible by funding from an SERDP grant to the University of

Hawaii – Hilo (UHH), and NSF - Hawai`i EPSCoR Grants No. EPS-09038, No.

0554657, and No. NSF EPSCoR 0237065. I would like to thank Peter Vitousek for

providing me with the opportunity to become involved with this project and advising me

throughout this research. The project would also not have been possible without

assistance from a variety of sources both at UHH and Institute of Pacific Islands Forestry

(IPIF), including Becky Ostertag and Susan Cordell. Thanks to Tara Holitzki and the

UHH Analytical Lab, who provided important assistance with KCl extraction analysis

and lab space. I would like to acknowledge Nicole DiManno and Amanda Uowolo for

important mentorship with lab and field training. Jodie Schulten and Laura Warman

assisted with field sample collection, and Ivan Martes Martinez, Amy Kim, and Nicole

Rodriguez also assisted with lab and field work.

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Table of Contents 4 Acknowledgements

5 Table of Contents

6 List of Tables and Figures

7 Abstract

8 Introduction

12 Materials and Methods

15 Results

18 Discussion

23 References

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List of Tables

10 Table 1: Species and Classification

13 Table 2: Sampling Locations

List of Figures 15 Figure 1: Rate of Carbon Efflux from Humus 16 Figure 2: Total Carbon Efflux over Incubation Period 17 Figure 3: Leaf Litter Decomposition

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Abstract High levels of invasion and disturbance in the lowland wet forests of Hawaii have led to a

need for novel management strategies for the conservation of biodiversity, carbon (C)

storage, and other ecosystem services. The hybrid ecosystems approach represents a

method by which species traits can be used in the creation of ecosystems which optimize

desirable characteristics. In the lowland wet forests of Hawaii, C storage is one of these

important traits, given the continual increase in atmospheric carbon and global

environmental change. In this study we examined soil humus decomposition and leaf

litter decomposition. We asked whether patterns seen in rates of litter turnover across

nine species currently present in the lowland wet forests of the big island of Hawaii

would be present in the turnover of humus originating from single species stands. We

used a soda-lime absorption measurement assay to quantify soil decomposition over a

period of four weeks and performed a four-month common site litter decomposition

experiment. Total C efflux values for the full four week period ranged from 13.4 – 16.9 g

C/kg soil, with little difference among species. Litter decomposition had greater

variability. The fastest decomposition occurred in Thespesia populnea (milo) litter, with

17.6% of mass left at the end of the decomposition period, while Persea americana

(avocado) litter was the slowest to decompose and maintained 78.6% of its mass. Our

results suggest a decoupling of litter and humus decomposition rates. These findings

provide additional understanding on the extent to which hybrid ecosystems may be able

to influence C storage and turnover through species treatments.

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Introduction

The modern era of mobility and globalization has brought about profound changes in

society and technology, allowing for unprecedented movement of goods and ideas. It has

also brought about significant and increasing impacts, both biotic and abiotic, on the

natural systems of our planet. Recent research estimates that humans have impacted 83%

of all the land surfaces on our planet, though that statistic is likely closer to 100% when

global distributed changes are considered (Hobbs et al. 2009). The breakdown of

biogeographic barriers as a result of both intentional and accidental human movement of

species has led to widespread biological invasion, a significant component of human-

caused global change that has only recently received substantial attention (Vitousek et al.

1997) but continues to represent a serious threat to biodiversity, ecosystem services and

human well-being (Simberloff et al. 2013 and Pejchar and Mooney 2009).

Invasion is an especially potent threat for island ecosystems because of their natural

biogeographic isolation. The lowland wet forests of the Hawaiian Islands are an example

of an ecosystem that is highly vulnerable to invasion due to its extreme isolation.

Evolving in the most remote island system in the world, Hawaiian lowland wet forests

lack species in certain niches, including nitrogen fixers and pioneer species capable of

quickly utilizing available nutrients. Such characteristics make these Hawaiian forests

highly susceptible to invasion (Ostertag et al. 2009 and Denslow 2003). Forests on the

eastern side of the island of Hawaii, like many lowland wet forests all across the Earth’s

tropics, have been subjected to significant degradation through clearing and land-use

change. The result is that very little forest remains, and, even where it does, it is often

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highly invaded by non-native species (Hobbs et al. 2009 and Zimmerman et al. 2008). In

the lowland wet forests of Hawaii, much of the remnant forests are so heavily invaded,

and native species regeneration so diminished, that removal of non-native species can

result in an open forest that will support mainly fast invasive growth unless continual

labor and funding are put into weeding and maintenance of restoration plots (Ostertag et

al. 2009).

