Institute of Grassland Science, Key Laboratory of Vegetation Ecology of the Ministry of Education, Jilin Songnen Grassland Ecosystem National Observation and Research Station, Northeast Normal University, Changchun, China.
Ecol Appl. 2023 Mar;33(2):e2803. doi: 10.1002/eap.2803. Epub 2023 Jan 11.
Grassland degradation caused by increases in livestock grazing threatens a variety of ecosystem services. Understanding changes in plant community assembly during the process of grassland degradation in the presence of grazing is important to help restore degraded grasslands worldwide but has received little attention thus far. The grassland degradation process is typified by heterogeneous degradation, that is, gradual formation of degraded patches (hereafter "patchy degradation"). Here, we experimentally examined the effects of herbivore grazing and patchy degradation on plant community assembly using nine pairs of non-degraded (intact) and patch-degraded (fragmented) grasslands subject to grazing by different-sized herbivores (i.e., NG, no grazing; SG, sheep grazing; CG, cattle grazing) over 4 years. Using a null-model approach, we estimated the relative magnitude of deterministic processes of community assembly by comparing the observed and expected β-diversity. We found that in the absence of herbivore grazing, deterministic processes played a greater role in community assembly, regardless of whether patchy degradation had occurred. However, the deterministic processes resulted in plant communities being more spatially similar in non-degraded grasslands while being more dissimilar in patchy degraded grasslands. Compared with non-degraded grasslands, species with strong competitive abilities (i.e., Leymus chinensis) were less dominant in patchy degraded grasslands, indicating relaxed competition and a reduced role of species interactions over plant communities. Instead, patchy degradation added the role of environmental variables over plant communities. SG consistently promoted more stochastic plant community assembly in both non-degraded and patch-degraded grasslands, while CG promoted more stochastic plant community assembly only in the non-degraded state, having no effect in the patch-degraded state. Our study offers important insights into changes in plant community assembly during ongoing patch-degradation of grasslands, indicating the role of increased environmental filtering of soil and reduced species interactions in driving plant community dynamics with increasing grassland patchy degradation. We also uncovered an herbivore species-specific effect on plant community assembly during the process of grassland degradation, which will better inform and improve future grassland restoration planning efforts.
草地退化导致放牧家畜数量增加，从而威胁到多种生态系统服务。了解放牧条件下草地退化过程中植物群落组装的变化对于帮助恢复全球退化草地至关重要，但迄今为止，这方面的研究还很少。草地退化过程的特点是异质性退化，即逐渐形成退化斑块（以下简称“斑块状退化”）。在这里，我们使用 9 对非退化（完整）和斑块退化（破碎）草地进行了实验，研究了食草动物放牧和斑块状退化对植物群落组装的影响，这些草地受到不同大小食草动物（即 NG，不放牧；SG，绵羊放牧；CG，牛放牧）的放牧，持续了 4 年。我们使用零模型方法，通过比较观察到的和预期的 β多样性来估计群落组装的确定性过程的相对大小。我们发现，在没有食草动物放牧的情况下，无论是否发生斑块状退化，确定性过程在群落组装中起着更大的作用。然而，确定性过程导致非退化草地中的植物群落在空间上更加相似，而在斑块状退化草地中的植物群落则更加不同。与非退化草地相比，具有较强竞争能力的物种（即赖草）在斑块状退化草地中的优势度较低，表明竞争放松，物种间相互作用在植物群落中的作用降低。相反，斑块状退化增加了环境变量对植物群落的作用。SG 始终促进非退化和斑块退化草地中更加随机的植物群落组装，而 CG 仅在非退化状态下促进更加随机的植物群落组装，在斑块退化状态下没有影响。我们的研究为正在进行的草地斑块状退化过程中植物群落组装的变化提供了重要的见解，表明随着草地斑块状退化的增加，土壤环境过滤的增加和物种间相互作用的减少在驱动植物群落动态方面起着重要作用。我们还揭示了食草动物物种特异性对草地退化过程中植物群落组装的影响，这将更好地为未来的草地恢复规划提供信息并加以改进。
