Grazing intensity and environmental factors effects on species composition and diversity in rangelands of Iran

Abstract

ABSTRACTThe aim of this study was to analyze the effects of grazing, precipitation and temperature factors on plant species dynamics (diversity and composition) in the semi-steppe of Isfahan semi-arid rangelands, Iran. The effects of Sheep grazing were studied in a controlled experiment along grazing gradient with seven intensities (from very heavily grazed to nongrazed) during six consecutive years (2006–2011). The results show that plant species dynamics varied among years, but were only slightly affected by grazing intensity. Since the experimental years were much dryer than the long term average, the abiotic constraints may have overridden any grazing effect. The differences among the years were predominantly determined by the abiotic factors of temperature and precipitation. Most of the variations in species dynamics and coexistence between the C4 and C3 species were explained by seasonal weather conditions, i.e., temperature and precipitation regimes during the early season (March-June). The dominant C3 species, Poa bulbosa, was highly competitive in March-June at low and high temperature and rainfall levels, respectively. In contrast, the most common C4 species Cynodon dactylon benefited from high and low early season temperature and rainfall levels, respectively. However, biomass of P. bulbosa was positively correlated with temperature in March, when effective mean temperature ranges from 0 to 5°C and thus promotes vernalization and vegetative sprouting. Our results suggest that, over a six-year period, it is temporal variability in temperature and precipitation rather than grazing that determines vegetation dynamics and species distributions in grazed semi-steppe ecosystems. Our results also support that the variability in the dominant species biomass, rather than diversity, determine ecosystem functioning. This study provides fundamental knowledge on the complex interaction of climate, vegetation, and grazing.