Vulnerability of Amazonian tree communities to global change

Tree mortality is the principal mechanism whereby forests lose living biomass. This process has been observed to have increased across the Amazon forest over recent decades. Greater tree mortality rates have been attributed largely to an increase in the frequency and intensity of droughts, and to the intensification of competition, as a consequence of greater tree growth stimulated by higher CO₂concentrations. Analysing the trends in mortality for different taxa allows us to test the contribution of these different drivers to the rise in tree mortality. Droughts are expected to kill wet-affiliated, large, and low wood density taxa. Increased competition is likely to affect slow growth, understory taxa. We assess data from over 30 years of forest monitoring across the Amazon to investigate the changes in mortality across different taxa, providing a greater understanding of the drivers of increased tree mortality across the basin and the vulnerability of these forests to water stress. We observed that the proportion of dead trees across different taxa has changed across the Amazon forest. We show an increase in the mortality of drought-vulnerable trees, particularly in those areas where dry climatic events have intensified over the last 30 years. However, the proportion of large taxa within the dead trees has not changed over the length of this study. We also observed indications of increasing competition-driven mortality represented by a decrease in abundance of slow-growth shade-tolerant species. A suite of mechanisms, varying regionally in importance, are acting synchronically to drive recent increases in tree death across Amazonia. The patterns and mechanisms observed here are amenable to incorporation within the latest generation of global vegetation models and Earth system models, providing a basis for improved simulations of forest dynamics in one of the world’s most carbon-dense ecosystems.