Response of understory plant species richness and tree regeneration to thinning in Pinus tabuliformis plantations in northern China

Background: Chinese pine (Pinus tabuliformis Carr.) is one of the major afforestation species in northern China and plays a key role in restoring forest ecosystems and preserving soil and water. However, most Chinese pine plantations are experiencing ecological problems such as the low diversity of understory plants and difficulty in natural regeneration. Thinning has been widely used to maintain and improve a variety of forest ecosystem services from plantations. To date, however, few studies have been conducted to systematically determine the effects of thinning on understory plant diversity and the regeneration of Chinese pine in plantations. Methods: We conducted a literature search, and selected 22 publications covering a total of 83 treatments related to thinning effects on the species richness of understory plants and 15 publications covering a total of 43 treat-ments related to thinning effects on the regeneration of Chinese pine, in tree plantations of northern China. The data from the literature were synthesized and evaluated with meta-analysis approach to determine the treatment effects. Results: Compared with the control stands, thinning increased the species richness of shrubs and herbs by an average of 25.3% and 26.5%, respectively. While the varying thinning intensities all had significantly positive effects on the species richness of understory plants, only moderate thinning (30%-50%) had a positive effect on the density of regenerating seedlings and saplings of Chinese pine (60.2%). The species richness of understory plants was greatest after 14 years of thinning with an increase of 36.3%, whereas the density of regenerating Chinese pine seedlings and saplings reached a maximum after >= 11 years of thinning with an increase of 76.5%, compared to that of the unthinned stands. Thinning in the half-mature plantations had the greatest effects on the understory shrub richness (44.1%) and the density of regenerating Chinese pine seedlings and saplings (86.5%). Both single and multiple thinning were found to significantly promote the species richness of understory plants and the density of regenerating Chinese pine seedlings and saplings, and the positive effects of thinning were greater in areas with a humidity index (HI) < 30 than in areas with an HI >= 30. In general, age group, planting density and recovery time were prominent factors affecting the species richness of understory plants, whereas the slope, HI and recovery time were the dominant controls of the density of regenerating Chinese pine seedlings and saplings, indicating differential effects of thinning on the species richness of understory plants and the regener-ation capacity of Chinese pine in plantations. Conclusion: Thinning appears to be a feasible management measure to improve the understory plant diversity and regeneration capacity of Chinese pine in plantations. We postulate that moderate thinning in half-mature forest stands with an HI < 30 can help effectively promote the species diversity of understory plants and the natural regeneration of Chinese pine, thereby maintaining a more resilient stand structure and the development of Chinese pine plantations.