investigation of the role of temporal distribution of precipitation on forage production value of the rangeland

plants cover composition, condition, trend, production, and grazing capacity of the rangeland change annually. in this research, the role of temporal rainfall distribution on rangeland production was assessed. this research had down in the rangelands of ilam (with semiarid to semi humid climate) and qom and markazi (with arid to semiarid climate) provinces. by attention on plant cover maps in each province 10 main types of plant cover in ilam, 8 types in qom and 10 types in markazi were selected. in these sites, relative factor to plant cover and soil contains: canopy cover, density, production, germination and, soil cover were measured along four 400m transects into 60 plots .the production of the graze able species was measured by clipping and weighting method. the regression techniques and correlation coefficient were used to the relation between production and precipitation. to determination the role of the temporal distribution of rainfall of the rangeland production, the relationship between rainfall and production was calculated in seven time steps contains. the results show that only 4% of the total forage production is explained by annual precipitation but in 96% cases there were related with start of growth season (march) and growth season(early march to late july).this result shows the importance of growth season precipitation on the forage yield in rangelands.