Competition and facilitation vary inversely along environmental stress gradients. Here we ask how environmental stress along a slope gradient influences plant-plant interaction in a semi-arid savanna. We attempted to link spatial patterns and ecological interactions, so we analysed the spatial distribution of three Vachellia (Acacia) tree species and assessed the consequences of their interaction on plant structural variables (height, basal area, biomass and canopy cover) at Matopos Research Station, Zimbabwe. Spatial patterns among and between the tree species were assessed using both univariate and bivariate analysis. To understand the extent of tree species isolation, we applied the species mingling index. Canopy cover varied significantly between species at both ends of the stress gradient, while basal area did not differ among the three species at any slope position. The woodland showed differential recruitment levels for the three species across the catena, which may indicate future changes in composition. The mingling index showed that there was species isolation in the bottom and middle catena. Pair-wise interactions of plants reflected two common spatial patterns, clustering, interpreted as suggesting facilitation, and random, interpreted as neutral. In some cases these patterns changed with spatial scale, indicating that the stress gradient hypothesis (SGH) may be influenced by the spatial scale at which species are interacting. Our univariate analyses corroborate the evidence that plants in semi-arid environments have a clustered distribution and that catenas can produce differential recruitment levels of the different species.

中文翻译：

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半干旱环境中稀树草原沿局部应力梯度的结构和空间相互作用

竞争和促进随着环境压力梯度成反比变化。在这里，我们询问沿斜坡梯度的环境压力如何影响半干旱稀树草原中的植物-植物相互作用。我们试图将空间格局和生态相互作用联系起来，因此我们在 Matopos 研究站分析了三种 Vachella (Acacia) 树种的空间分布，并评估了它们相互作用对植物结构变量（高度、基面积、生物量和冠层盖度）的影响，津巴布韦。使用单变量和双变量分析评估树种之间和树种之间的空间模式。为了了解树种隔离的程度，我们应用了物种混合指数。在应力梯度两端的物种之间冠层盖度变化显着，而基面积在任何坡位的三个物种之间都没有差异。林地在整个链中显示出三个物种的不同补充水平，这可能表明未来的组成变化。混杂指数表明下链和中链存在物种隔离。植物的成对相互作用反映了两种常见的空间模式，聚类，解释为暗示促进，随机，解释为中性。在某些情况下，这些模式随空间尺度变化，表明应力梯度假设 (SGH) 可能受到物种相互作用的空间尺度的影响。