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Establishment dates of live western juniper trees for plots within each plant association in which such trees occurred: pine–fescue (a–c), sage–fescue (d, e), sage–bluebunch (f ), juniper–needlegrass (g, h) associations. The date of the last fire is indicated by an arrow for each plot for which it is known. Trees were cored or sectioned 30 cm above ground level. Dead western juniper trees (not shown) occurred only in the juniper–needlegrass association.
Source publication
Coarse-scale estimates of fire intervals across the mountain big sagebrush (Artemisia tridentata spp. vaseyana (Rydb.) Beetle) alliance range from decades to centuries. However, soil depth and texture can affect the abundance and continuity of fine fuels and vary at fine spatial scales, suggesting fire regimes may vary at similar scales. We explore...
Contexts in source publication
Context 1
... association, indicating that it likely had a persistent grassy understorey in the past, consistent with its deep, loamy soils and our model of the rate of post-fire succes- sion. This association lacked live or dead western juniper trees >140 years old, indicating that such trees likely did not occupy this plant association before 1860 (Table 1, Fig. 4a-c). Rather, the overstorey likely contained only scattered ponderosa pine. ...
Context 2
... lack of live or dead western juniper trees with basal flut- ing in this association suggests that historical fire intervals were short enough to limit the development of large western juniper trees or woodlands (Fig. 4d, e). Tree age structure and the pres- ence of mollic soil horizons suggest that the plant composition was historically dominated by grasses or codominated by grasses and shrubs. Consistent with its deep, loamy soils, the modern abundance and continuity of fine fuels we measured in this association were similar to those of the pine-fescue ...
Context 3
... plots lacked them, indicating that grasses did not persist as a dominant component of this plant association in the past. Rather, fire intervals were possibly much longer than 25 years resulting in persistent shrub cover, and low grass cover in the past. Although there were no live old western juniper trees in this plant association (Table 1, Fig. 4f ), scattered large western juniper snags with well-developed basal fluting (18 ha −1 , likely killed by the 1941 fire) indicate that some fire intervals were long enough in the past to allow the establishment of large individual trees (>80 years between fires) but not long enough for closed woodland to ...
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Citations
... The prescribed fire area assigned to big sagebrush TRGs without PJ expansion 6.3%) was less than that with PJ expansion (8.8%) and the majority of the area with PJ expansion was characterized by Phase I expansion. The big sagebrush TRGs with higher resilience and resistance that were assigned prescribed fire have generally higher productivity, fuel, and burn probabilities (Dillon et al. 2023), and burned more frequently historically, regardless of the occurrence of PJ expansion (Miller and Rose 1999;Miller and Heyerdahl 2008;Miller et al. 2013). Fire return intervals in these big sagebrush TRGs have been lengthened due to a combination of fire suppression and removal of fine fuels by domestic livestock grazing (Burkhardt and Tisdale 1976;Miller et al. 2013Miller et al. , 2019. ...
Background
Sagebrush shrublands in the Great Basin, USA, are experiencing widespread increases in wildfire size and area burned resulting in new policies and funding to implement fuel treatments. However, we lack the spatial data needed to optimize the types and locations of fuel treatments across large landscapes and mitigate fire risk. To address this, we developed treatment response groups (TRGs)—sagebrush and pinyon-juniper vegetation associations that differ in resilience to fire and resistance to annual grass invasion (R&R) and thus responses to fuel treatments.
Results
We developed spatial layers of the dominant sagebrush associations by overlaying LANDFIRE Existing Vegetation Type, Biophysical Setting, and Mapping Zone, extracting vegetation plot data from the LANDFIRE 2016 LF Reference Database for each combination, and identifying associated sagebrush, grass, shrub, and tree species. We derived spatial layers of pinyon-juniper (PJ) cover and expansion phase within the sagebrush associations from the Rangeland Analysis Platform and identified persistent PJ woodlands from the LANDFIRE Biophysical Setting. TRGs were created by overlaying dominant sagebrush associations, with and without PJ expansion, and new indicators of resilience and resistance. We assigned appropriate woody fuel treatments to the TRGs based on prior research on treatment responses. The potential area to receive woody fuel treatments was constrained to 52,940 km ² (18.4%) of the dominant sagebrush associations (272,501 km ² ) largely because of extensive areas of low R&R (68.9%), which respond poorly and were not assigned treatments. Prescribed fire was assigned to big sagebrush associations with moderate or higher resilience and moderately low or higher resistance (14.2%) due to higher productivity, fuels, and recovery potential. Mechanical treatments were assigned to big sagebrush associations with moderately low resilience and to low, black, and mixed low sagebrush associations with moderately low or higher R&R (4.2%) due to lower productivity, fuels, and recovery potential. Persistent PJ woodlands represent high value resources and were not assigned treatments (9%).
