The role of local hydrology and leaf habit in leaf phenology patterns over 52 years of monitoring of canopy trees in central Amazonia, Brazil

Phenological cycles of tropical trees are shaped by environmental factors, yet the role of soil-water gradients on leaf phenology of equatorial forests remains poorly understood. We assessed the effects of water table depth (WT), classified into three categories based on Height Above Nearest Drainage (HAND) levels: Low (7-20m), representing shallow WT and greater root water access, Intermediate (21-60m) and High (61-89m), corresponding to deep WT and limited root water access, on leaf production and shedding of 547 canopy trees, monitored monthly for 52 years in Central Amazonia. Our results demonstrate that local hydrology is associated with phenological strategies and canopy persistence. Drier, high-HAND environments exhibited lower leaf flushing frequencies, a marked decline in mature leaves after 1990, and more frequent, prolonged canopy leaf-loss events. Conversely, wetter low-HAND sites showed higher leaf turnover and shorter flushing periods. Intra-annually, leaf flushing peaked during the dry season across all environments, but timing and intensity varied: low-HAND areas displayed bimodal patterns and earlier peaks (September), while high-HAND areas showed later peaks (October). Both deciduous and evergreen species responded to seasonal drought, but their temporal synchrony tended to be stronger in higher HAND environments, where climatic constraints are more severe. In wetter sites, weaker synchrony suggests reduced seasonal limitation on leaf production. We conclude that hydrological gradient and leaf habit jointly shape phenological dynamics, with drier environments experiencing greater canopy loss and tighter seasonal constraints on leaf renewal. These findings highlight the importance of local hydrological niches in forest responses to environmental change.