Degradation characteristics of swamps in Zoige Plateau induced by drainage based on quantitative classification of vegetation

Based on the field survey of swamp ecological characteristics and environmental quality and the ecological investigation of drained swamp transects in Zoige Plateau of Tibet in 2009,twenty typical swamp plots in the Plateau were classified into three types by TWINSPAN,i.e.,primary swamp,long-term drained degraded swamp,and short-term drained degraded swamp,and each type of the degraded swamps was divided into three degradation grades,i.e.,light degradation,moderate degradation,and severe degradation,with the degradation characteristics of vegetation and soil along the swamp degraded gradient studied.The swamp degradation in the Plateau was mainly driven by drainage pattern,drainage intensity,and soil moisture gradient,and the vegetation degradation was more obvious than the soil degradation.In the vegetation degradation,the structural change of hydro-type functional assemblage was most obvious,e.g.,the importance value of helophytes under the stress of long-term drainage and short-term drainage decreased from 0.920 to 0.183 and 0.053,while that of mesophytes increased from 0.029 to 0.613 and 0.686,respectively.The soil response to the swamp degradation was in hysteresis,i.e.,the soil physical and chemical properties presented definite variations but the differences were not significant among the swamps with different grades of degradation.The results of CCA indicated that soil moisture and nitrogen and potassium contents were the most important factors affecting the plant species distribution in drained degraded swamps in Zoige Plateau.