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）是全球热带稀树草原与草原管理者面临的普遍难题。该现象的潜在驱动因素涵盖多个维度：从局域的火、植食作用与人类直接活动的变化，到可能加速木本植物生长的全球气候或大气[CO2]变化。目前学界对局域与全球驱动因素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局域范围内限制木本植物入侵的成效边界。 本研究针对2007至2014年间南非赫卢赫卢韦-因弗洛齐公园（Hluhluwe-iMfolozi Park）近期发生的木本植物入侵展开调查。研究设置的植被样带覆盖了多样的土壤类型，并沿降雨、植食动物利用强度与火频的广泛梯度分布。 7年间，中等高度乔木（株高2至4米）的密度显著提升了46%，而小型与大型乔木的密度则保持稳定。中等乔木密度的增幅在沙土地区最为显著，这类区域的火灾发生频率较低且放牧压力有所上升。 即便在近期火频与历史火况相近的区域，乔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局域干扰历史的变化来解释，暗示存在局域管控范围之外的驱动因素发挥作用。 综合与应用：火可在一定程度上缓冲热带稀树草原的木本植物大范围入侵，但仅当管理者能够提升火频时，才能阻止入侵进一步扩散。放牧会降低火频与火强度，这可能与木本植物入侵的防控工作愈发冲突，为稀树草原管理者带来了两难抉择：是维持短期放牧种群的生产力，还是长期防控木本植物入侵。