Post-Fire Shrub Recovery Shifts from Disturbance Limitation to Structural Light-Capture Regulation in a Subtropical Forest

Wildfire and atmospheric nitrogen (N) deposition are increasingly important drivers of forest recovery under global change, yet their combined effects on understory shrub dynamics remain poorly understood in subtropical forests. We quantified temporal changes in shrub aboveground biomass (AGB) following low-severity fire and N addition in a subtropical conifer–broadleaf mixed forest in Central China, and evaluated the underlying mechanisms of resource and trait. Burning reduced shrub AGB by 58.3% in the first post-fire year, but this negative effect rapidly weakened, disappeared within two years, and shifted to a net positive effect (+28.2%) by year four. Nitrogen addition increased shrub AGB by 26.5%, with gradually strengthening effects over time. Understory light availability, soil available N, and community-weighted leaf area were the key predictors explaining variation in shrub AGB. Fire induced changes in shrub AGB was most strongly associated with leaf area rather than specific leaf area or foliar nutrient concentrations. Nitrogen addition improved shrub growth primarily through increased soil N availability and foliar N enrichment. Overall, post-fire shrub recovery was governed by a successional shift in limiting factors, transitioning from initial disturbance constraint to light-driven structural control with increasing importance of soil nutrient supply. These findings demonstrate that post-fire shrub recovery in subtropical forests is increasingly governed by canopy-mediated structural strategies rather than acquisitive leaf economics during succession. Therefore, increasing wildfire activity and atmospheric N deposition may reinforce shrub-dominated understory states, with important implications for post-fire regeneration trajectories and long-term forest carbon dynamics.