Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A.

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 Intermountain West, USA. 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-climat...干旱山区的多数森林均以低海拔林线为典型特征。明晰低海拔林线位置的调控机制，对于预测全球环境变化背景下森林分布的未来迁移格局具有重要意义。当前处于气候极限的低海拔林线，预计对气候变化更为敏感；而受非气候因子制约的低海拔林线，则较难直接响应气候变化，但可能对其他全球变化驱动因子更为敏感。本研究依托现有植被分类体系，为美国西部山间地带（Intermountain West）面积达170万平方千米的研究区域绘制了低海拔林线分布图。针对三种优势森林类型分别构建模型，解析低海拔林线位置的地形气候驱动因子，以此识别受地形气候制约的林线。随后，本研究借助土壤属性、近期火灾与土地利用的空间数据，识别出可能因非气候因素超出其生态生理极限而受到制约的低海拔林线。