Trait-based representation of drought-deciduous phenology in a demographic vegetation model

Drought-deciduous phenology is a key adaptive strategy shaping growth, mortality, and competitive dynamics in tropical forests, yet it remains poorly represented in most vegetation models. We introduce a mechanistic drought-deciduous phenology module in FATES-Hydro that links canopy leaf loss and refoliation directly to plant hydraulic states, enabling trait-based representation of drought responses within a vegetation demographic framework.
Leaf shedding is driven by the fraction of the canopy experiencing critically low leaf water potential, while refoliation is permitted only after sustained hydraulic recovery above a second threshold for multiple consecutive wet days. This formulation captures both rapid drought-induced canopy loss and delayed recovery constrained by hydraulic and carbon availability, and operates consistently with size-structured demography and carbon allocation.
We evaluate the model across six tropical forest sites spanning a strong moisture gradient and focus on a sensitivity analysis to identify trait-mediated controls on phenology and demographic outcomes. At seasonal dry sites, phenological parameters dominate canopy dynamics. The leaf-off water-potential threshold exerts first-order control over both leaf shedding and refoliation timing by regulating dry-season soil moisture depletion. The recovery threshold further delays or accelerates leaf-on timing, while the assumed distribution of leaf water potential within the crown primarily controls whether canopy loss occurs gradually or abruptly.
These phenological controls generate pronounced growth–mortality trade-offs along the moisture gradient. At dry sites, delayed leaf shedding enhances carbon uptake during the wet season but increases drought exposure and mortality, whereas earlier shedding reduces productivity while maintaining hydraulic safety.
Hydraulic trait sensitivities are strongly site dependent. Root distribution is the dominant control at seasonal sites, with deeper roots delaying leaf-off, shortening leaf-off duration, and advancing refoliation. This effect weakens in wetter forests. Rooting depth has little influence on canopy phenology at wet sites but increases gross primary productivity and evapotranspiration. Stomatal sensitivity (P50gs) plays a secondary role, regulating carbon gain and mortality differently across climates: risky stomatal strategies increase wet-season carbon fixation but elevate leaf loss and mortality at dry sites, while conferring higher growth without increased mortality at wet sites. In contrast, maximum xylem conductance has negligible influence on phenology, growth, or mortality. The lack of sensitivity differences between two wet sites with contrasting rainfall seasonality further indicates that light, rather than water, constrains wet tropical forest dynamics. These results highlight how integrating hydraulics, phenology, and demography enables trait-based predictions of tropical forest responses to increasing drought stress.