Effects of light intensity on the growth and photosynthetic physiology of Zelkova schneideriana seedlings

Zelkova schneideriana is an important timber species used in the transformation of monoculture coniferous forests in southern China. However, the optimal light conditions for cultivating Z. schneideriana under forest canopies remain to be determined. In this study, one-year-old container seedlings of Z. schneideriana were used in a field experiment under four artificially simulated light intensities: full sunlight (CK, 100% PAR), low shade (L, 70% PAR), medium shade (M, 51% PAR), and high shade (H, 22% PAR). The results showed significant differences in growth, morphology, and photosynthetic physiology of seedlings under different light intensities. Z. schneideriana seedlings exhibited a certain degree of shade tolerance during early establishment, achieving optimal growth performance under medium shade conditions. Compared to CK, the net increments in height and diameter at root collar of seedlings under M increased by 197.69% and 145.27%, respectively. Specific leaf area was greatest under high shade, indicating that leaves tend to utilize a larger leaf area to acquire carbon resources. Seedlings under M exhibited the highest values for photosynthetic pigment content, key photosynthetic enzyme activities, and photosynthetic nitrogen- and phosphorus-use efficiencies, which were consistent with their response to light-saturated photosynthetic rate. The phenotypic plasticity indices of the seedlings ranked in the following order: photosynthetic physiological traits > light response traits > morphological traits. Photosynthetic pigment content was identified as the physiological parameter most sensitive to changes in light intensity. Therefore, for the understory introduction of Z. schneideriana seedlings, it is recommended that canopy thinning be implemented to maintain understory light intensity at approximately 50% of full sunlight, thereby facilitating rapid seedling establishment and growth.