Ecological Factors and Response Pattern of Astragalus retamocarpus Boissier & Hohen. in Mazandaran Province Rangelands

Predictive models of plant species distribution are essential for assessing, restoring, protecting, and developing rangeland ecosystems. Understanding the ecological factors influencing plant composition, species response to environmental changes, and the potential of target species in specific areas is crucial for effective habitat management. This study employed multivariate analysis to identify ecological factors affecting plant composition, analyze species response to environmental changes, and determine the potential of Astragalus retamocarpus in Mazandaran Province's rangelands. The findings will contribute to recommendations for utilizing this species in rangeland improvement programs in similar habitats.

Vegetation sampling was conducted using a systematic-random method from 2016 to 2018. Five transects with equal distances between them were established, and six plots (2 x 2 meters) were placed on each transect. The vegetation study included measurements of canopy cover percentage, species density, litter percentage, bare soil percentage, and stone percentage within the plots. Soil samples were collected from the top 0-30 cm depth and analyzed for physical and chemical properties in the laboratory. Average annual rainfall and temperature data were obtained from weather stations. Detrended Correspondence Analysis (DCA) was performed to investigate the relationship between environmental variables and vegetation cover. A generalized additive model was utilized to predict species response to environmental changes. Canoco software version 4.5 was used for data analysis.

Astragalus retamocarpus was found to thrive at elevations ranging from 1800 to 2800 meters above sea level. The habitat exhibited electrical conductivity values of 0.98-1.1 ds/m, pH levels of 7.5-7.73, lime content of 1.6-2.6%, soil organic carbon ranging from 2.01 to 6.85%, nitrogen content of 0.28-0.54%, and loamy sand soil texture. Among the 22 primary variables studied, six variables were identified as significant factors influencing vegetation changes in the habitat. These factors, including litter percentage, clay percentage, slope percentage, average annual rainfall, organic carbon percentage, and phosphorus content, explained 36.3% of the variation in vegetation. The species exhibited a monoexponential response to variables such as sand percentage, silt percentage, electrical conductivity, altitude, average annual rainfall, and temperature. Conversely, the response to variables such as aspect, bare soil, stone, and gravel indicated a decreasing trend, with increased factors leading to decreased abundance and vegetation cover percentage. The response to variables such as organic carbon percentage, litter percentage, potassium and phosphorus content followed a monotonic increasing model. Lime percentage, saturated soil moisture, and soil apparent specific gravity demonstrated a bi-exponential response.

The generalized additive model employed in this study provides valuable insights into the ecological requirements of Astragalus retamocarpus. The findings can be utilized in vegetation management and rangeland improvement operations in similar areas. This species is significant in terms of high forage production and is recommended for increasing vegetation cover in rangelands.