Data from: Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A.
Urza, Alexandra; Weisberg, Peter; Dilts, Thomas (2020), Data from: Evidence of widespread topoclimatic limitation for lower treelines of the Intermountain West, U.S.A., Dryad, Dataset, https://doi.org/10.5061/dryad.g4f4qrfmw
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
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.
A ReadMe file has been uploaded to accompany the dataset.