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.
Methods
1. ltl_forest 

Description: line shapefile of the lower treeline between the Continental Divide and the Pacific Crest attributed with the predominant forest type from the USGS National GAP Landcover ECOLSYS_LU field

Methods: We mapped lower treelines across the entire study area using National Land Cover data from the Gap Analysis Program (US Geological Survey 2011), a 30m-resolution classification of major vegetation types from Landsat imagery. Land cover data were reclassified into a binary forest/non-forest raster. Pixels classified as “forest” included all ‘Warm or Cool Temperate Forests and Woodlands’ in the Formation class from the Gap Analysis Program National Land Cover data (US Geological Survey 2011). This dataset conforms with the National Vegetation Classification Standard, which uses both ‘forest’ and ‘woodland’ to indicate the dominance of the tree growth form, including various combinations of needle-leaved conifer, broad-leaved deciduous, and broad-leaved evergreen tree species of varying height and canopy spacing (Federal Geographic Data Committee 2008). Pixels classified as “non-forest” represented all other land cover types, including vegetation dominated by shrubs and herbaceous species, flooded or swamp forests, and areas used for agriculture or human development. Discrete patches of forest or non-forest smaller than 1,000 pixels were merged with their surrounding cover type, resulting in a minimum cartographic unit of 0.9 km2. The binary forest/non-forest raster was converted to polygons, and the edges between forest and non-forest polygons were converted to a polyline representing all forest edges. We used the contrast between the elevations for forest and non-forest patches bordering the forest edge to differentiate between upper and lower treelines for each polyline segment. Using Digital Elevation Models (US Geological Survey 2009), mean elevations of forest and non-forest pixels were calculated within a 4 km2 neighborhood surrounding each polyline vertex. For each polyline segment, the mean non-forest elevation was subtracted from the mean forest elevation. Lower treelines were identified as segments with a positive elevation contrast (where adjacent forested areas were at a higher elevation than adjacent non-forested areas). Lower treeline segments occurring within 100m of water bodies and segments that were outliers in elevation (mean elevation greater than 2500m a.s.l.) were removed. Finally, we visually inspected the resulting map with high-resolution aerial imagery, excluding segments that were not representative of lower treeline (e.g. sections around interior fires, harvested patches, or meadows), removing <1% of the treelines resulting from the automated process. All spatial processing required for mapping treeline was done in ArcGIS (ArcGIS Version 10.5; Computer Software, ESRI Redlands, CA, USA). Each lower treeline segment was attributed with the adjacent forest type from the USGS National GAP Landcover ECOLSYS_LU

2. wUS_studyarea

Description: polygon shapefile of the study area used in the paper "Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A."

Methods: The study area included the Intermountain West of the United States, defined here as the area between the Pacific Crest and the Continental Divide. Watershed boundaries were used to delineate the Pacific Crest and the Continental Divide, which represent the western and eastern boundaries. The northern and southern boundaries follow the borders of the United States of America.

干旱山地林区的诸多森林以存在海拔下林线（lower treeline）为典型特征。明晰下林线位置的调控机制，对于预测全球环境变化下森林分布的未来迁移格局至关重要。当前处于气候极限的下林线，对气候变化的响应敏感性更强；而受非气候因素约束的下林线，虽难以直接响应气候变化，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，对美国西部山间带（Intermountain West）面积达170万平方千米的研究区域开展下林线制图。研究针对三种优势森林类型分别构建下林线位置的地形气候驱动因子模型，以识别受地形气候限制的林线。随后，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过程超出自身生理生态极限而受约束的下林线。研究发现，美国松-桧木林地（pinyon-juniper woodlands）的下林线交错带整体受地形气候调控，对升温及伴随的干旱胁迫具有较高敏感性，不过该效应在研究区域内的空间分布存在异质性。与之形成对比的是，研究区域北部的干旱混交针叶林下林线，极少达到模型模拟的地形气候极限，这表明该森林类型的下林线因火灾、土地利用等非气候过程，被约束在其生理生态极限之上。本研究结果表明，美国西部山间带的多数下林线当前仍受气候条件限制，因此会对持续发生的气候变化产生响应。全球其他干旱或半干旱山地林区的下林线，同样可能对气候波动具有较强敏感性；不过林线对气候变化的响应，会在局地尺度上受到土壤属性、生物间相互作用以及自然或人为干扰的调控。本项区域下林线研究为识别下林线形成的驱动因子提供了分析框架，同时可为未来林线动态的更可靠预测提供支撑——这对预判森林生物群区的全球分布迁移至关重要。

