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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.
 

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The current study was conducted in the rangeland of Glandrood watershed in the province of Mazandaran. The main objective of this study was to investigate the responses of forbs species to some environmental gradients. For these purpose 153 plots of 1m² were established along the altitude gradient and Trasp. The sampling method was randomized-systematic. In the area sampled, frequency of forbs species, altitude and slope were recorded. Soil samples were taken from 0-20 cm in each quadrate. In each sample plot, pH, nitrogen (N), electrical conductivity (EC), organic matter, organic carbon, percentage of sand, silt and clay were measured. In this study, HOF function were used to describe the realized ecological niches of forbs species to the primary axis of classification or floristic gradient, as extracted by Detrended Correspondence Analysis (DCA) and the ecological variables were assessed with standard Spearman rank correlation tests. The data were analyzed by R _ver.3.0.3 software. The results showed that the most species have monotonically increasing or decreasing (19%), symmetric (15%), unimodals (23%) or skewed (35%) and only (4%) one species have flat response curves. For altitude variable of the 26 forbs species included in the general analysis, 4 species showed symmetric (HOF model IV), 9 skewed (model V), 5 species monotone (model II) and 7 species flat (model I) response curves. Among the forbs species, Colchicum kotschy, Medicago rigidula, Tecrium polium, platyloba Echinopora and Campanula stevenii had the narrowest niche ecological.
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