Tropical tree mortality has increased with rising atmospheric water stress

Evidence exists that tree mortality is accelerating in some regions of the tropics(1,2), with profound consequences for the future of the tropical carbon sink and the global anthropogenic carbon budget left to limit peak global warming below 2 degrees C. However, the mechanisms that may be driving such mortality changes and whether particular species are especially vulnerable remain unclear(3-8). Here we analyse a 49-year record of tree dynamics from 24 old-growth forest plots encompassing a broad climatic gradient across the Australian moist tropics and find that annual tree mortality risk has, on average, doubled across all plots and species over the last 35 years, indicating a potential halving in life expectancy and carbon residence time. Associated losses in biomass were not offset by gains from growth and recruitment. Plots in less moist local climates presented higher average mortality risk, but local mean climate did not predict the pace of temporal increase in mortality risk. Species varied in the trajectories of their mortality risk, with the highest average risk found nearer to the upper end of the atmospheric vapour pressure deficit niches of species. A long-term increase in vapour pressure deficit was evident across the region, suggesting that thresholds involving atmospheric water stress, driven by global warming, may be a primary cause of increasing tree mortality in moist tropical forests.

Automated summary

Evidence suggests that tree mortality is increasing in certain tropical regions, which could have significant impacts on the carbon sink and carbon budget. The mechanisms and species vulnerability behind these mortality changes are still unclear. A 49-year study of old-growth forest plots in the Australian moist tropics revealed that annual tree mortality risk doubled on average over the last 35 years, potentially reducing life expectancy and carbon residence time. Losses in biomass were not counteracted by growth and recruitment. Higher mortality risk was observed in less moist local climates, but local mean climate did not predict the rate of temporal increase in mortality risk. Species exhibited different trajectories in their mortality risk, and the highest average risk was found in species closer to the upper end of the atmospheric vapor pressure deficit niches. The long-term increase in vapor pressure deficit suggested that atmospheric water stress, driven by global warming, may be a primary cause of increasing tree mortality in moist tropical forests.