Shrub island effects on a high-altitude forest cutover in the eastern Tibetan Plateau

Context The roles of woody-plant islands are well documented in low-altitude regions, but research related to such shrub effects in high-altitude regions is scant. Aims Four common shrub species (Cerasus trichostoma, Ribes glaciale, Rosa omeiensis and Salix sphaeronymphe) in a high-altitude forest cutover of the eastern Tibetan Plateau, were chosen to evaluate the effects of both species and size of shrub islands on microhabitats, herbaceous communities and woody seedling regeneration. Methods Total 86 shrubs with different sizes were investigated; The shrub size, herb community structure and species composition, litter, soil nutrient and microclimate parameters beneath the shrub canopies were also measured. Results All shrubs significantly ameliorated microclimates, increased content of soil organic matter and total nitrogen, both grass and forb species richness, and litter cover and biomass, and promoted woody seedling recruitment (richness and number), but decreased cover and biomass of the herbaceous community beneath them. These effects were greater for larger shrubs, and also varied among shrub species with different crown architectures. We also found differences in species-dependency of the shrub effect for the responses of the herbaceous and woody seedling species, suggesting that shrubs also indirectly facilitate forbs and seedling regeneration through competition release of grasses. We conclude that shrub-island effects are size-and species-dependent. In order to accelerate natural succession and restoration in alpine cutovers, shrub island preservation and their effective utilization as reforestation microhabitats should be integrated into vegetation management procedures.