Grasslands managed for grazing are the largest land-use category globally, with a significant proportion of these grasslands occurring in semiarid and arid regions. In such dryland systems, the effect of grazing on native plant diversity has been equivocal, some studies suggesting that grazing reduces native plant diversity, others that grazing increases or has little impact on diversity. One impediment toward generalizing grazing effects on diversity in this region is that high levels of interannual variation in precipitation may obfuscate vegetative response patterns. By analyzing a long-term data set collected over a 20-yr period in a semiarid grassland, we explicitly evaluated the role of climate in regulating the effect of cattle grazing on plant communities, finding that climate interacted with grazing intensity to shape grassland communities. Community composition of plots that were intensively grazed varied considerably in response to climatic variation and native species richness was low relative to ungrazed and moderately grazed plots. Following a severe drought in 2002, exotic species richness rapidly increased in the high-intensity grazing plots. While this pattern was mirrored in the other treatments, exotic species richness increased to a greater extent and was slower to return to pre-drought levels in the high-intensity grazing plots. Overall, moderate grazing, even compared to grazing cessation, stabilized grassland communities through time, increased resilience to drought, and maintained the highest levels of native plant diversity and lowest levels of exotic diversity. These findings suggest that grazing, at moderate levels, may support grassland resilience to climate change in semiarid regions. However, grazing that exceeds tolerances, particularly in combination with extreme climatic events, like drought, can alter plant composition over relatively long timescales and possibly increase invasibility by nonnative species.

中文翻译：

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干旱加剧了半干旱草原上高强度牛放牧的负面后果。

经过管理的放牧草地是全球最大的土地利用类别，其中很大一部分草原位于半干旱和干旱地区。在这样的旱地系统中，放牧对本地植物多样性的影响是模棱两可的，一些研究表明，放牧会降低本地植物的多样性，而另一些研究表明，放牧会增加或对多样性几乎没有影响。普遍放牧对这一地区多样性的影响的一个障碍是，降水量的年际高水平变化可能会混淆植物的响应模式。通过分析在半干旱草原20年间收集的长期数据，我们明确评估了气候在调节牛群对植物群落的影响方面的作用，发现气候与放牧强度相互作用形成草地群落。集中放牧的地块的社区组成因气候变化而有很大差异，而相对于未放牧和中度放牧的地块，本地物种的丰富度较低。在2002年严重干旱之后，高强度放牧区的外来物种丰富度迅速增加。虽然在其他处理中也反映了这种模式，但在高强度放牧地带，外来物种的丰富度增加了更多，而恢复到干旱前的水平则较慢。总体而言，适度放牧（甚至与停止放牧相比）可以使草地群落随着时间的推移稳定下来，增强对干旱的抵御能力，并保持最高水平的原生植物多样性和最低的外来多样性。这些发现表明，适度放牧可在半干旱地区支持草地抵御气候变化的能力。但是，放牧超过容许范围，特别是与极端气候事件（例如干旱）相结合，可能会在相对较长的时间范围内改变植物组成，并可能增加外来物种的入侵性。