The fate of forests in a warming world is of major ecological, societal, and economic concern. Forests play a key role in the combined carbon-water-nutrient cycle, including important ecosystem services and feedbacks to the climate system. Historically, forests have been an important carbon sink because of an excess of net primary production (NPP) compared to ecosystem respiration, but the future of this carbon sink is increasingly in question, as NPP may decline in a warming world.