Fire severity shapes asynchronous herbaceous and shrub recovery along contrasting soil-recovery pathways in a subtropical Pinus massoniana plantation

Abstract

Post-fire ecosystem recovery depends on both fire severity and the coupling between soil and vegetation processes, yet herbaceous and shrub layers may follow different recovery pathways. We examined how fire severity shaped surface-soil recovery and understory vegetation dynamics in a subtropical Pinus massoniana plantation in Daye, central China. Unburned reference, low-severity, moderate-severity, and high-severity plots were surveyed at 0.5, 1.5, and 3.5 years after fire. The final dataset comprised 20 permanent plots repeatedly surveyed across three recovery stages, yielding 60 plot-stage observations. Soil physical and chemical properties, herbaceous and shrub diversity, and aboveground biomass were analyzed using linear mixed-effects models, principal component analysis, extended biomass models, and piecewise structural equation modeling. Higher fire severity was associated with strong early soil displacement. At 0.5 years, soil organic carbon declined from 37.33 ± 0.89 g kg⁻¹ in unburned plots to 22.72 ± 0.68 g kg⁻¹ in high-severity plots, whereas bulk density increased from 1.175 ± 0.013 to 1.314 ± 0.018 g cm⁻³. Herbaceous biomass showed the opposite response, increasing from 108.65 ± 30.94 g m⁻² in unburned plots to 442.92 ± 14.71 g m⁻² under high severity. In contrast, shrub biomass decreased from 720.92 ± 126.53 to 110.31 ± 2.95 g per 4 m² at 0.5 years and remained lower in high-severity plots than in the unburned reference after 3.5 years. Soil PC1 explained 68.4% of the variation in soil properties and represented an integrated soil recovery gradient, with positive loadings of soil water content, soil organic carbon, and total nitrogen and negative loadings of bulk density, pH, and available phosphorus. The final piecewise SEM showed an acceptable fit (Fisher’s C = 0.208, df = 4, P = 0.995) and indicated contrasting pathways: fire severity was positively associated with herbaceous biomass (standardized coefficient = 0.486) but negatively associated with shrub biomass (−0.355), whereas soil recovery was positively associated with shrub biomass (0.311). These results show that early herbaceous expansion may obscure delayed shrub and soil recovery, highlighting the need to evaluate post-fire recovery as a layer-specific soil-vegetation process rather than as a single greenness or biomass trajectory.