Data from: Do changes in grazing pressure and the degree of shrub encroachment alter the effects of individual shrubs on understorey plant communities and soil function?

1. Shrub encroachment has dramatically altered the structure and functioning of many dryland ecosystems worldwide. While positive effects of shrubs on their understorey are commonly found (patch-level effects), these effects can be either positive or negative when scaled up to the community or landscape level (landscape-level effects). These contrasting results are likely to be caused by differences in the degree of shrub encroachment or grazing pressure among studies. No study has addressed the relationship between the patch-level effects of shrubs and the degree of shrub encroachment (i.e., their density or cover) within the landscape, and how this relationship is influenced by grazing. 2. We examined how differences in grazing pressure and the degree of shrub encroachment, as measured by shrub cover, influences patch-level effects of shrubs on multiple soil and plant attributes (plant density, biomass and similarity between shrub-open areas, and soil stability, nutrient cycling, and infiltration) in two semi-arid Australian woodlands. 3. Shrubs had consistently positive effects on all plant and soil variables evaluated and, contrary to expectation, these positive effects did not decline with increasing shrub cover. Rather, these positive patch-level effects became larger or remained constant as shrub cover increased, depending on the studied variable and site. Heavy grazing negatively affected most of the variables studied, and altered the relationship between patch-level effects of shrubs and their landscape-level cover in half of the variables examined. In these cases, heavy grazing dampened the positive relationship between the strength of the positive patch-level effect and shrub cover. Synthesis: Shrubs positively affected plant and soil attributes, and this positive effect can generally be expected, or even increase, for stands up to 50% landscape-level cover. Heavy grazing, however, dampens the positive relationship between shrub effects at the patch level and their cover at the landscape level. Our results will help to reconcile the contrasting results found for the ecological effects of shrub encroachment in the literature and further refine our predictions on the effects of woody encroachment on ecosystem structure and functioning.

1. 灌丛扩张（shrub encroachment）已显著改变了全球众多旱地生态系统的结构与功能。尽管学界普遍观测到灌木对其林下层植被存在正向作用（斑块尺度效应），但当效应尺度扩展至群落或景观层面时（景观尺度效应），这类作用可表现为正向或负向。上述研究结果的矛盾性，大概率源于不同研究中灌丛扩张程度或放牧压力（grazing pressure）的差异。目前尚无研究探讨景观尺度下灌木斑块尺度效应与灌丛扩张程度（即灌木密度或盖度）之间的关联，以及放牧如何调控这一关联。
2. 本研究以澳大利亚两处半干旱林地为研究对象，探究以灌木盖度（shrub cover）衡量的灌丛扩张程度与放牧压力的差异，如何调控灌木对多种植物与土壤属性的斑块尺度效应——这些属性包括植物密度、生物量、灌丛与开阔区域间的相似性，以及土壤稳定性、养分循环与土壤入渗率。
3. 研究结果显示，灌木对所有待测植物与土壤变量均表现出持续的正向作用；且与预期相悖的是，这类正向效应并未随灌木盖度升高而减弱。相反，根据所研究的变量与样地的不同，这类正向斑块尺度效应会随灌木盖度增加而增强，或维持恒定。高强度放牧会对绝大多数待测变量产生负向影响，并在半数检测变量中改变灌木斑块尺度效应与其景观尺度盖度之间的关联。在这类情形中，高强度放牧会削弱正向斑块尺度效应强度与灌木盖度之间的正相关关系。
综合结论：灌木对植物与土壤属性具有正向作用，在景观尺度盖度不超过50%的灌丛群落中，这类正向效应通常可被观测到，甚至会随盖度升高而增强。然而，高强度放牧会削弱灌木斑块尺度效应与其景观尺度盖度之间的正相关关系。本研究结果有助于调和现有文献中关于灌丛扩张生态效应的矛盾结论，并进一步完善我们对木本扩张对生态系统结构与功能影响的预测。