Savanna grasslands are undergoing rapid land cover transformation as a consequence of woody plant encroachment. It still remains unclear how increasing woody plant density and cover in savannas influences tree-grass-soil interactions. This study determined the effect of increasing tree density on grass species richness, evenness, diversity and assessed how nutrients change along an encroachment gradient of a shallow plinthic savanna soil. Vegetation was quantified and soil samples analysed from eighteen randomly distributed 10 m × 10 m plots along a tree density gradient spanning from open, transitioning into intermediate and intensively encroached savanna grassland in northern South Africa. We found that an increase in tree density along the encroachment gradient decreased grass richness (GR) by 53% in the intensively woody-encroached grassland. Higher tree density and canopy cover led to a 90% dominance of Panicum maximum grass species. Conversely, higher tree density increased concentrations of carbon, nitrogen, phosphorus and exchangeable calcium and magnesium in the topsoil layer. This study highlights that woody plant encroachment decreases grass species richness and composition, but increases nutrients in the uppermost layer of a plinthic savanna soil. Such information is critical to improve understanding of the mechanisms underlying tree-grass interactions in savannas.

中文翻译：

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南非半干旱稀树草原的草种丰富度沿木本植物侵占梯度下降

由于木本植物的侵占，稀树草原正在经历快速的土地覆盖变化。目前尚不清楚稀树草原中木本植物密度和覆盖率的增加如何影响树-草-土壤相互作用。这项研究确定了增加树木密度对草种丰富度、均匀度、多样性的影响，并评估了养分如何沿着浅层稀树草原土壤的侵蚀梯度变化。对植被进行量化，并对 18 个随机分布的 10 m × 10 m 地块的土壤样本进行了分析，该地块沿树木密度梯度跨越了南非北部的开放、过渡到中间和密集侵占的稀树草原草原。我们发现，随着树木密度的增加，树木密度沿着侵占梯度增加，在密集的木本侵占草地中，草的丰富度 (GR) 降低了 53%。较高的树木密度和冠层覆盖导致 Panicum 最大草种占 90% 的优势。相反，较高的树木密度会增加表土层中碳、氮、磷以及可交换钙和镁的浓度。这项研究强调，木本植物的侵占会降低草种的丰富度和组成，但会增加底部稀树草原土壤最上层的养分。这些信息对于提高对稀树草原树-草相互作用机制的理解至关重要。这项研究强调，木本植物的侵占会降低草种的丰富度和组成，但会增加底部稀树草原土壤最上层的养分。这些信息对于提高对稀树草原树-草相互作用机制的理解至关重要。这项研究强调，木本植物的侵占会降低草种的丰富度和组成，但会增加底部稀树草原土壤最上层的养分。这些信息对于提高对稀树草原树-草相互作用机制的理解至关重要。