The prolonged retention of senescent branches and needles (canopy litter) in Cunninghamia lanceolata canopies is an evolutionary adaptation, yet its impacts on stand microenvironment and understory succession remain poorly quantified. To address this gap, we conducted a 5-year field experiment across six planting densities (1800, 2400, 3000, 3600, 4200, and 4800 trees·ha⁻¹), aiming to evaluate the effects of canopy litter removal on canopy structure, forest light environment, and understory biodiversity. Results demonstrated that leaf area index (LAI) and mean tilt angle of the leaf (MTA) significantly increased with density (p < 0.05), leading to marked reductions in photosynthetic photon flux density (PPFD) and light transmittance (T). Canopy litter removal significantly reduced LAI across all densities after 4–5 years (p < 0.05) and consistently enhanced PPFD and transmittance (p < 0.01). MTA and light quality parameters (red:blue and red:far-red ratios) both exhibited variable responses to litter removal, driven by density and time interactions, with effects diminishing over time. Understory vegetation diversity exhibited pronounced temporal dynamics and density-dependent responses to canopy litter removal, with increases in species richness (S), Simpson diversity (D), and Shannon–Wiener diversity (H), while Pielou Evenness (J) responded more variably. The most notable increase in species richness occurred in the 4th year, when 21 new species were recorded, largely due to the expansion of light-demanding bamboos (e.g., Indocalamus tessellatus and Pleioblastus amarus), heliophilic grasses (e.g., Lophatherum gracile) and pioneer ferns (e.g., Pteris dispar and Microlepia hancei). Correlation analyses confirmed PPFD as a key positive driver of all diversity indices (p < 0.01), whereas LAI was significantly negatively correlated with PPFD, light transmittance, and understory diversity (p < 0.01). These findings demonstrate that strategic management of canopy litter incorporating stand density regulation can improve understory light availability, thereby facilitating heliophilic species recruitment and biodiversity enhancement in subtropical coniferous plantations.

With global warming, understanding the effect of elevated temperature on the decomposition of Chinese fir needle litter has significant implications for nutrient cycling, yield, and management of economically important Chinese fir plantations. We conducted simulated warming decomposition experiments in incubators at 25 °C, 30 °C, and 35 °C on Chinese fir needle litter from middle-aged, mature, and overmature stands. Changes in litter mass and concentrations of some metallic elements and recalcitrant components were measured in litter sampled at different decomposition time-steps up to 264 days (d). Warming to 35 °C significantly increased the mass loss rate of needle litter from overmature stands throughout the experiment (except at 72 d). The effect of warming on litter mass loss rate for middle-aged and mature stands was lower and is attributed to higher litter quality in these stands. Compared to 25 °C, warming to 30 °C and 35 °C increased the needle litter decomposition rate across all developmental stages by 17.3% and 48.3%, respectively. Potassium (K), calcium (Ca), and magnesium (Mg) were mostly released during needle litter decomposition in all Chinese fir developmental stages. Lignin, condensed tannins, total phenols, and cellulose were enriched in needle litter, while the release of hemicellulose from near the start of the decomposition experiment was attributed to its lower molecular weight compared with other carbohydrates in litter. Compared with 25 °C, warming to 35 °C increased the release rates from litter of K, Ca, and Mg by 14.7%, 24.6%, and 21.5%, and the release rates of lignin, total phenols, cellulose, and hemicellulose by 7.5%, 8.8%, 10.4%, and 13.7%. Needle litter iron (Fe), aluminum (Al), and sodium (Na) in different development stages and manganese (Mn) in the overmature stands were mostly enriched during the experiment. Warming significantly promoted the enrichment of Fe, Al (except for mature stands), and Na, and reduced the enrichment of Mn. In summary, the sensitivity of needle litter to temperature in overmature stands is higher than that in middle-aged and mature stands, suggesting that forest managers can extend the rotation length of Chinese fir plantations to increase the yield of large-diameter timber, litter decomposition, and ecosystem nutrient return.