Climate Controls on Canopy Turnover: The Role of Atmospheric Demand and Hydrological Legacies in an Amazonian Lowland Forest

The Amazon rainforest, home to nearly 40% of Earth’s tropical forest biomass, plays a central role in the global carbon sink. Recent observations indicate a decline in net carbon uptake, attributed to intensifying hydro-climatic extremes, deforestation, and forest degradation. Seasonal droughts are lengthening, wet-season onsets are delaying, and strong El Niño events are becoming more frequent, yet the mechanisms by which these changes influence ecosystem processes remain poorly understood.

Leaf litterfall is a major pathway of carbon and nutrient transfer from vegetation to soils, integrating climate seasonality, atmospheric demand, and plant physiological strategies. Here, we analyze eight years of litterfall observations from a lowland Amazon rainforest together with meteorological data to identify seasonal and interannual climate controls on canopy turnover.

We find that litterfall dynamics differ systematically between years with typical and anomalous litterfall patterns. In years with typical seasonality, litterfall patterns are well explained by atmospheric moisture and pressure variables, with maximum relative humidity and barometric pressure emerging as dominant predictors. In contrast, anomalous years show distinct responses depending on hydro-climatic context: wetter wet seasons followed by intense dry seasons are associated with elevated litterfall, best explained by cumulative water deficit, whereas drier wet seasons are linked to suppressed litterfall driven by high evaporative demand, captured by potential evapotranspiration. Across all conditions, litterfall reflects the combined influence of antecedent hydrological states and immediate atmospheric demand, challenging phenology frameworks based primarily on precipitation or temperature alone.

Moisture-demand and pressure variables such as vapor pressure deficit, potential evapotranspiration, and barometric pressure may thus provide powerful yet underutilized insights into vegetation turnover, although better models will emerge from the integration of other biological processes such as stem flow and root dynamics. As climate change alters seasonality and increases hydro-climatic extremes, disruptions in canopy turnover are likely to influence vegetation dynamics, nutrient cycling, and the future resilience of the Amazon forests.