Herbivory has potential to modify vegetation responses to climatic changes. However, climate and herbivory also affect each other, and rarely work in isolation from other ecological factors, such as plant-plant competition. Thus, it is challenging to predict the extent to which herbivory can counteract, amplify, or interact with climate impacts on ecosystems. Here, we investigate how moose modify climatic responses of boreal trees by using experimental exclosures on two continents and modeling complex causal pathways including several climatic factors, multiple tree species, competition, tree height, time, food availability, and herbivore presence, density, and browsing intensity. We show that moose can counteract, that is, "cool down" positive temperature responses of trees, but that this effect varies between species depending on moose foraging preferences. Growth of preferred deciduous trees was strongly affected by moose, whereas growth of less preferred conifers was mostly driven by climate and tree height. In addition, moose changed temperature responses of rowan in Norway and balsam fir in Canada, by making fir more responsive to temperature but decreasing the strength of the temperature response of rowan. Snow protected trees from browsing, and therefore moose "cooling power" might increase should a warming climate result in decreased snow cover. Furthermore, we found evidence of indirect effects of moose via plant-plant competition: By constraining growth of competing trees, moose can contribute positively to the growth of other trees. Our study shows that in boreal forests, herbivory cooling power is highly context dependent, and in order to understand its potential to prevent changes induced by warming climate, species differences, snow, competition, and climate effects on browsing need to be considered.

中文翻译：

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像驼鹿一样酷：浏览如何抵消整个北方森林生态系统的气候变暖效应？

食草有可能改变植被对气候变化的反应。然而，气候和食草动物也相互影响，很少与其他生态因素（例如植物与植物的竞争）隔离开来。因此，预测食草动物可以在多大程度上抵消、放大或与气候对生态系统的影响相互作用是具有挑战性的。在这里，我们通过在两大洲使用实验性围栏并模拟复杂的因果途径，包括几种气候因素、多种树种、竞争、树高、时间、食物供应以及食草动物的存在、密度和浏览强度。我们表明驼鹿可以抵消，即“冷却”树木的积极温度反应，但这种影响因驼鹿觅食偏好而异。首选落叶树的生长受到驼鹿的强烈影响，而不太喜欢的针叶树的生长主要受气候和树高的影响。此外，驼鹿改变了挪威的罗文和加拿大的香脂冷杉的温度响应，使冷杉对温度更敏感，但降低了罗文的温度响应强度。雪保护树木不被浏览，因此如果气候变暖导致积雪减少，驼鹿的“冷却能力”可能会增加。此外，我们发现了驼鹿通过植物-植物竞争产生间接影响的证据：通过限制竞争树木的生长，驼鹿可以对其他树木的生长做出积极贡献。我们的研究表明，在北方森林中，