Climate warming is facilitating the expansion of many cold-sensitive woody species in woodland-grassland ecotones worldwide. Recent research has demonstrated that this range expansion can be further enhanced by positive vegetation-microclimate feedbacks whereby woody canopies induce local nocturnal warming, which reduces freeze-induced damage and favors the establishment of woody plants. However, this local positive feedback can be counteracted by biotic drivers such as browsing and the associated consumption of shrub biomass. The joint effects of large-scale climate warming and local-scale microclimate feedbacks on woody vegetation dynamics in these ecotones remain poorly understood. Here, we used a combination of experimental and modelling approaches to investigate the effects of woody cover on microclimate and the consequent implications on ecological stability in North American coastal ecosystems. We found greater browsing pressure and significant warming (~2°C) beneath shrub canopies compared to adjacent grasslands, which reduces shrub seedlings' exposure to cold damage. Cold sensitivity is evidenced by the significant decline in xylem hydraulic conductivity in shrub seedlings when temperatures dropped below -2°C. Despite the negative browsing-vegetation feedback, a small increase in minimum temperature can induce critical transitions from grass to woody plant dominance. Our framework also predicts the threshold temperature of -7°C for mangrove-salt marsh ecotones on the Atlantic coast of Florida. Above this reference temperature a critical transition may occur from salt marsh to mangrove vegetation, in agreement with empirical studies. Thus, the interaction between ongoing global warming trends and microclimate feedbacks may significantly alter woody vegetation dynamics and ecological stability in coastal ecosystems where woody plant expansion is primarily constrained by extreme low temperature events.

中文翻译：

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通过北美沿海生态系统的小气候反馈向木本植物优势的关键转变

气候变暖正在促进世界范围内林地-草地交错带中许多对冷敏感的木本物种的扩张。最近的研究表明，这种范围的扩大可以通过积极的植被 - 小气候反馈进一步增强，即木本冠层诱导局部夜间变暖，从而减少冰冻引起的损害并有利于木本植物的建立。然而，这种局部正反馈可以被生物驱动因素抵消，例如浏览和灌木生物量的相关消耗。大规模气候变暖和局地小气候反馈对这些交错带中木本植被动态的共同影响仍然知之甚少。这里，我们结合使用实验和建模方法来研究木质覆盖对小气候的影响及其对北美沿海生态系统生态稳定性的影响。我们发现与邻近的草原相比，灌木树冠下方有更大的浏览压力和显着的变暖（~2°C），这减少了灌木幼苗暴露于冷害的风险。当温度降至 -2°C 以下时，灌木幼苗的木质部水力传导率显着下降，这证明了冷敏感性。尽管有负面的浏览植被反馈，最低温度的小幅增加可以诱导从草本植物到木本植物优势的关键转变。我们的框架还预测了佛罗里达大西洋沿岸红树林-盐沼交错带的阈值温度为 -7°C。根据经验研究，高于此参考温度可能会发生从盐沼到红树林植被的关键转变。因此，持续的全球变暖趋势和小气候反馈之间的相互作用可能会显着改变沿海生态系统的木本植被动态和生态稳定性，在这些生态系统中，木本植物的扩张主要受到极端低温事件的限制。