DOI: 10.5061/dryad.crjdfn348
title: Data from: Wildfire catalyzes upward range expansion of trembling aspen in southern Rocky Mountain beetle-killed forests

description: In this study, we assessed whether recent wildfires and spruce beetle
 outbreaks promoted upward range expansion of trembling aspen (Populus
 tremuloides) in the San Juan Mountains of southern Colorado. We assessed
 aerial imagery for presence/absence of aspen in the study area and ground
 truthed a subset of these points to compare the aerial imagery analysis
 with field records of aspen. From these data we determined the upper
 elevational limit of aspen in the study area before fire. We then
 conducted field surveys at and above the upper elevational limit of aspen
 in burned and unburned, but beetle killed areas and collected
 data on stand structure, surface cover, seedling density and microsite
 conditions associated with seedlings.   
description: <em>Aerial Imagery
 Analysis</em> To determine the upper elevational
 limit of aspen in the study area prior to the 2013 wildfires,
 high-resolution aerial imagery (ArcGIS World Imagery Basemap January 2014
 release; 0.3-m resolution) was assessed to identify aspen presence/absence
 from 7,494 circular plots placed along 520 randomly generated upslope
 transects that spanned from 3200 m elevation to the highest mountaintops
 in the study region. Pre-fire 2011 aerial imagery from the ArcGIS World
 Imagery Basemap (January 2014 release; 0.3-m resolution) was the primary
 source for assessment, but was supplemented with 2011 NAIP imagery (1-m
 resolution) if the primary source was obscured by presence of shadows,
 clouds, etc. To establish transects across the study area, we first
 generated a set of random points (n = 1,228) at 3200, 3300 and 3400 meters
 in elevation. Then, from each random point we established a transect that
 followed the terrain upslope, placing a circular plot (25-m radius) at
 horizontal increments of 60 m. We then selected a subset of these
 transects (n = 550) for sampling, as follows. To ensure that we sampled
 areas containing aspen, we evaluated all transects that intersected areas
 classified as aspen in the 2012 LANDFIRE existing vegetation type data (n
 = 181), which uses a combination of field data, satellite imagery,
 biophysical layers and regression trees to produce a predictive model of
 vegetation cover on the landscape. We then added an additional random
 sample of transects (n = 369), for a total of 550 transects assessed.
 Circular plots along each transect were visually examined using the aerial
 imagery to determine if aspen occurred anywhere within the 25-m radius
 circle. If so, this point was counted as an “aspen present” point. In the
 case where circular plots overlapped due to intersecting lines, only the
 first plot viewed was used for analysis. All imagery was collected during
 the summer months, when aspen leaves were present and could be easily
 visually distinguished from much darker conifer needles. Aspen is the only
 tall broadleaf species in this area and therefore we had high confidence
 that it was correctly visually classified. It was distinguished from
 broadleaf shrub species by the presence of shadows (indicating tree
 status) in the aerial imagery as well as landscape context (e.g. alpine
 areas with high willow cover were not classified as aspen).<br> We
 tested the accuracy of our ability to visually determine aspen presence by
 field sampling a subset of these plots (n = 400) for aspen presence (at
 least one aspen tree within the plot). In the summers of 2018 and 2019, a
 subset of plots (n = 400) were navigated to in the field using GPS
 coordinates, to test the accuracy of our aspen presence data derived from
 aerial imagery. Only plots outside of the fire perimeters that had not
 been disturbed (by fire or harvesting) since 2011 were used for field
 validation. Plots that were near roads and most easily accessible were
 selected. At each plot, we established a 25-m radius circular plot and
 recorded the presence or absence of canopy aspen within the
 plot. <i>Seedling Surveys </i>
 To assess whether aspen
 upslope expansion has occurred following fire across our study area, we
 field-sampled a total of 47 burned and 41 unburned sites across eight
 burned and seven unburned elevational transects. Although the unburned
 sites did not experience mortality from the West Fork Fire Complex, all
 were impacted by the spruce beetle outbreak of the 2000’s and therefore
 still experienced severe canopy mortality before our survey. From the
 aerial imagery analysis, we identified locations where aspen existed at or
 near its conservative upper limit (3506 m) prior to the fire. From the
 locations of each of these aspen stands, an elevational transect was
 established running upslope to the top of the mountain or a total increase
 of 240 m in elevation, whichever occurred first, maintaining the same
 aspect. The first site on each elevational transect was established
 immediately upslope and adjacent to the pre-fire aspen stand, though one
 of these first sites was determined to be hazardous and therefore excluded
 from analysis. Subsequent sites were established every 40 m of elevation
 gain on the transects, which translated to 82 – 1,226 m of horizontal
 distance on the ground. Minor adjustments to this protocol were necessary
 on a few sites to avoid hazardous conditions or to keep the transect in
 its intended burn category. Slope, aspect, elevation, latitude, and
 longitude were recorded for each site in the field. Linear distance to the
 nearest live adult aspen stand was calculated in ArcMap using post-fire
 (2016 & 2018) aerial imagery to find the nearest stands.