Conclusions
Mapped TRGs can help identify the dominant sagebrush associations and determine appropriate fuel treatments at intermediate scales and provide the basis for quantitative wildfire risk assessments and outcome-based scenario planning to prioritize fuel treatment investments at large landscape scales.
... Note: Values were calculated for each regional extent from the model input data layers (Appendix S1: Table S1). communities in our study area (Mata-Gonz alez et al., 2018;Miller & Heyerdahl, 2018). ...
Abstract Environmental relationships can differ across the geographic range of species, especially for widespread generalists. Because habitat specialists are more vulnerable to environmental changes, incorrect assumptions about consistent habitat associations could hinder strategic conservation efforts. We used species distribution models (SDMs) to evaluate intraspecific variation in habitat associations for a habitat specialist of conservation concern, the pygmy rabbit (Brachylagus idahoensis), which is endemic to the sagebrush biome of the western United States. Our goal was to model habitat associations for pygmy rabbits across a portion of their range to evaluate regional variation and contrast predictions with results from a model developed at the rangewide extent. We created inductive SDMs using maximum entropy methods within five ecological regions that encompassed about 20% of the species rangewide distribution and spanned diverse environmental gradients. We included a suite of environmental predictor variables representing topography, vegetation, climate, and soil characteristics. Results of the regional models identified substantial variation in habitat associations across the five regions, with each retaining a unique set of environmental predictors. Bioclimatic variables were the most influential environmental parameters in all five regions, but the specific variables differed. The models developed at regional extents predicted smaller areas of habitat (an average of 15% less for suitable habitat and 80% less for primary habitat) than predictions generated from a model developed at the rangewide extent. Because bioclimatic variables were effective in discriminating areas used by pygmy rabbits, they also provided an opportunity to assess potential changes in habitat distribution by incorporating future climate projections. Distributions modeled under two mid‐century emission scenarios projected substantial reductions in suitable habitat for pygmy rabbits across most regions and pronounced variation among regions in the magnitude and direction of the climate effects. Collectively, results of this work underscore the need to incorporate regional variation in habitat associations into planning for current and future conservation and management strategies.
... probabilities) and burned more frequently historicallyRose 1999, Miller andHeyerdahl 2008). Prescribed re treatments in moderate or higher resilience areas usually result in increases in perennial grasses and forbs, moderately high recruitment of sagebrush, and little to no increase in invasive annual grasses in big sagebrush associations without PJ expansion(Ellsworth and Kaufman 2017, Chambers et al. 2017) and with PJ expansion (Roundy et al 2018, Freund et al. 2021). ...
Background
Sagebrush shrublands in the Great Basin, US, are experiencing widespread increases in wildfire size and area burned resulting in new policies and funding to implement fuel treatments. However, we lack the spatial data needed to optimize the types and locations of fuel treatments across large landscapes and mitigate fire risk. To address this, we developed Treatment Response Groups (TRGs) – sagebrush and pinyon-juniper vegetation associations that differ in resilience to fire and resistance to annual grass invasion (R&R) and thus responses to fuel treatments.
Results
We developed spatial layers of the dominant sagebrush associations by overlaying LANDFIRE Existing Vegetation Type, Biophysical Setting, and Mapping Zone, extracting vegetation plot data from the LANDFIRE 2016 LF Reference Database for each combination, and identifying associated sagebrush, grass, shrub, and tree species. We derived spatial layers of pinyon-juniper (PJ) cover and expansion phase within the sagebrush associations from the Rangeland Analysis Platform and identified persistent PJ woodlands from the LANDFIRE Biophysical Setting. TRGs were created by overlaying dominant sagebrush associations, with and without PJ expansion, and new indicators of resilience and resistance. We assigned appropriate fuel treatments to the TRGs based on prior research on treatment responses. The extent of potential area to receive fuel treatments was constrained to 52,940 km2 (18.4%) of the dominant sagebrush associations (272,501 km²) largely because of extensive areas of low R&R (68.9%), which is expected to respond poorly to treatment. Prescribed fire was assigned to big sagebrush associations with moderate or higher resilience and moderately low or higher resistance (14.2%) due to higher productivity, fuels, and recovery potential. Mechanical treatments were assigned to big sagebrush associations with moderately low resilience and to low, black, and mixed low sagebrush associations with moderately low or higher R&R (4.2%) due to lower productivity, fuels, and recovery potential. Persistent PJ woodlands represent high value resources and were not assigned treatments (9%).
