One of the most basic questions in ecology is: Where does each species occur And following on that: What environmental factors determine where a particular species occurs Despite the seeming simplicity of these questions, they remain unanswered for the vast majority of species. This lack of knowledge is especially troubling given the implications for understanding how species will respond to climate change through either shifts in where they occur or through physiological adaptations. This study will address these and other questions through a series of experimental and observational studies conducted in the Andean mountains of Peru, where many species are unusually vulnerable to small changes in temperature and rainfall. More specifically, the project will measure altitudinal ranges and thermal tolerances of nearly two dozen cloud forest tree species. The investigators will also determine the extent to which small populations of each species are tightly adapted to particular elevations ("local adaptation") and each species' ability to change its thermal tolerance ("plasticity"). Data will be incorporated into models designed to predict where species currently occur and where they might start to occur as climates change. This project is one of the first to systematically test several key assumptions critical to the assessment how climate change will impact where species are able to live. It will be the first-ever study to simultaneously measure the thermal tolerances of individuals, populations, and species of tropical trees. The results will greatly expand our understanding of the complex factors determining where species occur, and will strengthen our ability to predict species responses to future climate change.The research will be integrated with an extensive set of educational, training, and outreach programs in collaboration with established domestic and international environmental education organizations. Outreach programs are designed to help increase the participation of US minorities and international students in the biological sciences, and to broaden public awareness of the potential impacts of climate change on tropical forests around the world.