Species diversity and interspecific association in development sequence of Hippophae rhamnoides plantations in the Loess Hilly Region, China

Based on our field investigation,we analyzed the characteristics of species diversity and interspecific association at different development stages of Hippohae rhamnoides plantations.The results show that the species diversities of the shrub layer,grass layer and the whole community of the H.rhamnoides plantations all fluctuated in an“S”shape pattern.At different development stages,the species richness and diversity were higher in the grass layer than in the shrub layer.The shrub species diversity was lower on bare land,but increased gradually with development of the community.The shrub evenness index was higher in 13-year forest stand,while the grass diversity index was higher in the 3-year and 25-year plantations and lower in the 8-year plantation.The positive and negative absolute values of interspecific association between H.rhamonides and other principal species changed in a parabolic pattern.The significance level and intensity of interspecific association were lower or weaker in the13-year plantation,indicating that the species substitution rate slowed down,competition became less intense and community composition and its structure reached a relatively stable state.Therefore,to improve the ecological environment,priority should be given to the protection of the H.rhamnoides plantations younger than 13 years,while for those of 25 year plantations,moderate thinning should be made to promote their regeneration.