Phase 1 - Tree Species Effects on Ecosystem Processes in Lowland Costa Rica


National Science Foundation Awards: DEB 0236502 and 0236512


This is a collaborative project between:

  • Iowa State University, Ames, IA (ISU)
    Department of Ecology, Evolution and Organismal Biology (EEOB)
    Department of Natural Resource Ecology and Management (NREM)
  • Organization for Tropical Studies (OTS), Duke University, Durham, NC

 Main Questions:

  • How much do tree species differ in their cycling and accumulation of carbon in a lowland tropical rainforest environment?
  • Which species-level attributes are responsible for these differences?
  • Can species-specific traits be incorporated into global biogeochemical models such as CENTURY, to improve model predictions of ecosystem processes?


  • Experimental design using long-term tropical tree plantations
  • Field and laboratory measurements
  • Integrate findings with process-based modeling, using CENTURY


  • Improve models of tropical forest carbon and nutrient cycling
  • Restoration of degraded tropical landscapes
  • Design more sustainable and productive forestry systems
  • Promote long-term research, education and training in a tropical ecology.

Conceptual Framework:

Plant species differ in traits involving resource capture and use abilities, and hence have the potential to influence ecosystem processes. But to what extent do they impact ecosystems, and by what mechanisms? Our conceptual model for addressing these questions focuses on selected plant traits that influence ecosystem carbon cycling, and which have the potential to influence nutrient and global carbon cycling (Fig.1). Our framework incorporates the effect of the following plant species' traits on the corresponding processes:
1) Production capacity, in interaction with environmental conditions, determines the amount of organic inputs to the system (i.e., net primary productivity, NPP).

2) Plant allocation patterns determine above- and belowground partitioning of photosynthate, with consequences to detrital dynamics.

3) Root and leaf mortality rates influence detrital input fluxes.

4) Root and leaf tissue chemistry influence decomposition, hence soil carbon and nutrient fluxes.

5) Fluxes of photosynthate to roots influence rates of C cycling through soils and soil respiration.
Within this framework, we focus on quantifying differences among tree species in growth and biomass production, above- and belowground allocation patterns, and tissue chemistry. Our goal is to quantify the effects of these species' traits on effects on detritus production, leaf and root effects of these species' traits on effects on detritus production, leaf and root decomposition, soil organic matter mineralization, and ecosystem carbon stocks.

Fig. 1. Conceptual model of regulation of soil carbon dynamics by plants. Species traits have the potential to influence the following processes (currves connected to circles) include: 1) Net primary productivity (NPP); 2) allocation; 3) detrital fluxes; 4) decomposition; and 5) soil respiration that includes CO2 derived from living and dead roots, aboveground litter and soil organic matter.


To address the extent of, and mechanisms by which, tree species influence ecosystems, we propose to integrate field studies of plant traits, ecosystem processes, and soil properties with process-based modeling. Our replicated sites of six species, where climate, soils, land use history, and tree densities are uniform, are well suited for these studies. Based on our conceptual model (Fig. 1), we identify below three broad objectives that relate to: 1) plant traits that influence carbon cycling; 2) transfer from plant to soil carbon pools; and 3) soil carbon dynamics. The measured plant traits and ecosystem properties can be modeled in Century, providing the basis for Objective 4, to evaluate the variability in species' effects on ecosystems and the underlying mechanisms.

OBJECTIVE 1: To quantify differences among species in growth, detritus production, and aboveground:belowground allocation.

Rationale: Species have individual characteristics that define, within boundaries, their growth potentials and structural attributes within acceptable environments. Within a particular environment, differences among species in physiology, growth, C allocation to above- and belowground structures, and tissue turnover rates will affect above- and belowground biomass accumulation, rates of detritus production, and location (above or in soil) of detrital production

OBJECTIVE 2: To investigate differences among tree species in leaf and root tissue chemistry and decomposition rates and detrital pools.

Rationale . All other factors being equal, greater production of leaf and fine-root detritus will generate larger surface-litter and dead-root detrital pools. Differences among species in fine-root and aboveground litter production (Objective 1) thereby provide one clear mechanism by which species may influence the size and location of detrital accumulations. But detritus mass is controlled also by rates of decomposition, which are influenced by the chemical and structural nature of the detritus (e.g., Heal et al. 1997). Which of these two processes, i.e., litter production or decay rate, is more important in controlling detrital accumulations will depend upon which process varies relatively more. We suggest that detritus accumulations are more sensitive to loss rates than to input rates, and propose in this study to test this idea, which may be a general principle governing patterns of C sequestration and SOM dynamics in tropical forests.

OBJECTIVE 3: To investigate effects of tree species on soil organic matter (SOM) characteristics, and to evaluate the relative importance of quantity and quality of detrital inputs on SOM.

