Growing evidence shows that tropical forests are highly sensitive to climate change and variability. There are pronounced negative effects of warm and dry conditions during El Niño years (Lewis et al. 2011). Unusually warm and dry years have become more common lately, probably explaining the decline of the Amazon carbon sink since the early 1990s (Brienen et al. 2015). These negative effects of hot and dry conditions on tropical forest biomass were caused by increased tree mortality rather than by decreased productivity of living trees. Improved understanding of the causes of tropical tree mortality is thus critical for more reliable predictions of the global carbon cycle in a changing climate. 

The mechanisms causing drought-induced tree mortality is a matter of intense debate in the literature (McDowell et al. 2011). Two competing hypotheses suggest that it is either primarily caused by hydraulic failure of the water conducting tissue and subsequent desiccation, or by stomatal water-saving responses leading to decreased photosynthesis and subsequent tissue carbohydrate starvation. Controlled experiments have shown that both mechanisms may trigger tree mortality, and that they are highly interdependent. However, we still know little regarding why trees in nature die under dry and hot conditions. What are the functional traits predisposing trees to mortality? Do the same traits control both heat- and drought-induced mortality? Can tree mortality be predicted by a relatively simple trait-based model, suitable for future incorporation into dynamic vegetation models?