Widespread woody encroachment is a prominent concern for savanna systems as it is often accompanied by losses in productivity and biodiversity. Extensive ecosystem-level work has advanced our understanding of its causes and consequences. However, there is still debate over whether local management can override regional and global drivers of woody encroachment, and it remains largely unknown how encroachment influences woody community assemblages. Here, we examined species-level changes in woody plant distributions and size structure from the late 1980s to the late 2000s based on spatially intensive ground-based surveys across Kruger National Park, South Africa. This study region spans broad gradients in rainfall, soil texture, fire frequency, elephant density, and other topographic variables. Species-level changes in frequency of occurrence and size class proportion reflected widespread woody encroachment primarily by Dichrostachys cinerea and Combretum apiculatum, and a loss of large trees mostly of Sclerocarya birrea and Acacia nigrescens. Environmental variables determining woody species distributions across Kruger varied among species but did not change substantially between two sampling times, indicating that woody encroachers were thickening within their existing ranges. Overall, more areas across Kruger were found to have an increased number of common woody species through time, which indicated an increase in stem density. These areas were generally associated with decreasing fire frequency and rainfall but increasing elephant density. Our results suggest that woody encroachment is a widespread but highly variable trend across landscapes in Kruger National Park and potentially reflects an erosion of local heterogeneity in woody community assemblages. Many savanna managers, including in Kruger, aim to manage for heterogeneity in order to promote biodiversity, where homogenization of vegetation structure counters this specific goal. Increasing fire frequency has some potential as a local intervention. However, many common species increased in commonness even under near-constant disturbance conditions, which likely limits the potential for managing woody encroachment in the face of drivers beyond the scope of local control. Regular field sampling coupled with targeted fire management will enable more accurate monitoring of the rate of encroachment intensification.

中文翻译：

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木质侵占是通过加强，而不是扩大非洲大草原的物种范围而发生的

广泛的木本侵占是稀树草原系统的一个突出问题，因为它通常伴随着生产力和生物多样性的损失。广泛的生态系统层面的工作加深了我们对其原因和后果的理解。然而，关于地方管理是否可以超越木材侵占的区域和全球驱动因素仍然存在争议，并且在很大程度上仍不清楚侵占如何影响木材群落组合。在这里，我们基于对南非克鲁格国家公园的空间密集型地面调查，研究了从 1980 年代后期到 2000 年代后期木本植物分布和大小结构的物种水平变化。该研究区域跨越了降雨、土壤质地、火灾频率、大象密度和其他地形变量的广泛梯度。Dichrostachys cinerea和Combretum apiculatum ，以及主要是Sclerocarya birrea和Acacia nigrescens的大树的损失. 决定克鲁格木本物种分布的环境变量因物种而异，但在两个采样时间之间没有显着变化，这表明木本入侵者在其现有范围内增厚。总体而言，随着时间的推移，发现克鲁格更多地区的常见木本物种数量增加，这表明茎密度增加。这些地区通常与火灾频率和降雨量减少但大象密度增加有关。我们的研究结果表明，木本侵占是克鲁格国家公园景观中普遍但高度可变的趋势，可能反映了木本群落组合中局部异质性的侵蚀。许多稀树草原管理者，包括克鲁格的管理者，旨在管理异质性以促进生物多样性，植被结构的均质化与这一特定目标背道而驰。增加火灾频率具有一定的局部干预潜力。然而，即使在几乎恒定的干扰条件下，许多常见物种的共性也会增加，这可能会限制面对超出当地控制范围的司机管理木质侵占的潜力。定期现场采样与有针对性的火灾管理相结合，将能够更准确地监测侵占加剧的速度。这可能会限制面对超出当地控制范围的司机管理木质侵占的潜力。定期现场采样与有针对性的火灾管理相结合，将能够更准确地监测侵占加剧的速度。这可能会限制面对超出当地控制范围的司机管理木质侵占的潜力。定期现场采样与有针对性的火灾管理相结合，将能够更准确地监测侵占加剧的速度。