A possible solution currently being explored is the design of “hybrid ecosystems,” which

incorporate both native and non-native species and are intended to optimize certain

important ecosystem services. Understanding that the emergence of novel ecosystems

across the planet is now inevitable given species movement and the increasing impact of

global climate changes, this approach aims to develop methods with which to direct

ecosystem change such that important environmental functions can be maintained in the

face of human-induced global change (Hobbs et al. 2009 and Ostertag et al. SERDP

Proposal 2010). I had the chance to join a team of investigators from University of

Hawaii at Hilo, Stanford University, and the USDA Forest Service – Institute of Pacific

Islands Forestry (IPIF) to research the potential to design hybrid ecosystems in the

heavily invaded lowland wet forest on the big island of Hawaii (Ostertag et al. SERDP

Proposal 2010). Recognizing both the human and non-human needs in this ecosystem, the

goals of this study are to create methods which optimize carbon storage, biodiversity and

mobility in the forest.

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Average monthly global atmospheric carbon dioxide concentrations are poised to

consistently surpass 400 ppm within the next couple years (Tans/NOAA 2014). The

enormity of global change associated with this rise in atmospheric CO2 and other

greenhouse gases motivates a careful study of carbon cycling dynamics and how we

continue to influence fluxes of carbon in and out of the atmosphere. Tropical forest

ecosystems represent a significant sink for organic carbon, mainly in the biomass of

vegetation and carbon content stored in organic-rich soils (Mcleod et al. 2011). At the

same time, decomposition of these forest soils represents a globally significant source of

carbon dioxide to the atmosphere (Schlesinger and Andrews 2000). Previous research has

shown that changing forest composition, as occurs during significant biological invasion,

for example, can have a large impact on nutrient cycling within an ecosystem (D’Antonio

and Corbin 2003), suggesting both the importance of understanding the species effects

associated with different combinations of Hawaiian native and non-native plants as well

as the possibility of designing for specific nutrient and carbon cycling attributes through

careful selection of species in each hybrid ecosystem treatment.

Common Name Scientific name Classification Melastoma Melastoma septenervium Invasive Strawberry Guava Psidium cattleianum Invasive Mango Mangifera indica Non-native Ulu / Breadfruit Artocarpus altilis Native Hala Pandanus tectorius Native Niu / Coconut Cocos nucifera Native Ohia Metrosideros polymorpha Native Avocado Persea americana Non-native Milo Thespesia populnea Native Table 1. Species included in this study, with scientific names and status in Hawaiian Islands. Native, here, includes introductions by original Polynesian arrivals.

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The study presented here is situated within this framework of hybrid ecosystems as a

response to high levels of invasion and the need to gain a solid understanding of how

native and non-native species mixtures will influence carbon dynamics within the forest

ecosystem. The lowland wet forests on the eastern side of the island of Hawaii are

distinct among many tropical forests in that growth occurs on relatively young volcanic

flows, where large stocks of mineral soils do not yet exist. Instead, litter material

decomposes within holes in the rocky surface, leading to a layer of partially decomposed

organic matter, or humus, accumulating within the volcanic substrate. These conditions

imply that total carbon storage or efflux in soil is difficult to determine directly but also

that direct species effects on carbon turnover may be relatively higher as the

decomposition of this leaf and humus material represents the main source of C out of the

ecosystem.

Here, we examine leaf litter decomposition and carbon dioxide efflux during

decomposition of soil humus derived from single-species input material across nine

species present in Hawaiian lowland wet forests (Table 1), two of which are considered

highly invasive. By examining species-specific leaf litter and soil humus we aim to

quantify any differences that may exist among humus decomposition rates and, if

differences are present, compare this variation to variability observed in leaf litter

decomposition in this and other studies (Ostertag et al. 2009 and Allison and Vitousek

2004). If a positive correlation exists between litter and humus decomposition rates, it

would imply an important species effect on the overall C turnover of the ecosystem. On

the other hand, if the correlation is insignificant or negative we may be able to conclude

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that individual species, whether invasive or native, have more transient effects on

ecosystem C turnover. A fundamental understanding of how, and the extent to which,

individual species influence carbon turnover and storage within the Hawaiian lowland

wet forest ecosystem may help inform implementation of hybrid ecosystems moving

forward.

