The response of carbon assimilation and storage to long-term drought in tropical trees is dependent on light availability

Abstract

Whether tropical trees acclimate to long-term drought stress remains unclear. This uncertainty is amplified if drought stress is accompanied by changes in other drivers such as the increases in canopy light exposure that might be induced by tree mortality or other disturbances. Photosynthetic capacity, leaf respiration, non-structural carbohydrate (NSC) storage and stomatal conductance were measured on 162 trees at the world's longest running (15 years) tropical forest drought experiment. We test whether surviving trees have altered strategies for carbon storage and carbon use in the drier and elevated light conditions present following drought-related tree mortality. Relative to control trees, the surviving trees experiencing the drought treatment showed functional responses including: (a) moderately reduced photosynthetic capacity; (b) increased total leaf NSC; and (c) a switch from starch to soluble sugars as the main store of branch NSC. This contrasts with earlier findings at this experiment of no change in photosynthetic capacity or NSC storage. The changes detected here only occurred in the subset of drought-stressed trees with canopies exposed to high radiation and were absent in trees with less-exposed canopies and also in the community average. In contrast to previous results acquired through less intensive species sampling from this experiment, we also observe no species-average drought-induced change in leaf respiration. Our results suggest that long-term responses to drought stress are strongly influenced by a tree's full-canopy light environment and therefore that disturbance-induced changes in stand density and dynamics are likely to substantially impact tropical forest responses to climate change. We also demonstrate that, while challenging, intensive sampling is essential in tropical forests to avoid sampling biases caused by limited taxonomic coverage. A free Plain Language Summary can be found within the Supporting Information of this article.