Climatic drivers and internal connections among phenological events co-determine forest vegetative and reproductive phenology

Abstract Plant phenology plays a fundamental role in ecosystem functioning by regulating species interactions and biogeochemical cycles. While both climatic factors and internal phenological linkages influence plant life-history events, their relative contributions remain unclear, particularly in species-rich subtropical forests. Using 6 years of monthly litterfall data (leaves, flowers and fruits/seeds) from a subtropical evergreen broad-leaved forest, we combined cross-correlation analyses with generalized additive models (GAMs) into a unified framework to disentangle internal linkages among multiple phenological events and nonlinear climatic effects. We reveal three key findings. First, we identified distinct climatic drivers for each phenophase: temperature and wind speed primarily controlled leaf fall, day length regulated flowering and the El Niño-Southern Oscillation (ENSO) strongly influenced seed production. Second, internal phenological linkages exerted equally strong or stronger effects than climate, with flower production driving leaf abscission, photoperiod triggering flowering and both floral initiation and resource remobilization released from leaf abscission jointly determining seed production. Most notably, we discovered cross-year reproductive trade-offs mediated by resource allocation constraints, where current high seed output suppressed flowering in the following year. Our results demonstrate that community-level phenology emerges from the interplay between phenophase-specific climatic responses and endogenous physiological linkages across phenophases. These findings challenge climate-centric phenological models and highlight the need to integrate internal plant resource dynamics when forecasting ecosystem responses to environmental change.