Materials and Methods Sample Collection and Study Sites This study was carried out on the Island of Hawaii, primarily in the South Hilo and Puna

districts on the eastern side of the island. Samples from the species avocado, mango and

ulu were collected from the western side of island, in the South Kona district. All humus

samples were collected from young (< 750 years old) flows of ‘a‘a lava on sites which

contained a monospecific overstory. A mixed understory was ignored if it was clear that

contributions to the organic material from this younger and smaller vegetation were

negligible compared to the major input from the established overstory trees. Humus

samples were combined to a single source bag for each species, then subsampled for

replication in lab studies. Leaf litter samples were obtained by collecting the freshest

naturally fallen leaves on the forest floor. Leaf collection was randomized by establishing

a straight transect through each stand and collecting all suitable fallen leaves within a 1 m

by 1 m box every 5 m along the transect until a suitable quantity of leaves was obtained.

Table 2 shows the coordinates of the sampling location of soil humus and leaf litter for

each of the nine species included in this study.

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Humus Samples Leaf Samples Species Coordinates District Coordinates District Melastoma 19.699616 N, 155.095594 W South Hilo 19.699616 N, 155.095594 W South Hilo Straw. Guava 19.618533 N, 154.959820 W Puna 19.425343 N, 154.951011 W Puna Mango 19.494125 N, 155.916351 W South Kona 19.494125 N, 155.916351 W South Kona Ulu 19.494125 N, 155.916351 W South Kona 19.494125 N, 155.916351 W South Kona Ohia 19.367495 N, 154.958490 W Puna 19.367495 N, 154.958490 W Puna Hala 19.395459 N, 154.928960 W Puna 19.395459 N, 154.928960 W Puna Niu 19.429130 N, 154.878071 W Puna 19.429130 N, 154.878071 W Puna Avocado 19.494125 N, 155.916351 W South Kona 19.494125 N, 155.916351 W South Kona Milo 19.455001 N, 154.841351 W Puna 19.455001 N, 154.841351 W Puna Table 2. Shows GPS coordinates and district of sampling humus and leaf litter for all species. The leaf litter decomposition study was carried out at the Keaukaha Military Reserve

(KMR) in the district of South Hilo. The forest at KMR is some of the last remaining

lowland rainforest on the big island of Hawaii, but has seen a high level of invasion by

species such as Clidemia hirta, Melastoma candidum and Macaranga mappa

(Zimmerman et al. 2008). This forest is the site of the larger hybrid ecosystem study, but

litter bags were placed in natural forest, not within the experimental treatment plots. The

forest grows on an ‘a‘a lava flow that is about 750 years old, and the average rainfall at

the site is 3280 mm y-1. Mean annual temperature at the site is about 22 degrees Celsius

(Ostertag et al. 2009).

Soil Humus Respiration The soil humus respiration study was carried out over a four-week period, with data

points taken at time zero, one, two, three, and four weeks. However, after the data was

collected, it was clear that an error during the drying of the soda lime to constant weight

for the t = 1 week time point had occurred. Therefore, data for t = 1 week shown in the

results has been interpolated between measurements taken at t = 0 and t = 2 weeks. Soil

organic material was incubated in 1L glass mason jars for the entirety of this period. To

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begin the study, humus source bags were wetted to 90% of the field capacity. The 90% of

field capacity moisture level was determined for each humus type by measuring moisture

content and field capacity of each sample. Moisture content was determined by drying

soils for 24 hours at 105 degrees C, while field capacity was determined by

oversaturating soils and allowing excess water to drain by gravity through filter paper

over a period of 12 hours. Each mason jar was then filled with 60.5 +- 0.5g of wetted

humus. Each species-specific humus was subsampled into 10 replicates, with 10 blanks

also run simultaneously. Jars were then covered with plastic wrap to prevent excessive

drying of the soil, and soil moisture was monitored to maintain the same wetness every 3-

4 days.

At each weekly time point the soil respiration rate was measured through a soda lime

absorption measurement (Keith and Wong 2006). Soda lime of 8-12 mesh grade was

placed in an aluminum soil tin and oven dried at 105O C to constant weight, then weighed

and sealed into the humus-containing jar for 24 hours to absorb the carbon dioxide being

respired from the humus within the jar. Upon removal from the incubation jar the soda

lime is once again oven dried to constant weight at 105O C and weighed to determine

mass gain. Soda lime weight change was converted to CO2 efflux by multiplying by a

factor of 1.69 to account for water loss as carbon dioxide is absorbed by the soda lime

(Grogan 1998). Replicates at each time point were averaged and rates were then

integrated over the time series to calculate total carbon efflux. Significance was

determined using a paired, one-tailed t-test comparing based on time point during the

incubation.