 At each site, belt transects
 measuring 50-m long by 8-m wide were established horizontally across the
 slope and searched for aspen juveniles (&lt;1.4-m tall). The exception
 to this was at the pre-fire aspen stand sites (the lowest site on each
 elevational transect), where variable-width belt transects measuring 50 m
 in length were established. These belt transects were a minimum of 0.5-m
 wide and were widened by 0.5 m until at least 14 aspen juveniles were
 found, or a maximum width of 8 m was surveyed. This was done to account
 for the large number of aspen suckers at these pre-fire aspen-dominated
 sites. For each
 aspen juvenile found at a site, we recorded whether it was a seedling or
 sucker by gently excavating the roots. Aspen suckers sprout from lateral
 roots that can usually be found within 10 cm of the soil surface and can
 extend up to 30 meters away from the parent stem (Day, 1944; USDA, NRCS,
 2020). Therefore, the presence of a lateral root within 10 cm of the soil
 surface on a juvenile surrounded by other aspen juveniles indicated sucker
 status. Seedlings were characterized by multiple fine roots with no thick
 lateral root and generally occurred in isolation away from clumps of other
 aspen juveniles. In addition, one to five of the aspen seedlings found at
 the sites were excavated across each elevational transect and were
 confirmed to have a branching, fine root system characteristic of aspen
 established from seed. The height and diameter at soil surface were
 measured for each juvenile encountered, and any browse damage was noted.
 Microsite characteristics within 5 cm of the rooting location were
 recorded for each juvenile, including the presence or absence of a nurse
 object (defined here as rocks and logs &gt; 5-cm diameter), the
 substrate type, the presence and depth of litter, and the presence or
 absence of vegetation. Canopy cover above the juvenile was measured with a
 spherical densiometer. Across the 50-m length of the belt
 transect, the line point-intercept method was used to quantify the
 abundance of nurse objects, substrates, litter, understory vegetation, and
 canopy cover at the site. A pin flag was dropped every 0.5 m and all
 vegetation touching the pin flag (or touching the imaginary line ascending
 from the pin flag) was recorded according to functional group (grass,
 forb, shrub, tree) and status (live or dead). If multiple species of the
 same functional group touched the pin, the functional group was only
 recorded once for that point. Substrate type (soil, duff, boulder – rocks
 &gt; 20-cm diameter and not easily lifted, or coarse woody debris
 (CWD) – wood &gt; 20-cm diameter and not easily lifted), and presence
 of rocks (&gt; 5-cm diameter and easily lifted), wood (&gt; 5-mm
 diameter and easily lifted) and litter at each point was also recorded.
 The percent cover of all vegetation, substrate types and nurse objects at
 each site was calculated by dividing the number of points where the
 vegetation/substrate was recorded by the total number of points sampled (n
 = 100) and multiplying this by 100. A point classified as “bare soil” in
 the analysis had to have a substrate of soil and no litter, duff, moss,
 rock, or wood present at the point. Stand structure was additionally
 measured in three 100-m<sup>2</sup> circular plots
 (5.64-m<sup> </sup>radius) centered on the middle of the belt
 transect at 7 m, 25 m, and 43 m. The diameter at breast height (DBH),
 height, and status (live or dead) was recorded for each tree within the
 circular plot.   <em>Data
 Processing</em> All data processing and analysis
 was done in RStudio with the included R script. 
description: In the line-point-intercept data, some of the belt transects are
 missing surface cover codes (for example, each belt transect should have
 100 points total, but some only have 98 or 99). This can be easily
 remedied when calculating percent cover by just dividing by the number of
 points that were sampled and multiplying by 100.  For
 three belt transects (P231, P233, P235), the topcanopy code in the
 line-point-intercept (LPI) data was recorded incorrectly. Therefore, we
 used a linear regression between the % tree cover as calculated from LPI
 and the average canopy density as calculated from spherical densiometers
 in the three circular tree plots across the belt transect. We then used
 the model coefficients to calculate new % tree cover values for the three
 belt transects that were mis-recorded.   In all data
 files, a value of "NA" indicates that data was not collected
 because it was not applicable, and a value of "null" indicates
 that the data should have been collected, but was not (i.e. missing
 data).    A readme file has been created to assist in
 using this data
 (wildfire_catalyzes_aspen_range_expansion_readme.txt)