Objective Pinus tabuliformis is the main afforestation tree species in northern China. This paper aims to elucidate the internal mechanism of the influence of forest gap formed by tending and thinning on the natural regeneration of Chinese pine plantation and to explore the correlation between environmental factors and the growth of regeneration established.

Method We established four forest gap classes based on the forest-gap breadth to the average height (H) of surrounding trees, which was classified into small (0.75 H), medium (1.00 H), large (1.25 H) gaps and closed canopy reference plots (CK) in a plantation. Three repetitions were set for each gap size class. Each gap was divided into B(gap center area), C(gap edge area) and D(gap surrounding area).We investigated the regeneration performance of Chinese pine and measured the environmental factors (photosynthetically active radiation, air temperature, air humidity, soil temperature and soil humidity) during the growth peak season. We then analyzed the difference of species composition and growth status among the different gap size classes, and their relationship with environmental factors.

Result The number of Chinese pine regeneration seedling accounted for more than 82.6% of that of the total regenerated tree species. No significant difference was found in the proportion of Chinese pine regeneration number to all tree species among different gap size classes (P > 0.05). Gap sizes significantly influenced their mean density,mean height and mean current height (P < 0.05). The average density decreased with the increase of gap size, and the maximum value was 1.72 plant/m²; the average height increased with the increase of the gap size, and the maximum value was 120.41cm; the maximum value of average current height was 14.41 cm, which distributed in the L-II gap. The average density was significantly affected by different sections within the same gap size class (P < 0.05), the maximum density frequently distributed in section C no matter what gap size was. The tree current height was significantly different only in different sections within the L-II (P < 0.05). A significantly positive correlation existed between the photosynthetic active radiation and the height, and current height of regeneration in the L-I and L-II (P < 0.05). A significant negative correlation between regeneration seedling density and soil temperature only occurred in L-I (P < 0.05). air humidity and soil moisture were significantly correlated with some regeneration characteristics only in certain forest gaps.

Conclusion The gap generated from thinning for Chinese pine plantation has obvious promoting effect on natural regeneration, but has little effect on plant species composition; The gap size class mainly affected the performance of Chinese pine regeneration seedlings by affecting the photosynthetically active radiation. Among the five environmental factors, photosynthetically active radiation and soil temperature were the main factors affecting the regeneration seedlings.