Intercomparison of Soils, Nutrients, and Plants in Forest-Dynamics Research Plots of the Global Tropics

Robert Stallard: Telephone: +1-303-541-3022; FAX: +1-303-447-2505
e-mail: stallard@colorado.edu

Bob Stallard in front of a tree in the Forest Dynamics Plot, Barro Colorado Island, Panamá

(6) Intercomparison of soils, nutrients, and plants in forest-dynamics research plots of the global tropics: Despite current interest in tropical forests as key components of the global carbon cycle and as centers of biodiversity, we remain largely ignorant of the inter-relationships among forest attributes (e.g., dynamics, diversity, structure), and the physical properties of the environments in which these forests grow. In recent years data sets have become available describing growth, mortality and recruitment of approximately 10% of the global diversity of tropical tree species. These data result from the development of a network of large-scale forest dynamics plots coordinated by the Center for Tropical Forest Science (CTFS), and established using methods identical to those used to survey and census the 50-ha Forest Dynamics Plot on Barro Colorado Island (BCI), Panama. Just as data from the BCI plot have played a prominent role in our understanding of the community-level consequences of ecological processes played out at local scales, we will show how these plots now provide an unprecedented opportunity to examine a venerable question in tropical ecology: How do soil-borne resources influence the variation in forest structure and demographic turnover rates observed among plots, and among habitat types within plots?

To achieve this objective we propose three sets of measurements. At nine plot sites we will characterize variation in soil-moisture availability through the year and throughout the plots using a hydrological model (TOPMODEL). This model predicts soil-moisture saturation deficit based on rainfall, stream-flow data and local topography. Second, at five plots currently lacking soils data we will sample soil-chemical properties within standardized, topographically-defined habitat types. Third, at the same plots we will perform seedling growth experiments using mycorrhizal and non-mycorrhizal pioneer species to assess plant-availability of soil nutrients for each habitat type. These measurements will allow us to address questions at two spatial scales. At the among-plot scale we will be able to ask how stem density and basal area, as well as community-wide patterns of growth, mortality and recruitment, correlate with moisture availability and soil fertility. At the within-plot scale we will ask whether species exhibit specialization to particular hydrological niches, and whether habitat types differ in soil fertility, in turnover rates, and in local species richness. Resolution of these broad questions concerning correlations among soil-borne resources and characteristics of the vegetation will in turn permit us to refine future hypotheses aimed at understanding the mechanistic basis for interspecific variation in demographics and distribution.

This study will provide the first standardized large-scale measurements of the environmental context within which tropical forests grow. Our trans-continental approach to testing questions of ecosystem function and community organization has rarely been attempted, but it will be essential if we are to improve our understanding of the biogeographic and biophysical limits to our ecological generalizations. Understanding how variation in water and nutrient availability determines forest structure, composition and dynamics, and potentially influences local diversity through niche partitioning, will be essential to predicting future vegetation responses to climate change and will also provide a first step in guiding management to protect forest diversity. Wide dissemination and application of the results of this project is ensured by the active participation of in-country collaborators, and by data sharing through CTFS.

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