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. 大型哺乳动物植食动物（Large mammalian herbivores, LMH）对热带稀树草原中的植物具有强烈的调控作用，且诸多野生LMH种群正处于衰退之中。然而，预测这些种群衰退对植被结构的影响仍颇具挑战。
2. 相关实验表明，在排除LMH后，树木覆盖率可快速提升。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LMH衰退的生态影响——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，理论预测放牧者与啃食者对树木覆盖率的作用截然相反，这进一步加大了预测群落尺度种群衰退后果的难度。
3. 本研究以莫桑比克内战（1977-1992年）期间戈龙戈萨国家公园内放牧与啃食型LMH几近灭绝的事件作为自然实验，验证大型动物种群崩溃是否会提升树木覆盖率。我们对两张卫星影像中的草本植被盖度与树木盖度进行了分类：一张拍摄于1977年内战爆发初期，另一张拍摄于敌对行动结束二十年后的2012年。
4. 在整个面积达3620平方千米的公园范围内，比例性树木覆盖率提升了34%（从0.29升至0.39），新增树木覆盖面积达362平方千米。公园五个主要生境带中有四个（包括米翁博林地、金合欢-风车子-棕榈稀树草原以及泛滥平原草原）的树木覆盖率增幅更为显著（51%~134%），其中生态关键的东非大裂谷生境的平均增幅达94%。仅东部的切林戈马高原——历史上野生动物密度极低的区域——的树木覆盖率下降了5%。
5. 对上述结果最简约的解释为：啃食压力降低促进了树木的生长、存活及/或更新补充；我们未发现降雨量或火动态存在可解释树木覆盖率上升的方向性趋势。
6. 综合分析：莫桑比克这座旗舰级国家公园内发生的灾难性大型植食动物种群崩溃后，随之而来的是35年的林地扩张，且在战前野生动物生物量最高的区域扩张最为严重。本研究结果表明，啃食释放效应在调控生态系统尺度的树木覆盖率方面超越了放牧者-草本植物-火的反馈循环，这与小规模实验结果相符，但二氧化碳肥化的潜在协同效应仍无法完全排除。未来在戈龙戈萨国家公园开展的研究将揭示：恢复中的LMH种群能否逆转这一趋势，抑或是木本植物扩张会阻碍当前正在进行的生态修复工作。