Conclusions
Mapped TRGs can help identify the dominant sagebrush associations and determine appropriate fuel treatments at project area scales and provide the basis for quantitative wildfire risk assessments and outcome-based scenario planning to prioritize fuel treatment investments at landscape scales.
... Miller and Tausch [1] estimated fire frequency in mesic mountain big sagebrush communities at 11 to 25 years based on analysis of adjacent fire scarred trees. Fire return intervals in mountain big sagebrush were suggested to be greater than 80 years in the mountain big sagebrush-bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve)-Thurber needlegrass (Acnatherum thurberianum [Piper] Barkworth) plant association with lower fuel loads and associated with large western juniper snags [11]. Burkhardt and Tisdale [12] suggested that mountain big sagebrush currently encroached by western juniper must historically have had a fire return interval < 50 years because western juniper trees less than 50 years of age are readily killed by fire, which would inhibit woodland expansion. ...
Western juniper was often historically restricted to fire refugia such as rocky outcrops but has since Euro-American settlement expanded into areas previously dominated by sagebrush steppe. Wildfires in developed woodlands have been rare. In 2007, the Tongue-Crutcher Wildland Fire burned 18,890 ha in southwestern Idaho along a woodland development gradient, providing unique research opportunities. To assess fire effects on vascular plants, field data were collected in 2012/2013 and 2019/2020. Species richness was uniform along the sere, while species diversity declined in late woodland stages attributed to juniper dominance. The greatest changes in species composition following fire occurred in later woodland development phases. Herbaceous vegetation increased following fire, but sagebrush cover was still lower in burned plots 12–13 years post-fire. Many stands dominated by juniper pre-fire became dominated by snowbrush ceanothus post-fire. Juniper seedlings were observed post-fire, indicating that juniper will reoccupy the area. Our research demonstrates resilience to fire and resistance to annual grasses particularly in early successional stages, which provides opportunities for fire use as a management tool on cool and moist ecological sites. Loss of old-growth juniper to wildfire underlines the importance of maintaining and provisioning for future development of some old growth on the landscape given century-long recovery times.
... Millions of hectares in the western United States are now dominated by exotic, annual-herbaceous plants ( Bradley et al. 2018 ) that provide poor or inadequate habitat for native plant, insect, and wildlife species ( Knick et al. 2003 ;Wisdom et al. 2005 ). Historically, much of the sagebrush biome experienced fire relatively infrequently, with fire-free intervals typically lasting from a few to several decades in more mesic, productive sagebrush environments ( Miller and Heyerdahl 2008 ) to hundreds of years in more fuel-limited, warmer-drier sagebrush ecosystems ( Baker 2013 ). Most sagebrush species are not fire adapted, as they are easily top-killed by fire, lack the ability to resprout after disturbance, have limited seed-dispersal distance, and have relatively short-term seed viability ( Young and Evans 1989 ). ...
... Millions of hectares in the western United States are now dominated by exotic, annual-herbaceous plants ( Bradley et al. 2018 ) that provide poor or inadequate habitat for native plant, insect, and wildlife species ( Knick et al. 2003 ;Wisdom et al. 2005 ). Historically, much of the sagebrush biome experienced fire relatively infrequently, with fire-free intervals typically lasting from a few to several decades in more mesic, productive sagebrush environments ( Miller and Heyerdahl 2008 ) to hundreds of years in more fuel-limited, warmer-drier sagebrush ecosystems ( Baker 2013 ). Most sagebrush species are not fire adapted, as they are easily top-killed by fire, lack the ability to resprout after disturbance, have limited seed-dispersal distance, and have relatively short-term seed viability ( Young and Evans 1989 ). ...
