Effects of Topography and Soil Variables on Abundance of Onobrychis chorassanica Bunge. in Kardeh and Kurtian Rangelands, Mashhad, Iran

Environmental factors have major impact on the distribution and yield of plant species. For this purpose, the responses of Khorasanian sainfoin (Onobrychis chorassanica Bunge.) were evaluated with regard to some environmental factors in habitats of Khorasan Razavi province in 2018- 2020. Generalized Additive Model (GAM) was used to investigate the response of this species to soil and topographic factors. The results indicated that O. chorassanica exhibited a substantial response to some environmental factors in its habitat. The response pattern of this species includes the gradient of soil, Total Neutralizing Value (TNV) as well as soil clay percent, followed by the monotonic increase model. Therefore, with the increase in the values of these factors, its vegetation cover percent increased. In contrast, the response of this species along with the gradient of soil sand percent followed the monotonic decrease model and with enhancement of the factor amount, the presence of O. chorassanica decreased. Soil studies have revealed that this species is mainly distributed on loamy to sandy loam soils. The response pattern of O. chorassanica along with the gradient of Organic Carbon content (OC%) and soil litter percent complied with unimodal model and its optimal growth levels for these factors were 0.4% and 30%, respectively. The geographical response of this plant also displayed that the vegetation cover percent of the studied species increased in the western and north aspects and is rarely apparent in the southern and southeastern aspects. Investigation of O. chorassanica response on gradient of topographic and soil factors provided valuable information for determining ecological needs of this species, which can be considered in vegetation management and range improvement operations in similar areas.