Death from drought in tropical forests is triggered by hydraulics not carbon starvation

Drought threatens tropical rainforests over seasonal to decadal timescales (1-4), but the drivers of tree mortality following drought remain poorly understood (5,6). It has been suggested that reduced availability of non-structural carbohydrates (NSC) critically increases mortality risk through insufficient carbon supply to metabolism ('carbon starvation') (7,8). However, little is known about how NSC stores are affected by drought, especially over the long term, and whether they are more important than hydraulic processes in determining drought-induced mortality. Using data from the world's longest-running experimental drought study in tropical rainforest (in the Brazilian Amazon), we test whether carbon starvation or deterioration of the water-conducting pathways from soil to leaf trigger tree mortality. Biomass loss from mortality in the experimentally droughted forest increased substantially after > 10 years of reduced soil moisture availability. The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees. However, we find no evidence that the droughted trees suffered carbon starvation, as their NSC concentrations were similar to those of non-droughted trees, and growth rates did not decline in either living or dying trees. Our results indicate that hydraulics, rather than carbon starvation, triggers tree death from drought in tropical rainforest.
Drought-response observations from both field-scale experiments and natural droughts have demonstrated increased mortality over the short-term (1-3 years), with notably higher vulnerability for some taxa, and for larger trees (6,9,10). [...]