Data from: Fire prevents woody encroachment only at higher-than-historical frequencies in a South African savanna

Woody encroachment is a pervasive challenge facing savanna and grassland managers worldwide. Proposed drivers of the phenomenon range from local changes in fire, herbivory, and direct human impacts, to global changes in climate or atmospheric [CO2] that may be accelerating woody growth. The relative influences of local vs. global drivers and their interactions are largely unknown, but will determine the extent to which management can limit woody encroachment locally.
We examined recent woody encroachment in Hluhluwe-iMfolozi Park in South Africa from 2007–2014. Vegetation transects were distributed across broad gradients in rainfall, herbivore use intensity and fire frequency, on a variety of soils.
Density of medium trees (2–4 m tall) increased dramatically (by 46%) in seven years, while densities of small and large trees remained constant. Increases in medium tree density were largest on sandy soils, where fires were infrequent, and where grazing pressure increased.
Tree density increased even where recent fire frequency was similar to historical fire regimes. These potentially widespread increases, unexplained by changes in local disturbance history, suggest the possible influence of drivers outside the scope of local control.
Synthesis and applications. Fire can provide a limited buffer against generalized woody encroachment in savannas, but may only prevent further encroachment where managers can increase fire frequency. Grazing, which can limit fire frequency and intensity, may come increasingly into conflict with efforts to control woody encroachment, presenting a stark choice for savanna managers between maintenance of short-term grazer population productivity and longer-term prevention of woody encroachment.

木本植物入侵（woody encroachment）是全球范围内稀树草原与草原管理者面临的普遍挑战。目前学界针对该现象提出的驱动因子涵盖多个维度：从本地的火干扰、植食作用与直接人类活动影响，到可能加速木本植物生长的全球气候变化或大气二氧化碳浓度（atmospheric [CO2]）变化。目前学界对本地与全球驱动因子的相对影响及其交互作用仍知之甚少，但这些因素将决定管理者在当地限制木本植物入侵的可行程度。
本研究于2007至2014年间，针对南非赫卢赫卢韦-因弗洛齐公园（Hluhluwe-iMfolozi Park）近期发生的木本植物入侵展开调查。研究在多种土壤类型上布设了植被样带（vegetation transects），覆盖降雨、草食动物利用强度与火干扰频率的广泛梯度范围。
7年间，中等高度树木（株高2~4米）的密度大幅提升46%，而小型与大型树木的密度则保持稳定。中等树木密度的增幅在沙质土壤、火干扰频率较低且放牧压力升高的区域最为显著。
即便在近期火干扰频率与历史火制度（fire regimes）相近的区域，树木密度仍出现了上升。这类可能广泛发生的密度增长无法通过本地干扰历史的变化来解释，这表明存在超出本地管控范围的驱动因子的潜在影响。
综合与应用。火干扰可对稀树草原的普遍性木本植物入侵提供有限的缓冲，但仅在管理者能够提升火干扰频率的情况下，才能阻止入侵进一步扩散。放牧行为会限制火干扰的频率与强度，这可能会与木本植物入侵防控工作产生愈发尖锐的冲突，为稀树草原管理者带来了一项艰难抉择：是维持短期草食动物种群的生产力，还是长期防控木本植物入侵。