Department of Ecology & Evolutionary Biology, Yale University, New Haven, Connecticut, 06511, USA.
Yale Institute for Biospheric Studies, Yale University, New Haven, Connecticut, 06511, USA.
Ecol Appl. 2021 Dec;31(8):e02437. doi: 10.1002/eap.2437. Epub 2021 Sep 5.
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.
广泛的木质侵入是热带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丧失。广泛的生态系统水平的工作已经提高了我们对其原因和后果的理解。然而，关于局部管理是否可以超越木质侵入的区域和全球驱动因素，以及侵入如何影响木质群落组合，仍存在争议。在这里，我们根据南非克鲁格国家公园的空间密集地面调查，检查了 20 世纪 80 年代末至 2000 年代末木质植物分布和大小结构的种级变化。这个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。种级出现频率和大小类比例的变化反映了广泛的木质侵入，主要是由 Dichrostachys cinerea 和 Combretum apiculatum 引起的，以及大树木的损失，主要是 Sclerocarya birrea 和 Acacia nigrescens。决定克鲁格木质物种分布的环境变量因物种而异，但在两次采样之间没有发生实质性变化，这表明木质侵入者在其现有范围内变厚。总体而言，随着时间的推移，克鲁格的更多地区发现常见木质物种的数量增加，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。我们的结果表明，木质侵入是克鲁格国家公园景观中广泛但高度可变的趋势，这可能反映了木质群落组合中局部异质性的侵蚀。包括克鲁格在内的许多热带稀树草原管理者旨在管理异质性，以促进生物多样性，而植被结构的同质化则违背了这一具体目标。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，许多常见物种的普遍性也有所增加，这可能限制了在局部控制范围之外的驱动因素下管理木质侵入的潜力。定期实地采样加上有针对性的火灾管理将使更准确地监测侵入加剧的速度成为可能。
