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.

Spatial light heterogeneity within forest canopies drives organ-level competition and whole-plant functioning, yet how shade-induced lower-canopy senescence and retention affect upper-canopy photosynthesis in marcescent species remains unclear. In this study, three-year-old Chinese fir [Cunninghamia lanceolata (Lamb.) Hook.] seedlings were subjected to half-canopy (moderate shading, T1) and three-quarter canopy (severe shading, T2) treatments, alongside a non-shaded control (CK), to investigate the monthly and diurnal dynamics of intra-canopy photosynthetic and photochemical traits. The results showed that long-term shading significantly reduced lower-needle photosynthetic capacity and accelerated senescence, as limitations shifted from stomatal (during January–April) to non-stomatal (during May–June), ultimately impairing photosystem II (PSII) (indicated by a significant decline in F_v/F_m) and inducing needle retention. Concurrently, upper needles exhibited pronounced non-autonomous responses with the net photosynthetic rate (P_n), stomatal conductance (G_s), and water-use efficiency (WUE) decreasing with increasing shading intensity. Compared with the CK, the reduction in P_n reached 61.99% under severe shading (T2). Shading weakened the strong positive coupling between the upper and lower canopies observed in the control (r = 0.615 to 0.978), shifting specific parameters to an antagonistic relationship under T2 (e.g., lower-canopy P_n and upper-canopy L_s, r = −0.897), which indicates a transition from integrated coordination to functional differentiation. Chlorophyll fluorescence correlations further revealed that while moderate shading (T1) maintained PSII photochemical stability, severe shading (T2) induced divergent energy allocation, with lower-canopy F_v/F_m negatively associated with upper-canopy qP and NPQ (r = −0.361 and −0.355, p < 0.05). Furthermore, lower-canopy senescence exerted a 1–2-month time-lagged effect on upper-canopy photosynthesis, with gas exchange parameters demonstrating greater sensitivity than photochemical processes. Overall, the results from three-year-old potted seedlings suggest that shade-induced lower-canopy needle senescence and retention may regulate upper-canopy photosynthetic function, driving an intra-canopy transition from coordination to decoupling and antagonism through distinct time-lagged effects. Further field studies are needed to determine whether these patterns occur in mature C. lanceolata stands and how they influence plantation productivity and marcescence dynamics.