Many forests in dry mountain regions are characterized by a lower elevational treeline. Understanding the controls on the position of lower treeline is important for predicting future forest distributional shifts in response to global environmental change. Lower treelines currently at their climate limit are expected to be more sensitive to changing climate, whereas lower treelines constrained by non‐climatic factors are less likely to respond directly to climate change but may be sensitive to other global change agents. In this study, we used existing vegetation classifications to map lower treelines for our 1.7 million km² study region in the U. S. Intermountain West. We modeled topoclimatic drivers of lower treeline position for each of three dominant forest types to identify topoclimatically limited treelines. We then used spatial data of edaphic properties, recent fire, and land use to identify lower treelines potentially constrained above their ecophysiological limits by non‐climatic processes. We found that the lower treeline ecotone of pinyon–juniper woodlands is largely limited by topoclimate and is likely to be sensitive to increasing temperatures and associated droughts, though these effects may be heterogeneously distributed across the landscape. In contrast, dry mixed‐conifer lower treelines in the northern portion of the study area rarely reached their modeled topoclimatic limit, suggesting that non‐climatic processes, including fire and land use, constrain the lower treeline above its ecophysiological limits in this forest type. Our results suggest that much of the lower treeline in the Intermountain West is currently climate limited and will thus be sensitive to ongoing climate changes. Lower treelines in other arid or semi‐arid mountainous regions around the globe may also be strongly sensitive to climate, though treeline response to climate change will be mediated at the local scale by soil properties, biotic interactions, and natural or anthropogenic disturbances. Our regional study of lower treeline provides a framework for identifying the drivers of lower treeline formation and allows for more robust projections of future treeline dynamics, which are needed to anticipate shifting global distributions of the forest biome.

中文翻译：

---

美国西山间下部林线广泛的地形气候限制的证据。

干旱山区的许多森林的特征是海拔高度较低。了解下林线位置的控制对于预测未来森林响应全球环境变化的分布变化非常重要。预计目前处于气候极限的低树线对气候变化更敏感，而受非气候因素约束的低树线则不太可能直接对气候变化做出反应，但可能对其他全球变化因素敏感。在这项研究中，我们使用现有的植被分类来绘制170万公里^2的下部林线美国西部山间研究区。我们为三种主要森林类型中的每一个建模了较低林线位置的地形气候驱动器，以识别地形气候受限的林线。然后，我们使用了具有前卫性质，近期火灾和土地利用的空间数据，以识别可能由于非气候过程而受到其生态生理极限限制的林下线。我们发现，松树－杜松林地的较低树线过渡带在很大程度上受地形气候的影响，并且可能对温度升高和相关的干旱敏感，尽管这些影响可能在整个景观中分布不均。相比之下，研究区北部的干混针叶树下部乔木很少达到其地形气候极限，这表明非气候过程，包括火和土地利用，在这种森林类型中，将下部林木限制在其生态生理极限以上。我们的结果表明，西部山间地区的许多下层树线目前受气候限制，因此将对正在进行的气候变化敏感。尽管在其他地方，树势对气候变化的反应将通过土壤特性，生物相互作用以及自然或人为干扰来介导，但在全球其他干旱或半干旱山区的低树势也可能对气候非常敏感。我们对下林线的区域研究提供了一个框架，用于识别下林线形成的驱动因素，并允许对未来林线动态进行更可靠的预测，这是预测森林生物群落全球分布变化的必要条件。