I want to understand how plants will respond to rapid climate change at scales from cells to continents with the goal of building mechanistic tools to predict the future of Earth’s forests in the 21st century. The challenges of predicting forests’ responses to climate change based on fundamental tree physiology, while capturing the complex and non-linear dynamics of interactions among organisms, require an ambitious and cross-scale suite of experiments, observations, and modeling approaches. I will leverage recent theoretical advances, cross-system manipulations of drought stress, and newly available global datasets to construct robust and probabilistic models of forest ecosystems. Ultimately, rigorous, mechanistic forecasting of how changes in climate will affect ecosystems is one of the central aims of biology and will have manifold ramifications for the future of Earth’s ecosystems, societies, and climate.