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. 

Sagebrush fire effects: field methods

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 m² 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.

Vegetation data analysis

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. 

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