The present study investigates the ecological requirements of Astragalus curvirostris Boiss., with emphasis on determining the ecological factors that affect the distribution of plant species, and the species’ response to changes in ecological factors using a Generalized Additive Model (GAM) in the Iranian Province of Zanjan from 2017 to 2019. Randomized-systematic sampling was used to collect vegetation data. Data analysis was performed using SPSS17 and CANOC4.5 software. The results showed that the growth and development of A. curvirostris change according to environmental factors linked to the composition of the soil and the variety of the other species present. This model is indicative of a competitive limitation along the environmental gradient. By understanding all environmental parameters, the necessary steps could be taken towards planning proper management programs, including rangeland grazing management and determining the proper moment for seed collection, which will result in the conservation, improvement, and restoration of rangelands.

Abstract. Ferula ovina is a forage species, and of the species that is very important in soil conservation and water protection in the rangelands. It has become an endangered species in Iran due to regular cutting and overgrazing. The focus of this study is to investigate the effect of ecological factors on distribution of F. ovina in the study area. Samples were collected from three habitats (sites) where F. ovina was present along with three sites which did not have it. Three 150 m transects were established based on randomsystematically method on each site. On each transect, 10 square plots (2×2 m) were set up, and soil surface features were measured. Climatologic factors were assessed and physiographic factors were extracted for each site. Soil samples were collected from a 0-30 cm depth for each transect, and the values of soil were determined through laboratory analysis. According to the results, a significant difference was found for the parameters of elevation, slope, aspect, temperature, EC, pH, OC, phosphorus, clay, silt and sand (P≤0.01) between habitats where F. ovina were present and absent. Results of discriminant analysis suggested that the amount of sand and gravel on soil surface, phosphorous, clay, and EC were effective factors in the distribution of target species.