Plant species composition and soil characteristics around Iranian piospheres

Abstract

Most of Iran’s land area consists of arid and semi-arid rangelands, and because of this grazers depend on watering points. In the vicinity of the watering points, grazers generate areas of altered soils and vegetation known as piospheres. These rangelands are managed by three systems: Nomadic, Transhumant and Sedentary. To study the effect of grazing on piospheres, we sampled plant species composition, vegetation cover, height, soil nitrogen (N), soil organic matter (OM), soil pH and electrical conductivity (EC) over a two year period along 5000m-long transects radiating outward from watering points within desert-steppe (Lajaneh), steppe (Mojen) and mountain-steppe (Abr) ecosystems in Shahrood, Iran. Both univariate and multivariate methods were used to examine the influence of grazing on plant species and soil. One-way analysis of variance (ANOVA) was used to determine if differences existed among different distances. Soil N, OM, pH, EC and height of palatable species were the response variables. After using detrended correspondence analysis (DCA), an indirect gradient analysis technique, we used Canonical Correspondence Analysis (CCA), a direct gradient analysis technique, to examine the influence of grazing on plant species composition. Significance of species-environments correlations were tested by distribution-free Monte Carlo tests (999 permutations). Distance from the watering point, which we assumed to be inversely related to grazing pressures, was significantly related to species composition, cover and height of palatable species. Distance from water was also negatively correlated with N, pH, OM and EC. Livestock influenced species composition through defoliation and trampling. The most evident impact of overgrazing is the reduction of palatable species around watering points. Our results suggest that livestock may affect species composition along watering points by nutrient enrichment through voiding urine and feces. Our result demonstrates marked zonation in species composition along watering point (invaders, increasers and decreasers respectively). We argue here that, in studied areas, range managers should provide new water sources and reduce grazing pressure before the rangeland is damaged to a critical level. We suggest that piospheres deserve more researches and understanding their characteristics help us to manage arid and semi-arid rangelands.