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Leaf Litter Decomposition Leaf samples were air dried indoors at an ambient room temperature of 25O C, then

placed inside seam-sealed mesh bags of 1mm fiberglass window screen. Additional

leaves were then oven dried at 105O C for 2 days to determine the air dried-oven dried

conversion. Sealed litter bags were strung together on fishing line in groups of three, and

placed flat onto leaf-covered forest floor in the rainforest at KMR. Collection of the litter

bags occurred after 4 months, at which point the contents of the bag was oven-dried at

105O C and weighed to determine mass loss through decomposition. Percent leaf matter

remaining was then calculated by averaging replicates and correcting for an air-oven

conversion factor calculated previously. Significance was determined at the 95%

confidence level with an unpaired, one tailed t-test.

Results

Figure 1. Carbon efflux rate. X-axis shows time points of 0,1,2,3 and 4 weeks, when soda lime absorption measurements were taken. Y-axis shows rate of carbon efflux in mg C release per kg soil per day. Colors indicate soil humus of differing species origin, with black showing the overall average efflux rate among all species. (n=10 for each species)

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Soda lime incubation measurements showed initial carbon efflux out of the humus

samples in the range of 578 – 813 mg C/kg soil released per day (Figure 1). Efflux rate

measurements showed a generally decreasing trend, with a final range of 331 – 493 mg

C/kg soil per day after 4 weeks of incubation (Figure 1). When integrated over the full

period of incubation, total carbon respiration values fall in the range of 13.4 – 16.9 g

C/kg soil * 30 days, with an average of 15.5 g C/kg soil * 30 days (Figure 2). The species

with the lowest total efflux was avocado and the highest totals were strawberry and

Melastoma. Variation was significant among species in a one-way ANOVA (p = 0.0005),

however individual species differences were mostly insignificant when compared with t-

tests.

Figure 2. Total Carbon Efflux over 30 days. X-axis shows species of overstory from which humus was sampled. Y-axis shows total carbon respired from organic matter over the course of the 30 day incubation in g C per kg soil. The final bar shows the average across all species. Error bars show standard error. (n = 10 for each species) Leaf litter decomposition results showed greater variability than that of humus

respiration. Leaf decomposition was measured as the average proportion of leaf mass

remaining after the four month period in the field, and the results fell in a range from

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0.176 – 0.786 with milo on the lower end and avocado on the upper end (Figure 3). Both

milo and ulu were significantly different from all other species, and melastoma was also

significantly different from all species except mango (p = 0.0662) at the 95% significance

level. Four species in the midrange (guava, mango, hala and niu) did not differ

significantly from each other, but were significantly distinguished from the two species

with the slowest leaf litter decomposition, ohia and avocado (p = 0.0188, hala and ohia,

for example). Total carbon efflux and litter decomposition were not significantly

correlated (r = -0.55, p = 0.1249) based on these data.

Figure 3. Leaf Decomposition. X-axis shows species of leaf litter, y-axis shows the average portion of leaf mass remaining after the 4 month decomposition period. Error bars show standard error. Letters indicate statistically differentiated groups at 95% significance. *Melastoma and Mango showed a statistically insignificant difference (p = 0.66) likely due to the large standard error in the Melastoma data. (n = 10 for each species except mango and avocado, where n = 7) Discussion A number of previous studies in Hawaii have investigated the effect of invasive species

on carbon turnover through analysis of leaf litter decomposition rates. Their findings

b b b

c

d d

e%

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have indicated that invasive species exhibit faster litter turnover, suggesting the potential

for positive feedback to invasion since invasives in the low nutrient conditions of

Hawaiian forests are often also able to better take advantage of increased nutrient

availability (Allison and Vitousek 2004). Melastoma, in particular, has been shown to

have a higher rate of litter decomposition than many native species (Ostertag et al. 2009).

