Litter type and thickness modulate microclimate and seedling establishment in Cunninghamia lanceolata

Litter cover plays a crucial role in regulating seedling growth by modifying microclimate, yet the joint roles of litter type and thickness remain poorly understood. We used a pot experiment with three litter types (broadleaf Schima superba, needle Cunninghamia lanceolata, and a mixture), crossed with four thickness levels (0, 200, 400, 800 g·m^-2) to test how litter modulates microclimate and early performance of C. lanceolata seedlings. We monitored soil temperature and soil moisture, photosynthetic photon flux density (PPFD), and the red to far-red ratio (R:Fr) beneath the litter, and measured emergence, survival, morphology, and biomass. Litter cover significantly altered microclimate and in turn affected seedling growth. All litter cover reduced PPFD, with thick Schima superba litter nearly eliminating surface light, and R:Fr declined beneath litter. Litter buffered soil temperature, with daytime cooling and nighttime warming, and increased soil moisture, with stronger effects under broadleaf litter than under needle litter. Seedling responses depended on litter type and thickness. Thin to moderate cover enhanced emergence and survival, while thick cover suppressed them, especially under S. superba litter. Seedlings displayed shade avoidance, including taller height, reduced root growth, a shift in biomass allocation toward shoots and lower root-to-shoot ratio. Broadleaf litter exerted stronger effects than needle litter, and thickness responses were non-linear, with moderate cover facilitating growth but excessive accumulation inhibiting it. These results clarify the ecological role of litter in regeneration and suggest practical guidelines for forest floor management in subtropical conifer plantations.