Factors driving native tree species restoration in plantations and tree structure conversion in Chinese temperate forests

Abstract

Forests have been planted over large areas during forest restoration programs in Northeast China. An important challenge is transforming forest species structure by allowing colonization of native species in plantations, especially under the present policy of limited logging intensity and near-natural management practices. However, research on the entire process of native tree species restoration and conversion of tree species structure over time remains limited. Therefore, a multi-year field study was conducted to document native tree species restoration in plantations and different open land-cover types in Chinese temperate forests. We evaluated native tree species restoration in plantations (n = 30) and different types of natural recovery in open areas (n = 50) as controls. The first survey was conducted in 1990, with plots sampled in plantations of age 4–30 years, and repeat surveys were conducted in 1993, 2004, and 2016. We compared the importance of abiotic and biotic factors in the different survey periods using the Random Forest algorithm for restoration outcome (i.e., presence of ‘large trees’ (DBH ≥ 5 cm) of natural tree species in plantations) and the changes in plantation tree species composition during the survey periods. The aim was to test our hypothesis that abiotic and biotic factors, such as topography, habitat, and distance from nearest forest (i.e., seed source), differ in importance under different restoration periods. The results revealed that the basal area of large trees was the most important variable for native tree species restoration in plantations over the entire survey period. Abiotic factors, such as elevation and slope, differed in importance and relevance in the different recovery periods to the restoration outcomes in plantations and open areas. Increase in distance from seed source had a negative effect on natural tree species restoration in plantations and different types of open areas. Although the correlation of distance from seed source to restoration outcomes is consistent during the entire survey period, the importance of distance from seed source varied along a chronosequence. Different forest management measures require implementation. As succession progressed, the dominance of planted tree species in plantations gradually decreased. Compared with passive natural recovery, active planting of seedlings in open land not only promotes the restoration of coniferous species that no longer exist in an area owing to excessive logging, but also promotes the restoration of other native tree species.