Mid- to long-term influence of gaps on natural regeneration in Pinus tabuliformis plantations on the Loess Plateau

Chinese pine (Pinus tabuliformis) is one of the predominant tree species on the Loess Plateau, which is an area characterized by severe soil erosion and ecological fragility. Chinese pine plantations play crucial ecological roles in soil and water conservation and vegetation restoration on the Loess Plateau. However, most of these plantations currently face low species diversity and unstable stand structures. To promote rapid canopy recruitment in the mid- to long-term after gap formation, Chinese pine regeneration and related habitat factors were investigated in 94 14-year-old expanded gaps (hereafter referred to as gaps) in Chinese pine plantations in Huanglong County. The surveyed gaps were classified into 4 size levels, I (20, 40 m²], II (40, 80 m²], III (80, 200 m²] and IV (> 200 m²), to systematically study the responses of Chinese pine regeneration to gap size and microhabitats. Although gap size had no significant effect on the regeneration density, the height of the saplings exhibited a notable increasing trend with increasing gap size, reaching its maximum at level IV but showing no significant difference from that at level III. These findings suggest that the creation of gaps larger than 80 m² in near-mature stands could maintain effective Chinese pine regeneration in the mid- and long term. Soil available nutrients had a strong effect on the regeneration density, while the key factors influencing sapling density gradually changed from soil P to soil N and K as the gap size increased. In terms of seedling and sapling heights, the key factor was intraspecific competition. The existing number of Chinese pine regeneration in gaps (0.39–0.69 stem·m⁻²) generally meets the requirements for successful natural regeneration; however, their rapid growth may be limited by intraspecific competition during the later stages of development. Gap expansion is recommended to sustain Chinese pine regeneration and preserve other tree species with canopy replacement potential, gradually forming heterogeneous gap patches of various sizes in the stand to transform pure plantations into mixed forests.