Data from: Elephant damage, not fire or rainfall, explains mortality of overstorey trees in Serengeti

Generalizations about the drivers of tree demography in tropical savannahs continue to prove difficult because of the complex and dynamic interactions involved, and because multi-year datasets spanning meaningful gradients in potential drivers are lacking. Overstorey trees play disproportionate roles in the long-term dynamics and functioning of savannah ecosystems. Understanding demographic patterns in these trees is complicated by their resprouting ability after being top-killed and few studies have attempted to separate top-kill from true mortality events. We examined the interactive effects of fire frequency, mean annual precipitation and elephant herbivory on overstorey (>2m) tree mortality between 2009 and 2014 across 32 permanent vegetation plots in the Serengeti Ecosystem, Tanzania. Mean tree mortality over the study period was 0.28 ± 0.02 (0.07 yr−1). Among trees that were top-killed (19.1% of all individuals), 31.2% resprouted. Mortality was driven largely by elephant herbivory, though mortality rates varied considerably across space and tree species. Fire frequency and mean annual rainfall were weak predictors of tree mortality. Over the five year period, chronic elephant herbivory (i.e., repeated through time) was a stronger predictor of tree mortality than maximum elephant herbivory, suggesting that repeated, low-intensity damage from elephants was more important to mortality than acute, but infrequent, damage. Elephants disproportionately damaged certain tree species relative to their availability on the landscape, and the tolerance to damage differed by species as well. Acacia senegal was strongly selected and appeared to have low tolerance to damage when it occurred, resulting in a mortality rate of 0.736 ± 0.052 (0.234 yr−1) over the study period. Topkilling and resprouting rates varied across species, with Acacias generally having low resprout rates. Synthesis. Our study highlights the considerable role that chronic herbivory plays in structuring savannah tree populations, irrespective of prevailing fire and rainfall conditions, and provides important vegetative context to the dramatic recent declines and (in the case of Serengeti) increases in savannah elephant densities in sub-Saharan Africa.热带稀树草原（savannahs）中树木种群动态的驱动因子相关通用规律始终难以被明确界定，这是由于该系统中存在复杂且动态的相互作用，且缺乏覆盖潜在驱动因子有效梯度的多年期观测数据集。林冠层乔木（overstorey trees）在稀树草原生态系统的长期动态与功能维持中发挥着不成比例的重要作用。由于这些林冠层乔木在遭受顶枯后仍具备萌蘖更新能力，且鲜有研究尝试区分顶枯事件与真正的植株死亡事件，解析其种群动态模式变得极为复杂。本研究于2009至2014年间，在坦桑尼亚塞伦盖蒂生态系统（Serengeti Ecosystem）的32个永久性植被样地中，探究了火频、年平均降水量与大象植食压力（elephant herbivory）对高度大于2米的林冠层乔木死亡率的交互影响。研究期内树木的平均死亡率为0.28 ± 0.02（年均死亡率0.07 yr⁻¹）。在发生顶枯的个体（占总个体数的19.1%）中，有31.2%实现了萌蘖更新。尽管死亡率在空间尺度与树种间存在显著差异，但树木死亡主要由大象植食压力驱动。火频与年平均降水量对树木死亡率的预测能力较弱。在五年研究期内，长期持续的大象植食压力（即随时间反复发生的植食作用）对树木死亡率的预测能力强于极端大象植食压力，这表明大象反复造成的低强度损伤，相比偶发的急性损伤，对树木死亡的影响更为显著。大象对部分乔木树种的植食损伤程度与其在样地中的丰度不成比例，且不同树种对损伤的耐受能力也存在差异。阿拉伯胶树（Acacia senegal）被大象强烈偏好取食，且在遭受损伤后表现出较低的耐受能力，其研究期内死亡率达0.736 ± 0.052（年均死亡率0.234 yr⁻¹）。不同树种的顶枯率与萌蘖率存在显著差异，其中金合欢属（Acacia）植物普遍具有较低的萌蘖更新率。研究总结：本研究证实，无论当前火与降水条件如何，长期植食压力在塑造稀树草原乔木种群结构中均发挥着关键作用；同时也为撒哈拉以南非洲近期稀树草原大象种群密度的剧烈变化（塞伦盖蒂地区种群密度反而上升）提供了重要的植被生态学解释。