Data from: 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-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 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.

干旱山地的诸多森林普遍以发育更低的海拔下限林线（lower treeline）为特征。明晰下限林线位置的调控机制，对于预测全球环境变化背景下未来森林分布的迁移演化具有重要意义。当前处于气候极限状态的下限林线，预计对气候变化更为敏感；而受非气候因素约束的下限林线，则直接响应气候变化的可能性较低，但可能对其他全球变化因子更为敏感。本研究依托现有植被分类体系，对美国西部山间（Intermountain West）面积达170万平方千米的研究区域内的下限林线进行了制图。我们针对三种优势森林类型分别建模，解析了下限林线位置的地形气候（topoclimatic）调控因子，以识别受地形气候限制的林线。随后，我们结合土壤属性、近期火灾与土地利用的空间数据，甄别出那些因非气候过程而被约束在其生态生理极限之上的下限林线。研究发现，松桧林地（pinyon-juniper woodlands）的下限林线过渡带主要受地形气候调控，大概率对升温及伴随的干旱胁迫更为敏感，不过这种效应在研究区域内存在空间异质性。与之形成对比的是，研究区域北部的干旱混合针叶林下限林线，极少达到其地形气候模拟的极限值，这表明包括火灾与土地利用在内的非气候过程，在该森林类型中会将下限林线约束在其生态生理极限之上。我们的研究结果显示，美国西部山间区域的多数下限林线目前处于气候限制状态，因此将对持续的气候变化产生响应。全球其他干旱或半干旱山地的下限林线，或许也对气候扰动具有较强敏感性，但林线对气候变化的响应会在局地尺度上受到土壤属性、生物交互作用以及自然或人为干扰的调控。本项针对下限林线的区域研究，为解析下限林线形成的驱动因子提供了一套研究框架，同时能够更精准地预测未来林线动态——这对于预判森林生物群系的全球分布迁移至关重要。