CLIMATE-CHANGE, WATER-STRESS, AND FAST FOREST RESPONSE - A SENSITIVITY STUDY

Environmental crises provide ecology with a new impetus. Indeed, acid rain, massive changes in land-use, and the prospect of global climate change are giving the study of ecosystems a new meaning and new mandat es. Careful natural history descriptions are no longer sufficient for evaluating potential impacts of changing environmental conditions. Und erstanding how ecosystems work has become a priority. The present pape r investigates the role of water stress as an agent of rapid vegetatio n change. A sensitivity study is performed to explore the issue. Envir onmental changes are imposed, rather than obtained from atmospheric ge neral circulation model (GCM) output, to facilitate the interpretation of the results, to enable the identification of generic patterns, in particular, critical thresholds, and to foster the understanding of un derlying processes. A physically- and physiologically-based, climatica lly-sensitive, numerical simulation model of forest dynamics, the Ener gy, Water, and Momentum Exchange and Ecological Dynamics (EXE) model, is used for this purpose. The results of the sensitivity analysis unde rtaken with EXE document the interplay between physical and physiologi cal feedbacks and ecological forest responses to climatic changes. Mor eover, they explain the relative importance of temperature, water, and nitrogen limitations, indicating that water stress is responsible for some of the fastest vegetation changes.