Research and Publications
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Across plots that burned in multiple low- to moderate-severity fires, our findings indicated that post-fire outcomes in these systems are variable, resulting in a range of structural conditions following a first reburn (i.e., second fire). Areas with high levels of dead biomass burned at significantly higher severity in the third fire compared to those with higher shrub cover. Following a second fire, many plots exceeded historical estimates of stand structure metrics for yellow pine and mixed-conifer forests of the Sierra Nevada, particularly for coarse woody debris load, with some plots exceeding historical natural range of variation (NRV) estimates for live tree density. In plots with a history of varying fire severity in the initial and second fires, we found that snag basal area was associated with higher fire severity in the third fire.
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This study compares burned areas to climate and fuel conditions in three temperate regions: the desert, shrub, and forest ecoregions of western North America, west-central Europe, and southwestern South America. In each region the mean annual aridity index (AI, precipitation over potential evapotranspiration) spans arid to humid climates. We examined how the fraction of area burned from 2001 to 2021 varied with mean annual AI, mean aboveground biomass, and land cover type distributions. All three regions had low fractions of area burned for the driest climate zones (AI < 0.5), a sign of fuel limitation to burned area. Fraction of area burned increased with mean aboveground biomass for these dry zones. Fraction of area burned peaked at intermediate AI (0.7–1.5) for all regions and declined again in the wettest climate zones (AI > 1.5), a sign of climate limitation to burned area. Of the three regions, western North America had the highest burned area, fraction of area burned, and fire sizes. Fragmentation of vegetation patches by the high Andes Mountains in southwestern South America and by intensive land use changes in west-central Europe likely limited fire sizes. All three regions are at risk for future wildfires, particularly in areas where fire is currently climate limited.
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While substantial efforts have been made to conserve critical mule deer habitat, less attention has been given to mule deer habitat affected by invasive annual grasses (IAGs) and there is limited information about how mule deer respond to IAG invasions. We evaluated mule deer resource selection in a sagebrush grassland community impacted by IAGs in northeast Wyoming. We then used empirical model estimates to forecast how IAG management could impact mule deer habitat in the future following a strategic IAG framework focused on defending and growing sagebrush core areas most threatened by IAGs. We found that mule deer responded to IAGs in a nonlinear pattern across all seasons and strongly avoided areas once cover exceeded approximately 20%. When projecting results 20 yr into the future, we found that over half of the study area is expected to experience significant declines in mule deer habitat quality if IAGs continue to spread at the same rate observed over the past two decades. However, with targeted IAG treatments, we predicted widespread improvements in mule deer habitat, particularly in priority areas where ecological integrity can be restored with future IAG management. Our findings reinforce the emerging notion that ecosystem-based frameworks designed to defend and grow intact sagebrush steppe through strategic management efforts also have the potential to benefit species of conservation interest. As current conservation efforts to mitigate IAGs are not progressing fast enough to address the magnitude of the IAG problem in sagebrush across the west, strategic management efforts will be necessary to maintain important habitats for numerous sagebrush occurring wildlife.
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To assess relationships between fire spread rates and landscape burn severity patterns, we used satellite fire detections to create day-of-burning maps for 623 fires comprising 4267 single-day events within forested ecoregions of the southwestern United States. We related satellite-measured burn severity and a suite of high-severity patch metrics to daily area burned. Extreme fire spread events (defined here as burning > 4900 ha/day) exhibited higher mean burn severity, a greater proportion of area burned severely, and increased like adjacencies between high-severity pixels. Furthermore, increasing daily area burned also resulted in greater distances within high-severity patches to live tree seed sources. High-severity patch size and total high-severity core area were substantially higher for fires containing one or more extreme spread events than for fires without an extreme event. Larger and more homogenous high-severity patches produced during extreme events can limit tree regeneration and set the stage for protracted forest conversion. These landscape outcomes are expected to be magnified under future climate scenarios, accelerating fire-driven forest loss and long-term ecological change.
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To investigate the effects of extreme weather and forest management on fire severity, we used light detection and ranging (LiDAR) data to characterize pre-fire forest structure across five large wildfires which burned 460,000 ha in the northern Sierra Nevada, California, USA. We found that the odds of high severity fire occurrence in these fires were 1.45 times higher on private industrial land than in publicly owned forests, an effect equivalent to a three standard deviation decrease in fuel moisture. Next, we quantified the relationships between key forest structure metrics and the probability of high severity fire, as well as how these relationships were modified by extreme weather. We found that dense, spatially homogeneous forests with high ladder fuels were more likely to burn at high severity. Extreme weather magnified the effect of density, suggesting that treatments which remove overstory trees are especially important in extreme conditions. Forests managed by private industry were more likely to be dense, spatially homogeneous, and contain high ladder fuel loads than publicly owned forests, offering a potential explanation for the increase in high-severity fire occurrence on private industrial land. Overall, these results illustrate the need for comprehensive forest management to mitigate fire severity in a warmer future.
