Are tropical forest structure and dynamics too complex for universal explanations?
"When you walk through different tropical forests, or graph data from different forests, there are certain features that, at first glance, look similar everywhere '" the presence of some very large trees, or the general shape of the tree size distribution curve," explains first author, Helene Muller-Landau from the University of Minnesota. "So you start to think that there might be some general explanations. But the closer you look, the more differences you find '" differences that can'
Tropical forests give modelers a run for their money. Trees range from spindly understory shrubs to enormous buttress-rooted rainforest giants festooned with vines and peppered with orchids, ferns and bromeliads. Tropical trees depend on animals for pollination and seed dispersal and are constantly assaulted by insects, fungi, maelstrom and machete. Furthermore, soils, rainfall, the frequencies of hurricanes and fires, and assemblages of animals and plants in tropical forests all vary widely from one forest to another.
Prospecting in a gold mine of data
Twenty-five years ago a pair of dueling tropical forest ecologists (Robin Foster, now at the Chicago Field Museum and Steve Hubbell at the University of Georgia and the Smithsonian Tropical Research Institute, STRI) decided that the way to understanding was to measure and map every single tree in an area of lowland tropical forest on Panama'
At a U.S. National Science Foundation supported workshop, CTFS participants examined data from across the forest plot system and discussed possible multi-site comparisons. "A couple of the key papers on metabolic ecology had just come out and were generating a lot of excitement '" as well as considerable skepticism. And we thought, hey, we have an unprecedented opportunity to test these predictions, and to see whether these patterns are really similar across all these forests," remembers Muller-Landau. "If the proposed general rules truly applied, they would be tremendously useful - for example, they would help us predict how much above ground carbon is stored in tropical forests. There are some general similarities among sites in the tree size distributions, so it seemed plausible that there might be a general underlying explanation."
"Specifically, metabolic energy proposes a rule that total tree abundances scale as the -2 power of diameter, and we were hopeful that it would apply. But we tested that prediction on data from 14 sites around the world --over 2 million trees--and it just did not hold up anywhere," continues Muller-Landau.
The CTFS team subsequently tested other metabolic ecology-based predictions relating tree size to growth and mortality against data from 10 CTFS forests, and found that those were not supported either.
"The predictions of metabolic ecology were wrong in fundamental ways, and no one had noticed before because few datasets spanned the range from saplings to large trees. Most impressive was the consistency in the big discrepancies between observation and prediction, even in very different forests," articulates coauthor Richard Condit, STRI Staff Scientist, "For instance, diameter distribution always deviated by having far fewer large trees than the -2 slope predicts."
Shedding light on the process
The CTFS team did not stop at merely demonstrating that metabolic ecology failed to explain tropical forest structure '" they examined what alternative models might do better. Organisms live within the limits of their environment. Maybe by modifying the metabolic ecology model to include light as a limiting resource that varies with tree size, the predictions could be improved. The necessary light data were available from only one site, and, in that case, observed growth and mortality patterns were consistent with predictions. Similarly, tree size distributions corresponded more closely to a model based on demographic equilibrium theory that incorporated differences in growth and mortality among sites.
The results of these two studies clearly demonstrate that there are both qualitative similarities and significant quantitative differences in forest structure among tropical sites. This complexity defies simple characterization '" its explanation requires consideration of the diversity of environmental conditions and species traits in tropical forests, and complex feedbacks and interactions among species and resources. Muller-Landau and Condit are currently working on mechanistic models that attempt to better capture these complexities, and thereby attain a better understanding of what factors drive tropical forest structure and dynamics. Achieving such an understanding is particularly critical today, as tropical forests are threatened by global anthropogenic change, with still unknown implications for the integrity of their tremendous carbon stores.
The Smithsonian Tropical Research Institute (STRI), a unit of the Smithsonian Institution, headquartered in Panama City, Panama, furthers our understanding of tropical nature and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.
Helene C. Muller-Landau, Richard S. Condit, Kyle E.Harms, Christian O. Marks, Sean C. Thomas, Sarayudh Bunyavejchewin, George Chuyong, Leonardo Co, Stuart Davies, Robin Foster, Savitri Gunatilleke, Nimal Gunatilleke, Terese Hart, Stephen P. Hubbell, Akira Itoh, Abd Rahman Kassim, David Kenfack, James V. LaFrankie, Daniel Lagunzad, Hua Seng Lee, Elizabeth Losos, Jean-Remy Makana, Tatsuhiro Ohkubo, Cristian Samper, Raman Sukumar, I-Fang Sun, Nur Supardi M. N., Sylvester Tan, Duncan Thomas, Jill Thompson, Renato Valencia, Martha Isabel Vallejo, Gorky Villa Mu*oz, Takuo Yamakura, Jess K. Zimmerman, Handanakere Shivaramaiah Dattaraja, Shameema Esufali, Pamela Hall, Fangliang He, Consuelo Hernandez, Somboon Kiratiprayoon, Hebbalalu S. Suresh, Christopher Wills and Peter Ashton. 2006. Comparing tropical forest tree size distributions with the predictions of metabolic ecology and equilibrium models. Ecology Letters 9: 589-602.
Helene C. Muller-Landau, Richard S. Condit, Jerome Chave, Sean C. Thomas, Stephanie A. Bohlman, Sarayudh Bunyavejchewin, Stuart Davies, Robin Foster, Savitri Gunatilleke, Nimal Gunatilleke, Kyle E. Harms, Terese Hart, Stephen P. Hubbell, Akira Itoh, Abd Rahman Kassim, James V. LaFrankie, Hua Seng Lee, Elizabeth Losos, Jean-Remy Makana, Tatsuhiro Ohkubo, Raman Sukumar, I-Fang Sun, Nur Supardi M. N., Sylvester Tan, Jill Thompson, Renato Valencia, Gorky Villa Mu*oz, Christopher Wills, Takuo Yamakura, George Chuyong, Handanakere Shivaramaiah Dattaraja, Shameema Esufali, Pamela Hall, Consuelo Hernandez, David Kenfack, Somboon Kiratiprayoon, Hebbalalu S. Suresh, Duncan Thomas, Martha Isabel Vallejo and Peter Ashton. 2006. Testing metabolic ecology theory for allometric scaling of tree size, growth and mortality in tropical forests. Ecology Letters, 9: 575-588.