Understanding species responses to climate change is one of the central problems in ecology. Over the last decade, the researchers who will lead this project have used the elevation gradient spanning 3500 m from the Amazonian lowlands to the Andean treeline as a natural laboratory for investigating forest and ecosystem ecology. That previous work has documented tree migration in response to climate change, and predicted changes in the ranges of species under different scenarios of change. This project will now go further to investigate the underlying causes of such changes in where trees grow. The researchers will compare the physiology of plant species known to be migrating in response to climate change in the Andes and Amazon with the physiology of species that are holding their ground, and test the importance of environmental factors such as temperature, soil, herbivory and fungal pathogens. The results will help understand where tree species can survive, what affects their growth, and what limits their range along environmental gradients. The project will increase our ability to predict and model the effects of climate change on natural systems, particularly tropical montane forests, which are areas of high biodiversity.The broader impacts of this project include a suite of educational activities, resources, and opportunities aimed at teaching K-12 and college students about tropical ecology and the effects of global climate change; training opportunities for international and minority students which will assist them in pursuing advanced education and careers in the natural sciences; and the dissemination of results to a broad scientific and lay audience, including Spanish translations and versions of resources.