Spontaneous steppe restoration on abandoned cropland in Kazakhstan: grazing affects successional pathways

Abstract

After the collapse of the Soviet Union in 1991, 12 million ha of cropland were abandoned in the steppe zone of Kazakhstan. At the same time livestock numbers crashed, leaving large areas of steppe without any grazing. We aimed to investigate, to which degree spontaneous succession on former cropland leads to the restoration of native steppe vegetation, and how this process is influenced by changing grazing patterns. We recorded biomass and vegetation characteristics as well as local soil and landscape variables in 151 quadrats of 100 m2, covering 89 plots on former cropland abandoned in the early 1990s and 62 reference plots of near-natural steppe grassland that was never ploughed. About half of the plots of each category were located in a remote region where grazing has been absent for ca. 20 years, whereas the other half was located in a region with moderate livestock grazing. While there were no differences in the diversity, structure and plant life form composition of currently grazed and un-grazed near-natural steppe grasslands, corresponding successional plots on abandoned arable land exhibited significant differences. Grazed plots on former fields showed higher species richness and a higher cover of dwarf shrubs (mostly Artemisia spec.), ruderals and perennial herbs. At the same time, immigration of typical steppe species was much more successful. Contrary, in the absence of any grazing we found species-poor swards dominated by Stipa lessingiana and Leymus ramosus exhibiting an increasing frequency of wildfires due to litter accumulation. After 15–20 years, secondary steppe grasslands still differed substantially from their near-natural references. Our results suggest that grazing is mandatory to fully restore the original near-natural steppe vegetation and the underlying processes of pyric herbivory.