Effects of Canopy Litter Removal on Canopy Structure, Understory Light and Vegetation Dynamics in Cunninghamia lanceolata Plantations of Varying Densities

The prolonged retention of senescent branches and needles (canopy litter) in <i>Cunninghamia lanceolata</i> 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 (<i>p</i> < 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 (<i>p</i> < 0.05) and consistently enhanced PPFD and transmittance (<i>p</i> < 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 (<i>S</i>), Simpson diversity (<i>D</i>), and Shannon-Wiener diversity (<i>H</i>), while Pielou Evenness (<i>J</i>) 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., <i>Indocalamus tessellatus</i> and <i>Pleioblastus amarus</i>), heliophilic grasses (e.g., <i>Lophatherum gracile</i>) and pioneer ferns (e.g., <i>Pteris dispar</i> and <i>Microlepia hancei</i>). Correlation analyses confirmed PPFD as a key positive driver of all diversity indices (<i>p</i> < 0.01), whereas LAI was significantly negatively correlated with PPFD, light transmittance, and understory diversity (<i>p</i> < 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.