Modelling Tropical Dry Forest Phenology from a Plant Hydraulic Perspective

Plant responses to dry environments are shaped by diverse adaptive strategies linked to plant hydraulic traits, as reflected by the coexistence of deciduous and evergreen tree species in tropical seasonally dry forests. Empirical evidence suggests that leaf shedding is associated with declining leaf water potential, while leaf flushing depends on xylem rehydration. However, these physiological mechanisms are rarely incorporated into Dynamic Global Vegetation Models (DGVMs), which typically represent drought deciduous phenology using highly-simplified, threshold-based schemes with fixed rates of leaf phenological change. Here, we develop a stress–gradient based phenology scheme in which leaf shedding and leaf flushing are driven by the temporal gradients of leaf water potential and xylem water potential, respectively. This gradient–driven phenology mechanism was implemented in the LPJ-GUESS DGVM and validated with in-situ observations of phenological responses along hydraulic gradients. The new model has successfully reproduced phenological dynamics for the 7 selected locations and substantially improves model performance in simulating plant transpiration. We provide evidence that plant hydraulics are key controls for the phenological dynamics of tropical dry forests. The proposed stress-gradient phenological mechanism, linking phenology to plant hydraulic status, is an efficient approach to represent landscape phenology and improve simulations of water and carbon cycling over the tropical drylands. It may also help improve our understanding of forest response to drought stress, which remains largely unknown under warming climates.