Data from: Woody plant biomass and carbon exchange depend on elephant-fire interactions across a productivity gradient in African savanna

Elephants and fire are individually well-known disturbance agents within savanna ecosystems, but their interactive role in governing tree-cover dynamics and savanna–forest biome boundaries remains unresolved. Of central importance are the mechanisms by which elephants vs. fire affect tree biomass and cover, and how – over long time periods – both factors interact with rainfall and soils to govern tree biomass and carbon dynamics. Here, we evaluated the response of woody vegetation to 56 years of fire manipulation in South Africa's Kruger National Park, with three fire regimes (annual, triennial and unburned) replicated across a productivity gradient and subject to two periods of contrasting elephant abundances (generated by the cessation of culling in 1994). Higher fire frequencies had a negative effect on woody biomass in the low-elephant period, but this effect was weak to negligible in the high-elephant period as the difference among fire treatments diminished. Moreover, elephants removed increasing amounts of woody biomass as productivity increased across study sites, but fire did not. We infer that elephant-induced tree mortality could overcome increases in woody-plant productivity, while fire-induced mortality alone could not. Elephants caused woody-plant carbon to shift from a sink to a source; this effect was independent of fire treatment, with highest rates of net carbon removal in the wettest and most productive site. Synthesis. Our results reveal a context-dependent interaction between fire and elephants as disturbance agents in savanna: the influence of fire on woody plants was sensitive to the abundance of elephants and diminished with increased plant productivity. In contrast, elephants were capable of shifting landscapes from relatively dense woodland to open savanna, even in unburned sites, and exerted strong impacts irrespective of site conditions and plant productivity.大象与火各自都是稀树草原生态系统（savanna ecosystems）中广为人知的干扰因子，但二者在调控树木覆盖动态以及稀树草原-森林生物群系边界（savanna–forest biome boundaries）的交互作用仍未明确。核心关键在于，大象与火分别通过何种机制影响树木生物量与覆盖度，以及二者在长期时间尺度上如何与降水、土壤协同作用，调控树木生物量与碳循环动态。 本研究针对南非克鲁格国家公园（Kruger National Park）内一项开展了56年的火烧操控实验（fire manipulation），评估了木本植被（woody vegetation）对该实验的响应。该实验设置了三种火烧制度（fire regimes）：年烧（annual）、三年一烧（triennial）与未火烧（unburned），在生产力梯度（productivity gradient）上进行重复设置，并经历了两段大象种群密度（elephant abundances）差异显著的时期——该时期差异由1994年停止大象猎杀（cessation of culling）计划所导致。 在大象种群密度较低的时期，更高的火烧频率对木本生物量（woody biomass）产生负向影响；但在大象种群密度较高的时期，由于不同火烧处理间的差异逐渐缩小，该影响减弱至可忽略不计。此外，随着研究样地生产力提升，大象移除的木本生物量逐渐增加，而火烧则未表现出这一趋势。据此我们推断，大象诱导的树木死亡率足以抵消木本植物生产力的提升，而仅靠火烧诱导的死亡率则无法达成这一点。 大象使得木本植物的碳汇（carbon sink）转变为碳源（carbon source）；该效应不受火烧处理的影响，且在最湿润、生产力最高的样地中，净碳移除（net carbon removal）速率达到最高。 综合分析。本研究结果揭示了稀树草原中作为干扰因子的火与大象之间存在环境依赖型交互作用：火烧对木本植物的影响对大象种群密度敏感，且随植物生产力提升而减弱。与之相反，即便在未火烧的样地中，大象也能够将景观从相对茂密的林地转变为开阔的稀树草原，且无论样地条件与植物生产力如何，其影响都十分显著。