Degradation process and plant diversity of alfalfa grassland in North Loess Plateau of China
黄土高原北部紫花苜蓿草地退化过程与植物多样性研究

Vegetation recovery and reconstruction is the key of ecosystem restoration in the North Loess Plateau. With the Liudaogou catchment of Shenmu County, Shaanxi Province as test area, this paper studied the dynamics of the plant diversity of artificial alfalfa (Medicago sativa) grassland during its degradation process. The results showed that the degradation process of the grassland could be divided into three stages, i.e., artificial alfalfa grassland stage (1-6 yrs), transitional stage from artificial to natural grassland (6-10 yrs), and secondary natural grassland stage dominated by Stipa bungeana ( >10 yrs). In the whole degradation process from 1-to 30-yrs, 32 species belonging to 13 families and 28 genera were found, of which, 90% appeared in the initial 6-yrs. The dynamics of accumulatively appeared family, genus, and species in the vegetation succession process were well described by logarithmic function. During the process of community succession, the species richness (Gleason index and Margalef index),plant diversity (Shannon-Wiener index), and Pielou evenness index were changed with a similar tendency, i.e., increased more rapidly at the first stage, and the climax phase appeared in the second stage. The plant diversity and evenness decreased slightly, and gradually became stable later. Planting alfalfa could markedly accelerate the natural vegetation succession process in the forest-steppe ecotone of Loess Plateau, mainly due to the intensive soil water consumption of artificial grassland, which accelerated the process of soil aridification. To develop artificial grasslands in the Loess Plateau is an optimal joint between accelerating natural vegetation recovery and increasing farmers' income by stockbreeding, and is also a favorable paradigm both for the ecological and for the economic benefits in the eco-environmental construction in West China.