Relating Tree Physiology to Past and Future Changes in Tropical Rainforest Tree Communities

Abstract

Predicting future changes in tropical rainforest tree communitiesrequires a good understanding of past changes as well as a knowledgeof the physiology, ecology and population biology of extant species.Climate change during the next hundred years will be more similarto climate fluctuations that have occurred in the last few thousandyears and of a much smaller magnitude than the extent of climatechange experienced during last glaciation or at the Pleistocene-Holocenetransition. Unfortunately, the extent to which tropical rainforesttree communities have changed during the last few thousand yearshas been little investigated. As a consequence we lack the detailedevidence for population and range shifts of individual tropical speciesresulting from climate change analogous to the evidence availablefor temperate zone forests. Some evidence suggests that the rateof tropical forest change in the last several thousand years mayhave been high. If so, then CO2 increases and the likely alterationsin temperature, forest turnover rate, rainfall, or severe droughtsmay drive substantial future forest change. How can we predict ormodel the effects of climate change on a highly diverse tree community?Explanations for the regulation of tropical tree populations ofteninvoke tree physiology or processes that are subject to physiologicalregulation such as herbivory, pathology or seed production. In orderto incorporate such considerations into climate change models, thephysiology of a very diverse tree community must be understood. Mywork has focused on simplifying this diversity by categorizing theshade-tolerant species into functional physiological groups. Mostspecies and most individual trees are shade-tolerant species, gap-requiringspecies being relatively uncommon. Additionally, in a regeneratinggap most of the individuals are shade-tolerant species that establishedbefore gap formation. Despite the fact that the shade-tolerant speciesare of major ecological importance, their comparative physiologyhas received little attention. I have found that shade-tolerant speciesdiffer substantially in their responses to light flecks, treefalllight gaps and drought. Furthermore, among phylogenetically unrelatedspecies, these differences in physiology can be predicted from leaflifetime. These results provide a general framework for understandingthe mechanics of tropical rainforests from a physiological perspectivethat can be used to model their responses to climate change.