Humans and elephants as treefall drivers in African savannas
Humans and elephants as treefall drivers in African savannas
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人类和大象是非洲大草原上树木倒塌的驱动力
DOI：
10.1111/ecog.02549
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发表时间：
2017
期刊：
影响因子：
5.9
通讯作者：
N. Vaughn
中科院分区：
文献类型：
--
作者：
Penelope J. Mograbi;G. Asner;E. Witkowski;B. Erasmus;K. Wessels;R. Mathieu;N. Vaughn
人类在改变稀树草原的结构和功能方面发挥了重要作用，越来越大的土地利用压力只会增加它们对木本植物覆盖的影响。然而，作为生态组成部分的人类往往被忽视。人类和非洲象Loxodonta africana都会通过移除大树和可能增加灌木覆盖率的活动来改变稀树草原的木质植被。人类和大象与火的相互作用也可能改变植被的结构和组成。在这里，我们利用一个宏大的实验机会--由大象主导的景观、人类利用的公共收割地和一个与人类和大象隔开的自然保护区并列而成--来研究人类和大象对特定高度的树木坠落动力学的影响。我们使用重复的机载高分辨率光探测和测距技术(LiDAR)调查了6812公顷的树冠，追踪了两年来453 685棵树冠的命运。人为因素引起的两年一度的树冠落叶率是背景树冠落叶率1.5%的2-3.5倍，而大象为7.6%的树冠落叶率是对照林地的5倍。树木坠落的模型预测表明，人类或大象的存在是最重要的变量，其次是地质和火灾频率之间的交互作用。树木下落的模式在空间上是不同的，大象驱动的树木下落与地质和地表水有关，而人类模式与人们认为的进入木材采伐区和定居点扩张的便利性有关。我们的结果表明，人类和大象利用了所有高度级别的木本植被，而在高度利用的公共土地上，大树的短缺已经将树木坠落的发生转移到了较短的植被上。大象和人类主导的景观与可能阻碍树苗存活的互动效应有关，再加上景观中树木的丧失，可能会损害林地的可持续性。
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Humans have played a major role in altering savanna structure and function, and growing land-use pressure will only increase their influence on woody cover. Yet humans are often overlooked as ecological components. Both humans and the African elephant, Loxodonta africana, alter woody vegetation in savannas through removal of large trees and activities that may increase shrub cover. Interactive effects of both humans and elephants with fire may also alter vegetation structure and composition. Here we capitalize on a macroscale experimental opportunity - brought about by the juxtaposition of an elephant-mediated landscape, human-utilized communal harvesting lands and a nature reserve fenced off from both humans and elephants - to investigate the influence of humans and elephants on height-specific treefall dynamics. We surveyed 6 812 ha using repeat, airborne high resolution Light Detection and Ranging (LiDAR) to track the fate of 453 685 tree canopies over two years. Human-mediated biennial treefall rates were 2-3.5 fold higher than the background treefall rate of 1.5% treefall ha-1 yr-2, while elephant-mediated treefall rates were 5 times higher at 7.6% treefall ha-1 yr-2 than the control site. Model predictors of treefall revealed that human or elephant presence was the most important variable, followed by the interaction between geology and fire frequency. Treefall patterns were spatially heterogeneous with elephant-driven treefall associated with geology and surface water, while human patterns were related to perceived ease of access to wood harvesting areas and settlement expansion. Our results show humans and elephants utilize all height-classes of woody vegetation, and that large tree shortages in a heavily utilized communal land has transferred treefall occurrence to shorter vegetation. Elephant- and human-dominated landscapes are tied to interactive effects that may hinder tree seedling survival which, combined with tree loss in the landscape, may compromise woodland sustainability.
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