Comparison of the response curve of Bromus tomentellus Boiss. and Achillea millefolium L. species to some environmental gradients using the Generalized Additive Model in the rangeland of Galandrood watershed

The plant species response to environmental gradients indicates that the impact of environmental factors on plants. This study was done in the rangeland of Galandrood watershed in Mazandaran province. The objective of this study was investigation on the response of Bromus tomentellus Boiss. and Achillea. millefolium L. species to some environmental gradients using the generalized additive model (GAM). Towards this attempt 153 quadrates of 1m2 along altitude gradient were taken. The sampling method was randomized-systematic. In the area sampled, presence of Br.tomentellus and A. millefolium species, altitude, slope and aspect were recorded. Soil samples were taken from 0-20 cm in each quadrate. In each sample, 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 programe. The results showed that the response curve of Br. tomentellus to the altitude gradient has monotonically increasing and then flat trend but for A.millefolium was monotonically decreasing trend. The highest presence of Br. tomentellus was recorded in 2417 m along altitude gradient but for A.millefolium has been recorded 2125 m. sand content in the soil is a parameter that had significant positive impact on Br. tomentellus species distribution but this variable significant negative impact on A.millefolium species distribution.