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）已极大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下层植被的正向作用（斑块尺度（patch-level）效应），但当研究尺度拓展至群落或景观尺度（landscape-level）层面时，这类效应可能表现为正向或负向。这类相悖的研究结果，大概率源于不同研究中灌丛扩张程度或放牧压力的差异。目前尚无研究探讨景观尺度下灌木的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，以及放牧如何对这一关联产生调控作用。 2. 本研究以澳大利亚两处半干旱林地为研究对象，以灌木盖度作为灌丛扩张程度的衡量指标，探究放牧压力与灌丛扩张程度的差异如何影响灌木对多种土壤与植物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群落相似性，以及土壤稳定性、养分循环与土壤入渗。 3. 研究结果显示，灌木对所有评估的植物与土壤变量均表现出持续的正向效应；与预期相悖的是，这类正向效应并未随灌木盖度的增加而减弱，反而会随盖度提升而增强或保持恒定，具体取决于所研究的变量与样地。重度放牧会对多数研究变量产生负面影响，并使半数观测变量中灌木的斑块尺度效应与其景观尺度盖度之间的关联发生改变。在这类情形中，重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向关联。 综合分析结果：灌木对植物与土壤属性具有正向作用，这一正向效应在景观尺度盖度不超过50%的灌丛群落中普遍存在，甚至可能随盖度提升而增强。然而，重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向关联。本研究结果有助于调和学界关于灌丛扩张生态效应的相悖研究结论，并进一步完善我们对木本扩张如何影响生态系统结构与功能的预测。