Range Ecology and Range Management, Botanical Institute, University of Cologne, Cologne, Germany.
Glob Chang Biol. 2015 Mar;21(3):1258-70. doi: 10.1111/gcb.12777. Epub 2014 Nov 19.
Projected global change will increase the level of land-use and environmental stressors such as drought and grazing, particularly in drylands. Still, combined effects of drought and grazing on plant production are poorly understood, thus hampering adequate projections and development of mitigation strategies. We used a large, cross-continental database consisting of 174 long-term datasets from >30 dryland regions to quantify ecosystem responses to drought and grazing with the ultimate goal to increase functional understanding in these responses. Two key aspects of ecosystem stability, resistance to and recovery after a drought, were evaluated based on standardized and normalized aboveground net primary production (ANPP) data. Drought intensity was quantified using the standardized precipitation index. We tested effects of drought intensity, grazing regime (grazed, ungrazed), biome (grassland, shrubland, savanna) or dominant life history (annual, perennial) of the herbaceous layer to assess the relative importance of these factors for ecosystem stability, and to identify predictable relationships between drought intensity and ecosystem resistance and recovery. We found that both components of ecosystem stability were better explained by dominant herbaceous life history than by biome. Increasing drought intensity (quasi-) linearly reduced ecosystem resistance. Even though annual and perennial systems showed the same response rate to increasing drought intensity, they differed in their general magnitude of resistance, with annual systems being ca. 27% less resistant. In contrast, systems with an herbaceous layer dominated by annuals had substantially higher postdrought recovery, particularly when grazed. Combined effects of drought and grazing were not merely additive but modulated by dominant life history of the herbaceous layer. To the best of our knowledge, our study established the first predictive, cross-continental model between drought intensity and drought-related relative losses in ANPP, and suggests that systems with an herbaceous layer dominated by annuals are more prone to ecosystem degradation under future global change regimes.
预计全球变化将增加土地利用和环境压力因素（如干旱和放牧）的水平，尤其是在干旱地区。然而，干旱和放牧对植物生产的综合影响仍知之甚少，这阻碍了对其进行充分预测和制定缓解策略。我们使用了一个由来自 30 多个干旱地区的 174 个长期数据集组成的大型跨大陆数据库，量化了干旱和放牧对生态系统的影响，最终目的是提高对这些响应的功能理解。基于标准化和归一化地上净初级生产力（ANPP）数据，评估了生态系统稳定性的两个关键方面：对干旱的抵抗力和干旱后的恢复力。干旱强度使用标准化降水指数来量化。我们测试了干旱强度、放牧制度（放牧、未放牧）、生物群落（草原、灌木、稀树草原）或草本层的主要生活史（一年生、多年生）的影响，以评估这些因素对生态系统稳定性的相对重要性，并确定干旱强度与生态系统抵抗力和恢复力之间的可预测关系。我们发现，生态系统稳定性的两个组成部分都可以通过草本层的主要生活史更好地解释，而不是通过生物群落。随着干旱强度的增加（准）线性降低了生态系统的抵抗力。尽管一年生和多年生系统对增加的干旱强度表现出相同的响应速率，但它们在抵抗力的总体幅度上有所不同，其中一年生系统的抵抗力约低 27%。相比之下，以一年生植物为主的草本层系统具有更高的恢复力，尤其是在放牧的情况下。干旱和放牧的综合影响不仅仅是相加的，而是受草本层主要生活史的调节。据我们所知，我们的研究建立了第一个预测性的、跨大陆的干旱强度与与干旱相关的地上净初级生产力相对损失之间的模型，并表明在未来的全球变化中，以一年生植物为主的草本层系统更容易受到生态系统退化的影响。
