Resilience under prolonged drought

Abstract

To test rainforest resilience, the researchers revisited the Caxiuanã throughfall exclusion (TFE) experiment. Conducted in the eastern Amazon, this is the only tropical forest experiment implementing precipitation exclusion continuously over decades, and at ‘ecosystem’ scale (the one-hectare area of forest was artificially droughted for 23 years). A loss of 34% of rainforest biomass was observed, which was primarily due to the mortality of large trees during the first 15 years of experimental drought. Biomass then stabilized from 2017 to 2023, attributed to the recovery of water availability per tree to levels observed in the control forest study area. The loss of large trees reduced water competition during this stabilization phase, shifting the ecosystem from a probable carbon source to a small biomass sink.

The researchers further monitored individual trees during the 2023–2024 growth seasons, which included pronounced wet and dry seasons influenced by the effects of global warming and the strong El Niño in 2023–2024. They found that the droughted TFE trees displayed stable hydraulic function, with transpiration rates similar throughout the year to control trees, and an even smaller reduction in function during dry seasons. Measurements of leaf water potential, relative water content and stem water content all indicated that the TFE trees that had survived the preceding multi-decadal drought had now achieved hydraulic homeostasis. This homeostasis was linked most strongly to ecosystem-level structural changes — reduced water demand and increased water availability resulting from the prior loss of large trees.