Restoring temperate secondary forests by promoting sprout regeneration: Effects of gap size and within-gap position on the photosynthesis and growth of stump sprouts with contrasting shade tolerance

To improve the productivity and ecological functions, it is essential to recover secondary forests, the major forest resources in the world, by promoting the regeneration of dominant tree species. Forest gaps are a dominant form of small-scale disturbances in secondary forests, and sprout regeneration commonly occur after the gap formation from logging. Within-gap position and gap size are two key characteristics affecting tree regeneration by changing micro-environments. Promoting the sprout regeneration of dominant tree species under forest gaps with various sizes and within-gap positions is a key measure to recover secondary forests. Twelve artificial gaps were created in March 2015 and the photosynthesis and growth of stump sprouts of three dominant tree species with varying levels of shade tolerance (Quercus mongolica, Acer mono, and Tilia mandshurica) were monitored in 2016. The results showed that within-gap position and gap size had significant effects on the photosynthetic ability of stump sprouts of Q. mongolica, i.e., the moderate light condition at the center parts of large gaps was more beneficial to its photosynthesis with the maximum PNmax of 26.49 μmol m-2 s-1. Gap size significantly affected the biomass of stump sprouts of both Q. mongolica (shade intolerant tree species) and A. mono (intermediate shade tolerant tree species), e.g., the aboveground biomass of these two tree species in large gaps (178.90 g for Q. mongolica and 158.42 g for A. mono, respectively) were significantly higher than those in small gaps (50.52 g for Q. mongolica and 56.95 g for A. mono, respectively) (P < 0.05). It can be concluded that of the three tree species in this study, only Q. mongolica and A. mono are sensitive to the changing environments caused by the gap size and within-gap position at the early stage of gap formation, and their photosynthesis and growth can be promoted in large gaps and at the central part of gaps with moderate light conditions. Consequently, when logging trees to create gaps, forest managers can control the gap size and within-gap position where target trees are located to promote their stump sprouts regeneration. This study may provide a new insight for the directed cultivation and restoration of temperate secondary forests.