Eco-Geological Mechanism of Patch Degradation in Ejina, the Lower Heihe Kiver Basin

摘要

To characterize the effect of plant underground habitat on distribution and dynamics of vegetation patches in desert area, and to find the drivers of patch degradation, we analyzed relationship between plant community structure and its underground habitat inside and outside of vegetation patches in Ejina. Data about plant community structure and its underground habitat were collected from 64 sample sites in riverside, Gobi, and lake-basins, while landscape data were collected from the infrared color photo in 1996 from TM satellite and remote sensing image in 1999 and 2000 from America Landsat ETM. At regional scale, we found that vegetation was arranged, for horizontal heterogeneity of its underground habitat, in a two-phase mosaic composed of patches with high plant cover interspersed in a low-cover matrix. At small scale of vegetation patches, by providing different micro-environments for the root masses of different plant life-forms, vertical heterogeneity of plant underground habitat played a leading role in the establishment of plant community structure. The root mass zone of one-year gramineous plant started from ground surface and reached a depth at 0.4 m, while the one of perennial plant ranged from 0.4 m to 1.0 m in depth. Our results also demonstrated that it was true but rough of attributing patch degradation to the loss of soil resources, and the basic mechanism of patch degradation was the redistribution of water, nutrients and salt in vertical section of plant underground habitat, which resulted in the regressive succession of plant community in patches. For heterogeneity of anti-jamming ability in patches, which came from the heterogeneity of vegetation and its underground habitat, fragmentation of the old patches was the major pattern in the degradation. Wind was presumed as the major driver of patch degradation in Ejina, while the falling of water level caused by exploitation of water resources was presumed as inducement.