EFFECTS OF CANOPY STRUCTURE ON GROWTH AND BELOWGROUND/ABOVEGROUND BIOMASS OF SEEDLINGS IN UNEVEN-AGED TROJAN FIR STANDS

ABSTRACT Growth and survival of understory seedlings are influenced by their interactions with the environmental variables. The understanding of the relationships between stand density, canopy structure, and understory seedlings’ growth help forest managers conduct successful establishment through natural regeneration. In this study, the influence of stand basal area (BA) (m2.ha-1), photosynthetically active radiation transmitted through canopy (PART), leaf area index (LAI), height-to-crown base (HCB) and aspect on the root-collar diameter (RCD) (mm) growth, height growth (cm) and belowground/aboveground biomass (g) of seedlings is observed in uneven-aged Trojan fir stands. Seedling growth and belowground/aboveground biomass models were developed for five growing seasons after germination. PART, aspect and HCB were found to be the most strongly related to RCD and height growth of Trojan fir seedlings. PART and aspect explained the most variance in belowground/aboveground biomass of the seedlings. The data also showed that the light transmitted through the canopy was the most significant predictor of understory seedlings growth; thus, PART reaching understory can accelerate the development of sub-canopy layers in uneven-aged Trojan fir stands. Although survival of Trojan fir seedlings is usually high under canopy, results suggest that complete canopy closure should be avoided even in stands of shade-tolerant species such as Trojan fir to increase a seedling’s vigor for its successful recruitment into middle and overstory. The findings of this study will be helpful for forest managers to enhance the establishment and recruitment of Trojan fir seedlings in uneven-aged stands.

【摘要】林下层幼苗的生长与存活，与其所处环境变量间的相互作用密切相关。明晰林分密度、冠层结构与林下层幼苗生长之间的关联，有助于森林经营者通过天然更新实现林分的高效营建。本研究针对异龄特洛伊冷杉（Trojan fir）林分，观测了林分断面积（stand basal area, BA）（单位：m²·ha⁻¹）、冠层透射光合有效辐射（photosynthetically active radiation transmitted through canopy, PART）、叶面积指数（leaf area index, LAI）、枝下高（height-to-crown base, HCB）及坡向，对幼苗根颈直径（root-collar diameter, RCD）生长（单位：mm）、树高生长（单位：cm）及地下/地上生物量（单位：g）的影响。研究针对发芽后5个生长季的幼苗，构建了生长及地下/地上生物量预测模型。结果显示，PART、坡向与HCB与特洛伊冷杉幼苗的根颈直径及树高生长相关性最强；PART与坡向可解释幼苗地下/地上生物量的最大变异量。研究数据同时表明，冠层透射光照是林下层幼苗生长最关键的预测因子，因此抵达林下层的PART可加速异龄特洛伊冷杉林分中亚冠层的发育进程。尽管特洛伊冷杉幼苗在冠层下的存活率通常较高，但本研究结果提示，即便对于特洛伊冷杉这类耐荫树种，也应避免林分完全郁闭，以提升幼苗活力，使其成功更新进入中林层与上层林冠。本研究成果可为森林经营者提升异龄林分中特洛伊冷杉幼苗的定植与更新效率提供科学参考。