Response strategies of woody seedlings to shading and watering over time after topsoil translocation in dry-hot karst region of China

Translocating topsoil from an appropriate donor forest is a promising strategy to restore seriously degraded sites. However, some dominant or constructive species did not survive due to plant stress in the seedling stage, which ultimately led to a poor community structure and low similarity between new communities and donor forest. In this study, we replaced the topsoil of karst rocky desertification with forest topsoil and imposed different shading and watering treatments following quadratic saturation D-optimum design. Richness, abundance, height and base diameter of woody species (trees + shrubs) were measured yearly to construct mathematical equation between them and shade degrees (SD) and water quantity (WQ) by using multiple stepwise regression. Results displayed a positive linear relationship between richness of woody species and SD at the second year of topsoil translocation, which changed to a parabolic relationship at the fifth year with a maximum 30.8 species when SD was 37.5 %. The positive linear relationship between density of woody species, shrubs and trees and SD at the second year transformed into a parabolic relationship at the fifth year, and density reached maximum 6.3, 3.7 and 3.8 seedlings/m(2) when SD were 41.7, 38.9 and 45.0%, respectively. In contrast, the density of Paliurus orientalis and Sophora viciifolia turned into a negative linear relationship with SD at the fifth year. Moreover, although the effect of SD and WQ on height of woody species increased over time, shading inhibited base diameter of shrubs and height of Quercus cocciferoides, Sapium sebiferum and Fraxinus malacophylla at the fifth year. We concluded that no-shading and heavy shading reduced species diversity, and light to moderate shading should be recommended in first 2-3 years after topsoil translocation to enhance woody species survival and accelerate similar forest community establishment providing guidance for the management, regeneration and restoration of native forest in semi-arid areas.