DOI: 10.5061/dryad.g4f4qrfmw
title: Data from: Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A.

description: 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.
description: 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 km<sup>2</sup>. 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 km<sup>2</sup> 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 &lt;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. 
description: A ReadMe file has been uploaded to accompany the dataset.