Worldwide, open grass areas of savannah ecosystems are being transformed into shrubland. This woody plant encroachment is likely a result of factors such as rainfall, fire and secondary dispersal by ungulate herbivory. However, few experiments have been conducted to disentangle and quantify the role of these factors for seed germination in savannahs. We assessed in situ germination success of Vachellia drepanolobium seeds under simulated rainfall variability patterns, fire treatments and dung experiments in Maswa Game Reserve, Tanzania. Fire reduced seed germination by more than 13%, whereas germination in buffalo and elephant dung increased by 1% and 3% respectively. Additionally, intermediate simulated rainfall was more beneficial for seedling emergence success than large, infrequent simulated rainfall amounts, while shoot growth was twice as high under frequent and intermediate simulated rainfall treatments than under large infrequent simulated rainfall. Our results provide insights that bush fires, drought stress, and large rainfall events can suppress V. drepanolobium seedling emergence and growth. Hence, bush encroachment may be linked to management practices such as fire regimes and climatic conditions, i.e., frequent low rainfall conditions. Our results can help predict future patterns of encroachment under varying rainfall and fire events.

中文翻译：

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降雨、火灾和大型哺乳动物诱发的 Vachlia drepanolobium 建立驱动因素：对坦桑尼亚马斯瓦木本植物侵占的影响

在世界范围内，稀树草原生态系统的开阔草地正在转变为灌木丛。这种木本植物的侵占可能是降雨、火灾和有蹄类食草动物的二次传播等因素的结果。然而，很少有实验来解开和量化这些因素对稀树草原种子萌发的作用。我们评估了Vachlia drepanolobium的原位发芽成功率坦桑尼亚马斯瓦禁猎区模拟降雨变化模式、火灾处理和粪便试验下的种子。火使种子发芽率降低了 13% 以上，而水牛和大象粪便中的发芽率分别增加了 1% 和 3%。此外，与大的、不频繁的模拟降雨量相比，中等模拟降雨更有利于幼苗出苗成功，而在频繁和中等模拟降雨处理下的芽生长比在大不频繁模拟降雨下高两倍。我们的研究结果表明，丛林大火、干旱压力和大降雨事件可以抑制V.  drepanolobium幼苗的出苗和生长。因此，灌木侵占可能与火灾状况和气候条件（即频繁的低降雨条件）等管理措施有关。我们的结果可以帮助预测不同降雨和火灾事件下未来的侵占模式。