Evaluation on the response of Bromus tomentellus Boiss and Festuca ovina L., to some environmental variables using the Generalized Additive Model(GAM) in the rangeland of Galandrood watershed in Mazandaran province, Iran

Investigation on the shape of the curve for responses of plant species to environmental gradients is one of the basic topics in the science of rangeland ecology. The objective of this study was investigation on the response of Bromus tomentellus and Festuca ovina species to some environmental variables using the generalized additive model (GAM) in the rangeland of Galandrood watershed in Mazandaran province. Towards this attempt, 153 quadrates of 1m2 along altitudinal gradient were taken. The sampling method was randomized-systematic. In the area sampled, presence of B. tomentellus and F.ovina, altitude, slope and aspect were recorded. Soil samples were taken from 0-20 cm in each quadrate. In each quadrate, soil properties including: pH, N, EC, organic carbon, the percentage of sand, silt and clay were measured. In order to study the shape of response curve in relation to the mentioned variables, GAM model was used with binomial distribution function. The data were analyzed by R ver.3.0.2 computer program. Results showed that altitude was the most important variable affecting the presence of B. tomentellus, While variables such as temperature and slope were the most important factors affecting the presence of F.ovina species. Clay and silt content in the soil were parameters that had significant negative impact on B. tomentellus and F.ovina species distribution. So, with an increase in clay and silt content of the soil, the possibility of the B. tomentellus and F.ovina presence were reduced.