The study of habitats properties and response pattern of (Astragalus retamocarpus Boissier & Hohen.) to environmental factors in rangelands of Mazandaran province.

Background and objective: Today, the use of predictive models of plant species distribution plays an important role in the assessment, restoration, protection and development of rangeland ecosystems. It is one of the important tools to obtain information about the causes of species distribution and habitat suitability for plant species. In this research, using multivariate analysis, the ecological factors affecting in plant composition, species response to changes in environmental factors, and the potential of the target species in the study area were determined. Based on this, with a better understanding of the ecological niche of this plant, the necessary recommendations can be made for the use of the species in rangeland improvement programs in similar habitats. Methodology: Vegetation sampling was done by systematic-random method during 2016-2018. The number of five transects with the same distance to each other was used. Then, on each of them, six plots (2 x 2 meters) were established with the same distances.Vegetation studies was including the percentage of canopy cover, density of species, the percentage of litter, bare soil and stones  in the plots. In each of the plots, the soil sample was collected from the depth of 0 to 30 cm of the soil surface and analyzed in the laboratory for physical and chemical properties. Average annual rainfall and temperature were collected from weather station data. n order to investigate the relationship between the effective environmental variables and the vegetation cover and to choose the appropriate linear and non-linear method, DCA was performed on the vegetation data and the length of the gradient was determined. A generalized additive model was used to predict the response of plant species to changes in environmental factors. Canoco software version 4.5 was used to analyze the data in this section. Results: The height range of this plant habitat is 2400-2700 above sea level. The values ​​of Ec 0.98-1.1 ds/m, pH is 7.5-7.73, lime is 1.6-2.6%, soil organic carbon is 2.01-6.85%, nitrogen  is 0.54-0.28% and the texture of the soil is  loamy sand. Investigating the effect of a set of environmental factors on vegetation changes in the habitat led to the selection of 6 variables out of 22 primary variables. All environmental factors expressed  36.6% of the changes in vegetation and among them, the factors of litter percentage, clay percentage, slope percentage, average annual rainfall, organic carbon percentage and phosphorus content are significant. The response of the species to the variables of sand percentage, silt percentage, electrical conductivity, altitude, average annual rainfall and temperature is monoexponential. The response of the species to the variables of aspect, bare soil, stone and gravel is decreasing.  With the increase of these factors, the abundance and percentage of vegetation cover of the species also decreases. The response of this species to the variables of percentage of organic carbon, percentage of litter, amount of potassium and phosphorus, follows the monotonic increaser  model. Species response to lime percentage, saturated soil moisture and soil apparent specific gravity followed the bi-exponential model. Conclusion: The generalized incremental model provides valuable information to determine the ecological needs of the species, which can be used in vegetation management and rangeland improvement operations in similar areas using the data of this research. The studied species is significant in terms of high forage production and is suggested as one of the suitable species for increasing the vegetation cover of  rangelands.