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 in...

木本植物入侵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：从火灾、植食作用与人类直接活动的局地变化，到可能加速木本植物生长的气候或大气二氧化碳（[CO2]）全球变化。目前学界对局地与全球驱动因子的相对影响及其交互作用仍知之甚少，但这些因素将决定当地管理措施抑制木本植物入侵的可行限度。我们于2007年至2014年间，针对南非赫卢赫卢韦-因福洛齐公园（Hluhluwe-iMfolozi Park）近期发生的木本植物入侵展开研究。研究布设的植被样带覆盖了降雨、植食动物利用强度与火灾频率的广泛梯度，且样带分布于多种土壤类型之上。株高2~4米的中等树木密度在七年时间内显著提升46%，而小型与大型树木的密度则保持稳定。中等树木密度增幅最大的区域为火灾频次低、放牧压力上升的沙质土壤地带。树木密度在……