Litter Quality and Soil Microorganisms Mediate Reduced Litter Decomposition Following Understory Vegetation Removal in Forest Ecosystems

Understory vegetation is a critical component of forest ecosystems. Its removal can substantially alter litter decomposition processes, with cascading effects on carbon (C) and nutrient cycling in terrestrial ecosystems. However, the global response patterns of litter decomposition to understory removal and underlying controlling factors remain unclear. We conducted a meta-analysis of 330 observations from 29 peer-reviewed field litterbag studies to assess the effects of understory removal on litter decomposition. We evaluated the changes in decomposition rate, mass loss, and nutrient dynamics to quantify the impacts of understory removal on litter decomposition. We assessed the associated shifts in soil microbial communities, measured using phospholipid fatty acids (PLFAs), to examine how microbial responses mediate decomposition during understory removal. We examined whether canopy type moderated these responses and explored the key predictors of decomposition for understory removal. Understory removal significantly reduced litter decomposition rate and mass loss by an average of 29.6% and 14.8%, respectively, while increasing lignin remaining by 30.1%. Soil microbial biomass also declined, with total, fungal, and actinomycete PLFAs decreasing by 12.0%, 30.8%, and 27.5%, respectively. Across canopy types, understory removal decreased litter mass loss in both broadleaved and coniferous forests. However, the remaining N and P increased significantly in broadleaved forests but changed only marginally in coniferous forests. Random forest analysis showed that initial litter quality and variations in fungal biomass were the primary predictors of decomposition responses. Understory vegetation removal significantly suppresses litter decomposition by reducing fungal biomass, and interacting with litter quality constraints and canopy type strongly moderates these effects. This highlights the essential role of understory vegetation in sustaining nutrient cycling and microbial functioning in forest ecosystems and underscores its critical role in guiding sustainable forest management.