The response of seedlings and saplings to canopy structure and light in different gaps in a spruce-fir mixed stand in Changbai Mountains, China

Environmental heterogeneity caused by the formation of forest gaps has important implications for the establishment and growth of forest regeneration, and different tree species with different shade tolerance at various stages of regeneration respond differently to the environment. In this study, we investigated 48 forest gaps and classified them into four classes based on their area, including small (< 20 m2), medium (20 m(2) to 50 m(2)), large (50 m(2) to 120 m(2)), and extra-large (> 120 m(2)). We conducted an analysis of the differences and variations in canopy structure (canopy openness and leaf area index (LAI)) and light environment (direct light, diffuse light, and total light) for different sizes of gaps and along the north-south axis of the gaps. Additionally, we conducted a preliminary study of how the seedlings and saplings of different tree species (Korean pine (Pinus koraiensis), Ezo spruce (Picea jezoensis), and Khingan fir (Abies nephrolepsis)) respond to environmental factors such as canopy structure and light within gaps. Gap size significantly influenced the direct and total light in northern (N, N1) portion of gaps, while for diffuse light, gap size demonstrated a notable effect along the north-south axis, except for the N. Maximum direct light did not occur at the center of gaps, but rather near the center of the gap and tended to the crown edge. In contrast, the maximum diffuse light occurred at the center of gap. When the forest gap area < 20 m(2), the diffuse light dominated the northern part of the gaps. However, when the gap area > 20 m(2), the northern part was dominated by direct light. The variation pattern of total light showed a level of consistency with that of direct light. A redundancy analysis (RDA) showed that the density of shade-tolerant fir seedlings and saplings, as well as spruce seedlings, were primarily positively correlated with the LAI, while the density of spruce saplings was negatively correlated with direct light. The density of heliophilous species Korean pine seedlings and saplings was strongly and weakly positively correlated with the direct light, respectively. The mean ground diameter and height of spruce and fir seedlings and saplings positively correlated with diffuse light, whereas the mean ground diameter and mean height of Korean pine seedlings and saplings strongly positively correlated with the canopy openness and direct light, respectively. These findings suggest that the gap size and location within gaps can compensate each other in silvicultural practices based on the creation of gaps. Knowledge of the ecological characteristics of the target species, enables the selection of appropriate relative positions to achieve more suitable light conditions for their growth.