Post-Soviet shifts in grazing and fire regimes changed the functional plant community composition on the Eurasian steppe

Abstract

Globally, grasslands are shaped by grazing and fire, and grassland plants are adapted to these disturbances. However, temperate grasslands have been hotspots of land-use change, and how such changes affect interrelations between herbivory, fire and vegetation are poorly understood. Such land-use changes are widespread on the Eurasian steppe, where the dissolution of the Soviet Union in 1991 triggered the abandonment of cropland and pasture on globally relevant scales. Thus, to determine how relationships between plant functional composition, grazing and fire patterns changed after the Soviet Union dissolved, we studied a 358,000 km2 region in the dry steppe of Kazakhstan, combining a large field dataset on plant functional traits with multi-scale satellite data. We found that increases in burned area corresponded to decreases in livestock grazing across large areas. Furthermore, fires occurred more often with high cover of grasses with high leaf dry matter content and thus higher flammability, whereas higher grazing pressure favoured grazing-tolerant woody forbs and ruderal plants with high specific leaf area. The current situation of low grazing pressure represents a historically exceptional, potentially non-analogue state. We suggest that the dissolution of the Soviet Union caused the disturbance regime to shift from grazer to fire control. As grazing and fire each result in different plant functional compositions, we propose that this led to widespread increases in grasses and associated changes in steppe plant community structure. These changes have potentially occurred across an area of more than 2 million km2, representing much of the world’s largest temperate grassland area, with globally relevant, yet poorly understood implications for biodiversity and ecosystem functions such as carbon cycling. Additionally, future steppe management must also consider positive implications of abandonment (‘rewilding’) because reverting the regime shift in disturbance and associated changes in vegetation would require grazing animals to be reintroduced across vast areas.