Life form and preference can drive spatial relationships among plant species in semi-arid rangelands of middle Iran

Abstract

The interaction among species is one of the most important factors influencing the distribution of plants in a given habitat. Vegetation of arid rangelands often appears patchy with many herbaceous species associated with shrub species. It was hypothesised that plant interactions among different life forms together with the effect of grazing preference generate this type of spatial distribution of plant species in the semi-arid rangelands of middle Iran. Spatial patterns and associations were quantitatively analysed using Hopkins’ index and Chi-square tests to establish links between spatial relationships, plant life forms and preference by grazers on the south foothills of the Taleghan ranges, Alborz Province. Data were collected from three geographical sites with different dominant species and clumped patterns of plant spatial distributions for these species were detected. These spatial patterns were probably generated by the interplay of life form-dependent facilitative interactions among plants and the grazing pressure on preferred species rather than due to their intrinsic characters. In the study sites, grass species were significantly more frequently observed in the vicinity of nurse species than in open areas. It was also found that plant-plant interactions may change in response to changes in environmental conditions, such as soil characters and the benefits achieved by the beneficiary species from being associated with nurse species. All preferred perennial species were spatially associated with shrub species (Astragalus gossypinus Fischer and Thymus kotschyanus Boiss & Hohen), while nurse species that just physically protect the beneficiary species from grazing (e.g. A. gossypinus) did not associate with non-preferred species (e.g. Stipa barbata Desf). Instead, these non-preferred species may positively aggregate with nurse plants showing other beneficial characteristics such as nitrogen fixation and improvement of soil conditions.