Sagebrush ecosystems in the United States have been declining since EuroAmerican settlement, largely due to agricultural and urban development, invasive species, and altered fire regimes, resulting in loss of biodiversity and wildlife habitat. To combat continued conversion to undesirable ecological states and loss of habitat to invasive species fueled by frequent fire, a variety of fuel treatments, including networks of fuel breaks, are being implemented or proposed in sagebrush ecosystems, particularly in and around the Great Basin. In this forum paper we briefly review current knowledge of common fuel treatment approaches, their intended benefits, potential risks, and limitations. We additionally discuss challenges for fuel treatment strategies in the context of changes in climate, invasive species, wildlife habitat, and human population, and we explore how advances in geospatial technologies, monitoring, and fire behavior modeling, as well as accounting for social context, can improve the efficacy of fuels management in sagebrush ecosystems. Finally, given continued potential for ecosystem transformation, we describe approaches to future fuels management by considering the applicability of the Resist-Accept-Direct (RAD) framework. The intent of the paper is to provide scientists and land managers with key information and a forward-thinking framework for fuels science and adaptive management that can respond to both expected and unexpected changes in sagebrush rangelands.
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... In the interior western United States, native piñon and juniper woodlands (Pinus monophylla; Juniperus occidentalis, and osteosperma) have expanded into sagebrush steppe communities (Artemisia), leading to suppression of understory vegetation (Abdallah et al., 2020;Miller & Tausch, 2001;Ray et al., 2019;, altered hydrology (Ochoa et al., 2018;Pierson et al., 2010), altered fire regimes (Balch et al., 2012;Miller & Heyerdahl, 2008;Whisenant, 1990), and shifted patterns of biodiversity (Knick et al., 2014;McIver & Macke, 2014;Miller et al., 2011). In response to piñon and juniper expansion, land managers have undertaken large-scale tree removal in an effort to achieve a more desirable balance among vegetation functional groups (Miller et al., 2005(Miller et al., , 2019. ...
Over the past century, piñon and juniper trees have encroached into sagebrush steppe lands of the interior United States, and managers have for many years removed trees to stimulate the favored understory. While consistent understory response to tree removal in these semiarid lands suggests that trees outcompete other plants for water, no studies have linked increased soil water to understory response after tree removal. We tested the hypothesis that tree removal at six sagebrush steppe sites increased soil water, leading to increased understory plant cover. Using a structural equation model, we found that before tree removal, trees suppressed shrubs (standardized coefficient [SC] = −0.87), perennial deep‐rooted (SC = −0.50) and shallow‐rooted bunchgrasses (SC = −0.36), but had no influence on cheatgrass. The model explained between 2% (cheatgrass) and 40% (shrubs) of pretreatment cover variation. Measurement of the same plots six years post‐treatment showed that most cover variation was due directly to plant growth, with standardized coefficients between 0.51 (perennial shallow‐rooted grasses) and 0.72 (cheatgrass). Competition between cheatgrass and perennial deep‐rooted grasses was evident, with perennials having twice the influence on cheatgrass than vice‐versa (SC = −0.24 vs. −0.11). Spring soil water (wet‐degree days) increased significantly after tree removal, measured as cumulative over 6 years (SC = 0.30), and in the early Spring of year six (SC = 0.16). Treatment‐induced increase of cumulative Spring wet degree‐days explained variation in shrub cover at year 6 (SC = 0.12) and the increase of early Spring wet degree‐days at year 6 led to increases in perennial deep‐rooted grasses (SC = 0.24) and cheatgrass (SC = 0.23). We detected no influence of Spring wet degree‐days on perennial shallow‐rooted grasses. The post‐treatment model explained between 34% (shallow‐rooted perennial grasses) and 69% (deep‐rooted perennial grasses) of variation in understory cover. Most variation was explained by re‐measurement of the same populations, followed by treatment effects mediated through increased soil water availability, soil factors, and direct effects of the treatment itself. In conclusion, our model is consistent with the a priori hypothesis that additional wet degree‐days due to tree removal is a significant mechanism behind observed increases in understory cover.
... In the eastern steppe, fire is limited by horizontal fuel continuity. Similar to other steppes (Miller and Heyerdahl, 2008), fine fuels (e.g., grasses) are desiccated and readily burnable every summer in the Patagonian steppe (Kitzberger, 2012). However, when the amount of bare ground is above a certain threshold (<50%; e.g., overgrazed areas, rocky or stony outcrops), fire spread is limited. ...