Factors involved in decomposition rates include leaf characteristics such as specific leaf

area, nutrient concentration, and lignin content. Invasives have been shown to exhibit

higher specific leaf area, higher foliar concentrations of N and P, and lower construction

costs for leaf tissue, all traits which could lead to increased litter decomposition (Baruch

and Goldstein 1999, Allison and Vitousek 2004). Given these previous results, we

hypothesized that Melastoma and strawberry guava would be species with some of the

fastest litter decomposition, along with milo because of its physical leaf traits.

Conversely, species such as hala, niu, and ohia were expected to occupy the slow end of

the litter decomposition spectrum given that they are native species with much lower

specific leaf area.

Our data place milo as the species with the fastest litter decomposition, which is

consistent with our expectation based on leaf traits. In our study, decomposition of

Melastoma litter was third fastest, behind milo and ulu litter, but still significantly faster

than most species included in the study. Milo and ulu have been included in few

previous litter experiments as most work focuses mainly on montane forests rather than

lowland forests of Hawaii. Thus little data exist on foliar concentrations of nutrients or

specific leaf area that may explain why these litter types were the fastest to decompose.

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At the slower end of the spectrum there was less separation among species. Avocado and

ohia were the slowest to decompose, with mango, hala, niu, and strawberry guava

occupying the middle range. This result is fairly consistent with our understanding of

litter decomposition, as some of these represent native species with dense, slow growing

leaves. The slow turnover of strawberry guava litter is somewhat unexpected based on its

extreme invasive potential, but leaf characteristics commonly associated with the

Myrtaceae family, to which strawberry guava belongs, help support this result.

Overall, our data for litter decomposition could be improved through a longer

decomposition experiment in the field. In this case litter bags were left out for only four

months, and therefore we may not have resolved potentially significant differences in

litter turnover rates that could have been seen with a longer experiment. In order to

address these issues we plan to carry out follow up assays, primarily to determine nutrient

concentrations within the original leaf litter. Although differing foliar nutrient

concentrations can also be the result of either site or species effects, this will help us

better understand the trends we have observed in litter decomposition, especially in the

context of other litter quality data presented in other studies. If our strawberry guava

litter, for example, has low nutrient concentrations, it may help explain the slower than

expected turnover rate that we observed here.

While the species effect was significant in soil humus decomposition based on the one-

way ANOVA, the range and magnitude of the differences among species are small

compared to the differences seen in litter turnover. This low variation is an important

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result, as it suggests that, despite substantial variation in litter turnover across species,

organic rich soil humus turnover is relatively uniform regardless of the species from

which it was derived. Correlation between total C efflux from soil and litter turnover was

insignificant statistically. A study carried out on nutrient-poor granitic soils in the

Seychelles found that, while litter from invasive species was of higher quality and

decomposed more rapidly, soils beneath litter from various species did not also vary

similarly in quality and nutrient concentration. It is possible that soil fertility, and the rate

of decomposition of organic matter, is complex and perhaps co-limited by multiple

nutrients such that simply increasing turnover of litter material does not necessarily result

in an increase in soil quality or soil humus turnover (Kueffer et al. 2008). It may also be

that different communities of decomposers are acting on the larger organic materials of

litter and the more highly broken down components of soil humus. This could suggest

that organisms responsible for leaf degradation and decomposition are responding to

variable plant species input in the form of nutrient or ion concentrations while those

responsible for microbial respiration in soils are not, perhaps because components of

litter input are not limiting in this process.

The connection between nutrient cycling and carbon cycling may not be straightforward

in these systems. We carried out a nitrogen mineralization assay for the humus samples

collected in this study alongside our carbon efflux measurements and found that Nmin

varied much more greatly across species than C efflux (data not shown). We are waiting

for further assays into the nutrient concentrations and exchangeable ions present in these

soils before attempting to interpret these data. With initial N concentration, we may be

21

able to ascertain whether nutrient cycling and accumulation of N is correlated with litter

turnover even if soil carbon turnover is not.

In the conceptualization of hybrid ecosystems, these results indicate that variability in C

turnover observed in leaf litter across species may not be mirrored in soil organic material

turnover. This suggests, at least in the short term, that treatments may not affect carbon

dynamics to a substantial extent. Further assays will aid in the important de-coupling of

carbon and nutrient cycling rates, providing information about whether increases in

nutrient cycling in organic matter could be occurring in isolation from increases in C

turnover. This result might indicate that, while high nutrient inputs from rapid

decomposition of invasive litter may support faster turnover of nutrients such as N and P

in soils, it may not also increase the rate of humus decomposition, leaving C storage in

the ecosystem less affected by invasion.

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