研究方法
1. 下林线矢量图层（ltl_forest）
描述：分隔大陆分水岭（Continental Divide）与太平洋山脊步道（Pacific Crest）之间区域的下林线线状shapefile，属性字段包含美国地质调查局（US Geological Survey, USGS）国家间隙分析项目（Gap Analysis Program, GAP）土地覆盖数据中的ECOLSYS_LU字段，即优势森林类型。
研究方法：依托美国地质调查局2011年发布的间隙分析项目（GAP）国家土地覆盖数据开展全研究区下林线制图——该数据集基于陆地卫星（Landsat）影像生成，分辨率为30米，对主要植被类型进行了分类。研究人员将土地覆盖数据重分类为二元的“森林/非森林”栅格图层。被归类为“森林”的像元，包含间隙分析项目国家土地覆盖数据（美国地质调查局2011）中“形成类”下的所有“暖温带或寒温带森林与林地”类型。该数据集符合《国家植被分类标准》（联邦地理数据委员会，Federal Geographic Data Committee 2008），该标准以“森林（forest）”与“林地（woodland）”共同表征乔木生长型占优的植被，涵盖针阔针叶树、阔叶落叶树、阔叶常绿树等不同高度和冠幅间距的各类组合。被归类为“非森林”的像元则涵盖其余所有土地覆盖类型，包括以灌木和草本植物占优的植被、淹水森林或沼泽林，以及农业或城镇开发用地。将面积小于1000个像元的零散森林或非森林斑块与周边覆盖类型合并，最终得到最小制图单元为0.9平方千米的图层。将二元森林/非森林栅格转换为面要素，再将森林与非森林面要素的边界转换为线状要素，以此表征所有森林边界。研究人员依托森林边界两侧的森林与非森林斑块的高程差异，对每条线状要素段区分上林线与下林线。借助美国地质调查局2009年发布的数字高程模型（Digital Elevation Models, DEM），在每个线状要素顶点周边4平方千米的邻域范围内，计算森林与非森林像元的平均高程。针对每条线状要素段，用森林像元平均高程减去非森林像元平均高程。将高程差值为正的线状要素段识别为下林线（即相邻森林区域的海拔高于相邻非森林区域）。移除距离水体100米范围内的下林线线段，以及高程异常的线段（平均海拔高于2500米，a.s.l.）。最后，研究人员借助高分辨率航空影像对生成的下林线图进行目视校验，剔除不符合下林线特征的线段（例如内部火灾、采伐斑块或草甸周边的线段），最终移除的线段占自动化流程生成林线的比例不足1%。所有林线制图所需的空间处理均在ArcGIS（ArcGIS 10.5版本；环境系统研究所，美国加利福尼亚州雷德兰兹）中完成。每条下林线线段均关联了美国地质调查局国家间隙分析项目土地覆盖数据ECOLSYS_LU字段中的相邻森林类型。

2. 研究区域矢量图层（wUS_studyarea）
描述：本论文《美国西部山间带下林线普遍存在地形气候限制的证据》（"Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A."）中使用的研究区域面状shapefile。
研究方法：本研究区域涵盖美国西部山间带，本次研究中将其定义为太平洋山脊步道与大陆分水岭之间的区域。以流域边界划定作为东西边界的太平洋山脊步道与大陆分水岭，北界与南界则遵循美国本土的国境线。