Study of the relationship between plant specific leaf area (SLA) and water use efficiency (WUE) under different density conditions is necessary to understand the mechanisms of material circulation and energy flow through the soil,plants,and atmosphere.There were three main goals in this study:(1),to determine the relationship between plant SLA and WUE;(2),to analyze whether the relationship is variant along a gradient;and (3),to explain the causes of the relationship.The study site was located in the national wetland park conservation areas in Zhangye city,Gansu Province,China (100°26'50.72″E,38°58'42.9″N).The study area began at the lakeside and extended to the central section of the lake island,gently transiting through the wetland community,in which the common reed (Phragmites australis) was the single dominant species.The reed community was divided into 3 levels based on density,with a "bundle" forming the sampling unit:Low density (Ⅲ,50-70 bundle/m²),medium density (Ⅱ,130-150 bundle/m²),and high density (I,210-230 bundle/m²).Next,using a zigzag order from bottom to top,six (2 m×2 m) reed samples were selected per density,for an overall total of 18 samples (3×6).Then,community traits (height and density) and soil moisture were determined and 12 individuals of P.australis were taken to the laboratory to measure the leaf thickness.In addition,the leaf net photosynthetic rate (Pn) and transpiration rate (Tr) of the reeds were measured in each plot.Quadrat survey methods were used to determine the relationship between the specific leaf area and water use efficiency of the reeds under three density conditions.The results indicated that,as population density decreased from high,to lower,the wetland community soil moisture content gradually decreased,as well as reed height,leaf area,leaf dry biomass,SLA,and transpiration rate (Tr).However,the net photosynthetic rate (Pn),leaf thickness,and WUE gradually increased.There was a highly significant negative correlation (P < 0.01) between SLA and WUE at high and low densities (I,Ⅲ),whereas it was less significant (P < 0.05) at medium density (Ⅱ).Along the density gradients,the reeds adjusted the shapes of their leaf blades,causing the SLA and WUE to be negatively correlated,which reflected the resource utilization and photosynthetic product accumulation models that the plants adopted to become acclimatized to heterogeneous environmental factors,such as light and soil moisture.