Rationale . Because these tree species differ in growth and tissue chemistry characteristics, we expect that soil carbon dynamics will also differ under the species. Although both quantity and quality of detrtial inputs influence carbon stocks, we predict that chemistry of detritus will exert a relatively stronger influence on soil carbon stocks than will detritus production rates, because soil detrital decay rates will vary more among species than will inputs (Objective 2). We focus on species' effects on total SOM quantity because SOM strongly influences plant productivity via its effects on soil structure, water-holding capacity, cation exchange and nutrient release. In many tropical soils SOM is the dominant source of available N and P for plants, and mineralization of SOM plays a major role in promoting soil fertility in the tropics (Tiessen et al. 1994). Failure to convert tropical forest lands into permanent and productive systems is often related directly to declines in SOM (Parton et al. 1994, Woomer et al. 1994).
SOM quantity alone, however, does not necessarily correlate well with SOM turnover rate or capacity of the soil to supply nutrients (Sanchez & Miller 1986). Thus, we focus also on SOM characteristics, especially quantification of isolatable C fractions that have been previously defined by their kinetics, because they provide a means for evaluating soil carbon dynamics that have consequences for soil nutrient mobilization and carbon sequestration. Moreover, based on their turnover times, these fractions can be related to modeled pools in the Century SOM model.

OBJECTIVE 4: To evaluate the effects of dominant-tree characteristics on carbon storage and cycling in moist tropical forests.

Rationale : Evergreen broadleaved tropical forests cover only about 10% of the world's land surface (De Fries et al. 1998), but are very important in the global carbon cycle. About 36% of the Earth's terrestrial NPP occurs in tropical moist forests (Ajtay et al. 1979, Melillo et al. 1993), and they contain some 474 Pg of soil organic carbon (SOC), 20% of the global total (Jobbágy & Jackson 2000). Approximately 42% of the world's forests, containing ~60% (212 Pg C) of the global forest biomass, are in the tropics and subtropics (Dixon et al. 1994). Tropical deforestation, with its associated reductions in biomass and soil carbon pools, contributed an estimated 1.7-2.4 Pg C to atmosphere during the 1990s (Fearnside 2000, Houghton & Hackler 2000), in comparison with a mean fossil-fuel contribution of 6.2 Pg C (Marland et al. 2000). Quantifying the importance of different factors that control C accumulation and cycling in moist tropical forests is important to the evaluation of human-driven environmental changes.


Literature cited:

Ajtay, G. L, P. Ketner and P. Duvigneaud. 1979. Terrestrial primary production and phytomass. Pages 129-181 in B. Bolin, E. T. Degens, S. Kempe and P. Ketner (editors). The Global Carbon Cycle. John Wiley & Sons, Chichester.
De Fries R. S., M. Hansen, J. R. G. Townshend and R. Sohlberg. 1998. Global land cover classification at 8 km spatial resolution: the use of training data derived from Landsat imagery in decision tree classifiers. Intern. J. Remote Sens. 19: 3141-3168.
Dixon R. K., S. Brown, R. A. Houghton, A. M. Solomon, M. C. Trexler and J. Wisniewski. 1994. Carbon pools and flux of global forest ecosystems. Science 263: 185-190.
Fearnside, P. M. 2000. Global warming and tropical land-use change: greenhouse gas emissions from biomass burning, decomposition and soils in forest conversion, shifting cultivation and secondary vegetation. Climatic Change 46: 115-158.
Houghton, R.A., and J.L. Hackler. 2000. Carbon Flux to the Atmosphere from Land-Use Changes. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA.
Jobbágy E. G. and R. B. Jackson. 2000. The vertical distribution of soil organic carbon and its relation to climate and vegetation. Ecol. Applic. 10: 423-436.
Marland, G., T.A. Boden, and R. J. Andres. 2000. Global, Regional, and National CO2 Emissions. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA.
Melillo J. M., D. A. McGuire, D. W. Kicklighter, B. Moore III, C. J. Vorosmarty and A. L. Schloss. 1993. Global climate change and terrestrial net primary production. Nature 363: 234-240.
Parton, W.J., P.L. Woomer and A. Martin. 1994. Modelling soil organic matter dynamics and plant productivity in tropical ecosystems. In: Eds. Woomer, P.L. and M.J. Swift. The biological management of tropical soil fertility. J. Wiley, Chichester, UK.
Sanchez, P.A. and R.H. Miller. 1986. Organic matter and soil fertility management in acid soils of the tropics. International Society of Soil Science Transactions 13th Congress 6: 609-625.
Tiessen, H. E. Cuevas and P. Chacon. 1994. The role of soil organic matter in sustaining soil fertility. Nature 371: 783-785.
Woomer, P.L., A. Martin, A. Albrecht, D.V.S. Reseck amd H.W. Scharpenseel. 1994. The importance and management of soil organic matter in the tropics. In: Eds. Woomer, P.L. and M.J. Swift. The biological management of tropical soil fertility. J. Wiley, Chichester, UK.
Young, A. 1976. Tropical soils and soil survey. Cambridge University Press, Cambridge.