Data from: Ecological legacies of civil war: 35-year increase in savanna tree cover following wholesale large-mammal declines

1. Large mammalian herbivores (LMH) exert strong effects on plants in tropical savannas, and many wild LMH populations are declining. However, predicting the impacts of these declines on vegetation structure remains challenging. 2. Experiments suggest that tree cover can increase rapidly following LMH exclusion. Yet it is unclear whether these results scale up to predict ecosystem-level impacts of LMH declines, which often alter fire regimes, trigger compensatory responses of other herbivores, and accompany anthropogenic land-use changes. Moreover, theory predicts that grazers and browsers should have opposing effects on tree cover, further complicating efforts to forecast the outcomes of community-wide declines. 3. We used the near-extirpation of grazing and browsing LMH from Gorongosa National Park during the Mozambican Civil War (1977-1992) as a natural experiment to test whether megafaunal collapse increased tree cover. We classified herbaceous and tree cover in satellite images taken (a) at the onset of war in 1977 and (b) in 2012, two decades after hostilities ceased. 4. Throughout the 3620-km2 park, proportional tree cover increased by 34% (from 0.29 to 0.39)—an addition of 362 km2. Four of the park's five major habitat zones (including miombo woodland, Acacia-Combretum-palm savanna, and floodplain grassland) showed even greater increases in tree cover (51–134%), with an average increase of 94% in ecologically critical Rift Valley habitats. Only in the eastern Cheringoma Plateau, which had historically low wildlife densities, did tree cover decrease (by 5%). 5. The most parsimonious explanation for these results is that reduced browsing pressure enhanced tree growth, survival, and/or recruitment; we found no directional trends in rainfall or fire that could explain increased tree cover. 6. Synthesis: Catastrophic large-herbivore die-offs in Mozambique's flagship national park were followed by 35 years of woodland expansion, most severely in areas where pre-war wildlife biomass was greatest. These findings suggest that browsing release supersedes grazer-grass-fire feedbacks in governing ecosystem-level tree cover, consistent with smaller-scale experimental results, although the potentially complementary effect of CO2 fertilization cannot be definitively ruled out. Future work in Gorongosa will reveal whether recovering LMH populations reverse this trend, or alternatively whether woody encroachment hinders ongoing restoration efforts.

1. 大型哺乳类植食动物（Large mammalian herbivores, LMH）对热带稀树草原的植物具有强烈调控作用，而诸多野生大型植食动物种群正面临衰退。然而，精准预测此类种群衰退对植被结构的影响仍极具挑战。
2. 实验研究表明，在排除大型哺乳类植食动物的干预后，树木盖度可快速上升。但目前尚不清楚该实验结果能否外推至生态系统尺度，以预测大型植食动物衰退的影响——这类衰退往往会改变火制度、引发其他植食动物的补偿性响应，且常伴随人为土地利用变化。此外，理论预测草食性放牧者与木本枝叶啃食者对树木盖度的作用截然相反，这进一步增加了预测群落尺度衰退后果的难度。
3. 本研究借助莫桑比克内战（1977-1992年）期间戈龙戈萨国家公园（Gorongosa National Park）内放牧与啃枝型大型哺乳类植食动物近乎灭绝的自然实验，验证巨型动物种群崩溃是否会提升树木盖度。我们对两张卫星影像中的草本与树木盖度进行了分类：其一拍摄于1977年内战爆发初期，其二拍摄于停火20年后的2012年。
4. 在整个面积达3620平方千米的公园范围内，树木盖度比例上升了34%（从0.29升至0.39），新增树木覆盖面积达362平方千米。公园内五大主要生境带中有四个（包括米翁博林地、金合欢-诃子-棕榈稀树草原与泛滥平原草地）的树木盖度增幅更为显著（51%~134%），其中生态关键区域东非大裂谷（Rift Valley）生境的平均增幅达94%。仅历史上野生动物密度极低的东部切林戈马高原（Cheringoma Plateau）的树木盖度出现下降（降幅5%）。
5. 对该结果最简约合理的解释是：植食动物啃食压力的降低提升了树木的生长速率、存活率和/或更新成功率；我们未发现可解释树木盖度上升的降雨或火活动的方向性变化趋势。
6. 综合讨论：莫桑比克这一旗舰级国家公园内发生的巨型大型植食动物大规模死亡事件后，伴随了35年的林地扩张现象，且该现象在战前野生动物生物量最高的区域最为显著。本研究结果表明，植食动物取食释放效应在调控生态系统尺度树木盖度时，超越了草食动物-草本植物-火的反馈循环调控作用，这与小型尺度实验的结果一致；但二氧化碳施肥效应（CO2 fertilization）的潜在补充作用仍无法被完全排除。未来在戈龙戈萨国家公园的研究将揭示：恢复中的大型植食动物种群是否会逆转这一趋势，或是木本灌丛扩张（woody encroachment）会阻碍当前的生态修复工作。