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

Citation

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

Abstract

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. 

Usage Notes

A ReadMe file has been uploaded to accompany the dataset.