Warming trends are altering fire regimes globally, potentially impacting on the long-term persistence of some ecosystems. However, we still lack clear understanding of how climatic stressors will alter fire regimes along productivity gradients. We trained a Random Forests model of fire probabilities across a 5°lat × 2° long trans-Andean rainfall gradient in northern Patagonia using a 23-year long fire record and biophysical, vegetation, human activity and seasonal fire weather predictors. The final model was projected onto mid- and late 21st century fire weather conditions predicted by an ensemble of GCMs using 4 emission scenarios. We finally assessed the vulnerability of different forest ecosystems by matching predicted fire return intervals with critical forest persistence fire return thresholds developed with landscape simulations. Modern fire activity showed the typical hump-shaped relationship with productivity and a negative distance relationship with human settlements. However, fire probabilities were far more sensitive to current season fire weather than to any other predictor. Sharp responsiveness of fire to the accelerating drier/warmer fire seasons predicted for the remainder of the 21st century in the region led to 2 to 3-fold (RCPs 4.5 and 8.5) and 3 to 8-fold increases in fire probabilities for the mid- and late 21st century, respectively. Contrary to current generalizations of larger impacts of warming on fire activity in fuel-rich ecosystems, our modeling results showed first an increase in predicted fire activity in less productive ecosystems (shrublands and steppes) and a later evenly amplified fire activity-productivity relationship with it shape resembling (at higher fire probabilities) the modern hump-shaped relationship. Despite this apparent homogeneous effect of warming on fire activity, vulnerability to predicted late 21st century shorter fire intervals were higher in most productive ecosystems (subalpine deciduous and evergreen Nothofagus-dominated rainforests) due to a general lack of fire-adapted traits in the dominant trees that compose these forests.
... However, we found little direct evidence of fire in the preceding period to support this idea. Surface fires, if they had occurred, could have consumed dead and down woody fuel, eliminating evidence of fire, but if fire were an important driver of past tree demography at Horse Ridge, we would have expected to find charring on surface wood and old tree boles, and some fire scars, as western juniper can occasionally scar (Miller and Heyerdahl 2008). Although our lack of evidence of fire limits the support for this explanation, changes in fire activity may provide the most direct and simple causal mechanism linked to the patterns we see in the juniper establishment record. ...
Climate and Landscape Controls on Old-Growth Western Juniper Demography in the Northern Great Basin, USA
... However, we found little direct evidence of fire in the preceding period to support this idea. Surface fires, if they had occurred, could have consumed dead and down woody fuel, eliminating evidence of fire, but if fire were an important driver of past tree demography at Horse Ridge, we would have expected to find charring on surface wood and old tree boles, and some fire scars, as western juniper can occasionally scar (Miller and Heyerdahl 2008). Although our lack of evidence of fire limits the support for this explanation, changes in fire activity may provide the most direct and simple causal mechanism linked to the patterns we see in the juniper establishment record. ...
Western juniper (Juniperus occidentalis Hook.) woodlands have persisted for millennia in semiarid parts of the northern Great Basin, USA, providing critical habitat for plant and animal species. Historical records suggest that the establishment of western juniper is strongly associated with regional climatic variability. For example, the abundance of western juniper pollen and macrofossils measured in lake sediment cores increased rapidly in the mid-1500s, concurrent with a regional increase in winter precipitation. However, little is known about how climatic factors interact with landscape structure to control the spatial distribution of western juniper at fine scales and at lower treelines. We used tree rings to reconstruct a spatially distributed history of establishment for 421 western juniper trees across 130 ha on Horse Ridge in central Oregon. Establishment occurred between 845 and 1961 CE, but most trees established after the mid-1550s. The pronounced sixteenth century pulse of establishment represents a transition from more open wooded shrublands to persistent woodlands and coincides with an increase in cool-season moisture and generally cool summer temperatures. Ancient trees that established before this were limited to certain microsites, suggesting that local topoedaphic conditions influenced juniper woodland demography and distributions, although we could not identify consistent environmental drivers. In the future, warmer and drier growing season conditions and a potential increase in wildfire activity may broadly limit western juniper recruitment and its distribution across the region, but at finer scales landscape features that buffer climate change impacts or provide fire safe niches may serve as refugia.
