DOI: 10.5061/dryad.f1vhhmh25
title: Data from: Historical fire regimes and contemporary fire effects within sagebrush habitats of Gunnison Sage-grouse

description: The historical role of fire in sagebrush (Artemisia tridentata) landscapes
 remains poorly understood yet is important to inform management and
 conservation of obligate species such as the threatened Gunnison
 Sage-grouse (GUSG; Centrocercus minimus). We reconstructed fire histories
 from tree-ring fire-scars at sagebrush-forest ecotones (10 sites, 111
 trees) to better understand the role of fire in sagebrush landscapes of
 the Upper Gunnison Basin (UGB), Colorado, and how fire may have changed
 following European-American settlement. We assessed likely influences of
 historical fire by surveying plant composition and structure at 100
 sagebrush sites with and without recent (2001–2020) fire. 
 Tree-ring fire-scars revealed a history of repeated low-severity fire at
 sagebrush-forest ecotones until 1892, followed by over a century without
 fire. Between 1684 and 1892, the mean fire interval (MFI) among sites
 averaged 41.3 years (ranging from 18.2 to 79.7 years). Fire over this
 period occurred synchronously at two or more sites on average every 23.6
 years, consistent with spread between sites. Most (70%) of the historical
 fires burned in the early growing season when strong winds can spread fire
 through sagebrush. Recent burns, relative to unburned sites exhibited
 greater reductions in sagebrush (Artemisia tridentata; 27% vs. 6%) and
 concomitant increases in herbaceous (40% vs. 55%) cover. These differences
 declined with time since fire but persisted for at least two decades.
 Burns were dominated by a suite of native perennial grasses, forbs, and a
 re-sprouting shrub species. Historically, such openings may have served as
 seasonal GUSG habitat. Burns exhibited slightly increased cover (4% vs.
 1%) of a widely-planted non-native perennial grass, crested wheatgrass
 (Agropyron cristatum).  Our results suggest that parts of the UGB
 sagebrush landscapes were characterized historically by frequent fire and
 dynamic vegetation mosaics that included open, grassy patches. These
 findings are consistent with the use of prescribed fire to restore and
 maintain this ecological process and vegetation heterogeneity. However,
 the contemporary context for fire has changed, and now includes
 substantially reduced (Endangered Species Act) ESA-listed GUSG
 populations, increased risk of non-native plant invasion, and climate
 warming. These circumstances highlight new risks, information needs, and
 opportunities for key knowledge co-production via management-research
 partnerships. 
description: <strong>Sagebrush fire effects: field
 methods</strong> We characterized fire effects
 on sagebrush vegetation in the UGB, from vegetation surveys in 50 recently
 burned and 50 adjacent, unburned sagebrush sites (Figure 1). Fire
 perimeter polygons were obtained from local US Forest Service (USFS) and
 Bureau of Land Management (BLM) offices, and included both prescribed
 burns and wildfires. Burned sites were sampled across 12 different fires
 occurring in eight different years between 2001 and 2021, facilitating a
 comparison of vegetation composition across a chronosequence of
 time-since-fire. We excluded from sampling any burns that were subject to
 post-fire management including reseeding or planting of sagebrush. Sites
 were randomly distributed within fire perimeters and adjacent areas within
 100 m of fire perimeters. For every burned site we sampled, an unburned
 site was sampled directly adjacent to the fire perimeter (Figure 1). All
 sites were within occupied GUSG habitat or directly adjacent to occupied
 habitat and within the sagebrush steppe (Figure 1). We
 used the BLM’s nationally standardized Assessment, Inventory, and
 Monitoring (AIM) protocols (Toevs et al., 2011) to conduct the vegetation
 surveys. These protocols include measures of plant cover and ground cover
 along line-point intercept (LPI) and continuous-line intercept (CLI)
 transects, and a survey of vascular plant species richness. We first
 established the plot center at each site, where spatial coordinates were
 recorded using a handheld GPS unit. We then established three, 25-m
 transects (facing 0°, 120°, and 240°, respectively), beginning 5-m from
 plot center, creating a 30-m radius plot (2827 m<sup>2</sup>
 or 0.7 acres). For line-point intercept transects, a pin flag was dropped
 every 0.5-m along the transect line. All vascular plant species touching
 the pin flag were recorded, and the ground surface cover (bare soil, rock,
 or plant basal hit) was recorded. Maximum herbaceous and woody plant
 heights within a 15-cm radius from the transect tape were taken every
 2.5-m. For continuous-line intercept transects, continuous shrub cover, by
 species, was recorded along the entire transect line, providing another
 estimate of shrub species cover. We began recording shrub cover where a
 threshold 50% cover over a 2-cm area was achieved; we stopped if there was
 a gap at least 5-cm long (Toevs et al., 2011). The data used in all our
 analyses came from the LPI data; CLI data exhibited the same patterns for
 shrub cover as the LPI data, and are not presented. To provide a separate
 measure of richness, all plant species within the 30-m radius plot
 (including those not occurring on a transect) were recorded via a
 subsequent walking census that was continued until no new species had been
 observed for two minutes. <strong>Vegetation
 data analysis</strong> We summed intercepts by
 species from the LPI transect data to calculate the total foliar cover of
 each species at each site. We also calculated total cover for each life
 form (woody, herbaceous, graminoid, and forb) as well as the absence of
 cover (bare ground), and total cover by native vs. non-native species. We
 averaged woody and herbaceous heights across each site. We conducted
 paired t-tests to contrast key vegetation metrics including life form
 (percent woody, herbaceous, graminoid, and forb cover), total foliar
 cover, percent sagebrush cover, total native species cover and percent
 bare ground in burned and unburned sites. We used Wilcoxon signed rank
 tests to contrast key vegetation metrics that were not normally
 distributed including average woody and herbaceous height and total
 non-native species cover. We calculated averages for sagebrush and woody
 cover in unburned sites, at 0, 5, 10, 15, and 20 years after fire, to
 estimate the duration of fire-induced changes. We also constructed linear
 models to assess effects of time-since-fire on the community attributes.
 We only tested for effects on burned plots in these models. We conducted
 Wilcoxon signed rank tests to contrast species-specific differences in
 cover in burned and unburned sites for the 12 herbaceous species with the
 highest total percent cover. We summed intercepts by
 species from the CLI transect data to calculate the total foliar cover of
 each species at each site. We also calculated total woody and sagebrush
 cover. We used paired t-tests to contrast woody cover and sagebrush cover
 between burned and unburned sites. Wilcoxon signed-rank tests were used to
 contrast species-specific differences in cover using the six woody species
 with the highest total percent cover. 
description: Excel and/or RStudio.