Abiotic environmental factors have a major impact on the distribution and performance of plant species. For this purpose, regarding the importance of Vicia variabilis in forage production and the possibility of using it in the improvement and reclamation of oak forest understory, this study aimed to investigate the effect of soil variables and topography on the response of this species using generalized additive models in the west of Iran. The results showed that the response pattern of V. variabilis along the gradient of clay, silt, and soil saturation moisture followed the monotonic increase model. Conversely, the response of this species along the gradient of sand percentage, bare soil percentage, and content of sulfate salts in the soil followed the monotonic decrease model. The species response pattern along the gradients of electrical conductivity, lime percentage, pH of soil, soil sodium ion, phosphorus, altitude, slope, stone and gravel percentage, and canopy cover percentage of the forest overstorey followed a unimodal model, and its optimal growth limit for these factors was 0.85 dsm^–1, 5.10%, 7.26, 0.38 meql^–1, 35 ppm, 1 650 m, 31%, 7%, and 25%, respectively. The results of phenological studies on V. variabilis during 2018 to 2020 showed that late June/early July was the most suitable time for livestock use of this species, and the best time to collect the seeds was mid-July to the early August. Regarding the response of this plant to the gradient of studied environmental factors, it is recommended to pay attention to its habitat characteristics and ecological requirements in improvement programs on oak forests understory vegetation.