Large tree mortality census KNP (2006-2018)

Fire and elephant herbivory are major drivers of large tree mortality in savanna ecosystems. While the spatial variation of these agents is well-studied, less attention has been paid to how disturbance history influences mortality risk for trees over time. In a long-term cohort study, we examined how the sequence of fire- and elephant-induced damage influences mortality of trees, and determined whether risk of mortality is compounded with time. Data on over 2500 large trees were collected from 22 transects in Kruger National Park, South Africa, in 2006, and trees were re-sampled in 2008, 2011, 2015, and 2018. Over the twelve year period, we recorded a cumulative death toll of 47.6% with an estimated annual mortality rate of 3-5% between 2006 and 2015, and a sharp increase to 8.8% in 2018. The main attributed agent of tree mortality was elephant damage, occurring either once or across multiple census periods. A classification tree (CT) analysis partitioned over different census periods showed that the probability of mortality for the ten most common species depended not only on the type and intensity of fire and elephant-induced damage, but also on the historical sequence of damage by these agents. In fact, elephant damage to the main stem incurred even up to 12 years earlier increases risk of mortality for large trees, especially in combination with fire damage. As expected, vulnerability to damage and risk of mortality varied between species, resulting in the potential for long-term changes in species composition at the landscape scale. Overall, this study highlights how multiple interacting agents cause emergent and lagging patterns of mortality risk for large trees in savanna ecosystems; a result that only becomes apparent through fine-scale long-term tracking of cohorts.

火与大象啃食是热带稀树草原（savanna）生态系统中大型树木死亡的主要驱动因子。尽管学界已对这两类干扰因子的空间变异特征开展了充分研究，但针对干扰历史如何随时间推移影响树木死亡风险的探讨仍较为不足。
本研究依托一项长期队列研究，剖析了火灾与大象引发的损伤时序对树木死亡率的影响，并明确了死亡风险是否会随时间推移而累积加剧。
研究于2006年在南非克鲁格国家公园的22条样带中采集了超过2500棵大型树木的基础数据，并分别于2008年、2011年、2015年及2018年对这些树木进行了重采样监测。
在长达12年的监测周期内，研究记录到整体累计死亡率达47.6%；2006至2015年间的年死亡率估算值为3%~5%，而2018年该数值骤升至8.8%。
导致树木死亡的主要归因因子为大象啃食损伤，该损伤可单次发生，也可在多次普查时段中持续出现。
分类树（classification tree, CT）分析针对不同普查时段进行分组后结果显示：十种最为常见的树种的死亡概率不仅取决于火灾与大象引发损伤的类型和强度，还与这两类干扰因子的损伤发生历史时序密切相关。
事实上，即便早在12年前发生的大象对树干主茎的损伤，仍会提升大型树木的死亡风险；若该损伤与火灾损伤同时发生，则死亡风险将进一步加剧。
正如预期，不同树种对干扰的敏感性及死亡风险存在显著差异，这意味着景观尺度下的物种组成可能发生长期变化。
综上，本研究揭示了多种交互作用的干扰因子如何在稀树草原生态系统中引发大型树木死亡风险的突发及滞后特征；这一结论唯有通过对树木队列的精细化长期追踪方可得以显现。