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Ventenata and medusahead are winter annuals that emerge in late fall, winter, and spring. These grasses mature early in the summer providing fine fuels for wildfires.Ventenata and medusahead are frequently found together where conditions allow. These invasive annual grasses increase wildfire danger in shrublands and woodlands of the American West.
- Fine, highly flammable fuel loads facilitate larger and more frequent fires.
- Relatively high silica content make these grasses less palatable for grazing (unlike cheatgrass which is palatable in its green phase), and creates a build-up of litter on the soil surface.
- These species can spread throughout areas that once acted as natural fire breaks.
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Co-varying disturbance and environmental gradients can shape vegetation dynamics and increase the diversity of plant communities and their features. Pinyon–juniper woodlands are widespread in semi-arid climates of western North America, encompassing extensive environmental gradients, and a knowledge gap is how the diversity in features of these communities changes across co-varying gradients in fire history and soil. In pinyon–juniper communities spanning soil parent materials (basalt, limestone) and recent fire histories (0–4 prescribed fires or managed wildfires and 5–43 years since fire) in Grand Canyon-Parashant National Monument (Arizona, USA), we examined variation at 25 sites in three categories of plant community features including fuels, tree structure, and understory vegetation. Based on ordinations, canonical correlation analysis, and permutation tests, plant community features varied primarily with the number of fires, soil coarseness and chemistry, and additionally with tree structure for understory vegetation. Fire and soil variables accounted for 33% of the variance in fuels and tree structure, and together with tree structure, 56% of the variance in understories. The cover of the non-native annual Bromus tectorum was higher where fires had occurred more recently. In turn, B. tectorum was positively associated with the percentage of dead trees and negatively associated with native forb species richness. Based on a dendroecological analysis of 127 Pinus monophylla and Juniperus osteosperma trees, only 18% of trees presently around our study sites originated before the 1870s (Euro-American settlement) and <2% originated before the 1820s. Increasing contemporary fire activity facilitated by the National Park Service since the 1980s corresponded with increasing tree mortality and open-structured stands, apparently more closely resembling pre-settlement conditions. Using physical geography, such as soil parent material, as a landscape template shows promise for (i) incorporating diversity in long-term community change serving as a baseline for vegetation management, (ii) customizing applying treatments to unique conditions on different soil types, and (iii) benchmarking monitoring metrics of vegetation management effectiveness to levels scaled to biophysical variation across the landscape.
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We leveraged work identifying sagebrush areas suitable for woody fuel treatments based on resilience to disturbance and resistance to annual grass invasion (R&R) and areas of sagebrush mapped as high conservation value. We used wildfire simulation modeling to estimate annual wildfire exposure (area burned), and identify areas where fire is transmitted to locations of high conservation value that are low R&R. We then optimized treatment location with the ForSys spatial planning system to prioritize treatment of wildfire exposure where treatments are ecologically suitable and explored how operational restrictions (e.g., distance to roads) limited the capacity to treat exposure. Overall, woody fuel treatments could be realistically implemented in only 7.6 % (2.5 million ha) of sagebrush dominated areas. We found that 24 % of the wildfire exposure across all sagebrush associations occurred where fuel treatments were ecologically suitable, but consideration of operational constraints reduced treatable exposure to 9 %. However, there was double the opportunity to reduce transmitted exposure to areas of high conservation value in the operational scenario despite restrictions. Leveraging treatment suitability and sagebrush conservation to strategically design implementable project treatment can help direct limited resources where they are likely to have the greatest ecological and risk reduction benefit.
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We created a regional-scale chronosequence of areas that burned only once from 1984 to 2017 using Landsat-derived burned area products, and collected species composition data across a gradient of 4–32 years since fire. We used linear mixed models to look for evidence of native plant recovery, and used indirect gradient analysis and joint species distribution models to examine the response of species occurrence to a) fire occurrence and timing and pre- and post-fire climate; and b) topography, grazing, and annual grass dominance.
Native diversity and perennial herbaceous cover were unrelated to time since fire and negatively associated with annual grass cover. The occurrence of a single fire had mostly negative associations with native species and mostly positive associations with non-native species. Grazing intensity did not affect the dominant post-fire annual grass, but non-native annual forbs sorted along a gradient towards two groups based on grazing intensity, annual grass cover, and topography.
Annual grass competition will likely maintain the post-fire invasive-dominated plant community even if management interventions successfully stop the grass-fire cycle.
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We showed that climate warming in southwest drylands has been associated with concurrent changes in vegetation and fuels and decreases in resistance and resilience. We provide an approach that allows managers to quantify the ongoing changes at management appropriate scales. We suggest climate smart management strategies to help direct ecosystems into conditions that can decrease fire risk, increase resistance to plant invasions, and reduce vulnerability to climate change.