Shifting Mosaics in Semi-Steppe Rangelands Driven by Interactive Effect of Human Made Disturbances

Abstract

Semi-steppe rangelands are a complex, highly dynamic and often multi-layered mosaic of grassland, shrubland, and intermediate communities. A few recent studies have explicitly or implicitly developed synthetic hypotheses about how interactive effects of human made disturbances initiate dynamic changes in plant community composition to cause a shifting mosaic of vegetation pattern across the landscape in rangeland ecosystems, yet to be tested in semi-steppe rangelands. The main goal of this study was to examine a conceptual model of plant community dynamic driven by the interactive effect of grazing and fire in semi-steppe rangelands in West-Iran. The study area includes shrubland, grassland and intermediate plant communities. Several patches within the study area were accidentally burned in 2006, 2008 and 2009. Burned patches were located along a gradient of animal grazing (from light to heavy grazing). We compared plant community composition and animal selections on burned and unburned patches (control) of each plant community. The results showed that if grazing intensity was low, a shift from shrublands to grasslands would be the observed pattern of community dynamics; otherwise with higher level of grazing intensity, change in vegetation structure caused by fire in shrublands was rather transient and this plant community returned to the former state of vegetation four years after the fire. We also observed a higher animal selection on recently burned areas compared to previously burned patches, a pattern that was the resulted of a series of positive and negative feedbacks in forage quality created by selective animal foraging behaviour. The results indicate that the effect of fire on plant community dynamics in semi-steppe rangeland is controlled by grazing intensity and the local changes in plant composition within each community. Both determinants cause a cyclical process of vegetation succession. Vegetation patterns represent the various states of recovery in vegetation and introduces a specific landscape composition in which each scrubland, grassland and intermediate vegetation patch can be described as part of a shifting mosaic process at landscape scale