Understanding the effects of a new grazing policy: The impact of seasonal grazing on shrub demography in the Inner Mongolian steppe

摘要

Grazing by livestock is a common land use in arid and semi-arid areas. Developing sustainable grazing regimes that conserve vegetation and maintain productivity is therefore important in these ecosystems. To solve environmental problems induced by overgrazing in Chinese semi-arid regions, the Chinese government has recently implemented a new policy of seasonal grazing, with no grazing from April to July. While this policy has been implemented in huge areas, its consequences for grazed plant populations have not been assessed so far. We evaluated the demographic consequences of seasonal grazing for Caragana intermedia, a long-lived dominant shrub serving as a main food source for livestock in Inner Mongolia, China. Controlled seasonally grazed and ungrazed populations were monitored during 2007-2009, and their vital rates were compared. We then constructed integral projection models (IPMs) to analyse the effects of seasonal grazing on population dynamics. Seasonal grazing negatively affected two vital rates: seedling survival and seedling recruitment were 25-71% and 69-91% lower in the seasonally grazed treatment than in the ungrazed situation, respectively. Seasonal grazing had a minimal effect on adult survival and growth, but improved juvenile survival by 8-31%. Despite its effects on several vital rates, seasonal grazing did not significantly affect long-term population growth rates (λ), which remained close to unity in both grazed and ungrazed areas based on deterministic and stochastic analyses. An elasticity analysis showed that population growth rate was mainly governed by the high survival of large adults. Results of Life Table Response Experiments (LTREs) revealed that variation in population growth rates across treatments and years was more strongly governed by temporal differences than by grazing. Synthesis and applications. Our study showed that the relatively large changes in vital rates induced by seasonal grazing did not affect population growth rates. Caragana intermedia populations can be sustained under the seasonal grazing regime probably because the grazing intensity is moderate and because this species has a high probability of adult survival under grazing. Plant species with similar life-history traits to C. intermedia are likely to offer good opportunities for sustainable seasonal grazing regimes in arid and semi-arid inland ecosystems. Our study showed that the relatively large changes in vital rates induced by seasonal grazing did not affect population growth rates. Caragana intermedia populations can be sustained under the seasonal grazing regime probably because the grazing intensity is moderate and because this species has a high probability of adult survival under grazing. Plant species with similar life-history traits to C. intermedia are likely to offer good opportunities for sustainable seasonal grazing regimes in arid and semi-arid inland ecosystems.