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Background Forest and nonforest ecosystems of the western United States are experiencing major transformations in response to land-use change, climate warming, and their interactive effects with wildland fire. Some ecosystems are transitioning to persistent alternative types, hereafter called “vegetation type conversion” (VTC). VTC is one of the most pressing management issues in the southwestern US, yet current strategies to intervene and address change often use trial-and-error approaches devised after the fact. To better understand how to manage VTC, we gathered managers, scientists, and practitioners from across the southwestern US to collect their experiences with VTC challenges, management responses, and outcomes. Results Participants in two workshops provided 11 descriptive case studies and 61 examples of VTC from their own field observations. These experiences demonstrate the extent and complexity of ecological reorganization across the region. High-severity fire was the predominant driver of VTC in semi-arid coniferous forests. By a large margin, these forests converted to shrubland, with fewer conversions to native or non-native herbaceous communities. Chaparral and sagebrush areas nearly always converted to non-native grasses through interactions among land use, climate, and fire. Management interventions in VTC areas most often attempted to reverse changes, although we found that these efforts cover only a small portion of high-severity burn areas undergoing VTC. Some areas incurred long (>10 years) observational periods prior to initiating interventions. Efforts to facilitate VTC were rare, but could cover large spatial areas. Conclusions Our findings underscore that type conversion is a common outcome of high-severity wildland fire in the southwestern US. Ecosystem managers are frontline observers of these far-reaching and potentially persistent changes, making their experiences valuable in further developing intervention strategies and research agendas. As its drivers increase with climate change, VTC appears increasingly likely in many ecological contexts and may require management paradigms to transition as well. Approaches to VTC potentially include developing new models of desired conditions, the use of experimentation by managers, and broader implementation of adaptive management strategies. Continuing to support and develop science-manager partnerships and peer learning groups will help to shape our response to ongoing rapid ecological transformations.

Author(s): Krawchuk, Meg A; Meigs, Garrett W; Cartwright, Jennifer M; Coop, Jonathan D; Davis, Raymond; Holz, Andres; Kolden, Crystal; Meddens, Arjan JH

Altered fire regimes can drive major and enduring compositional shifts or losses of forest ecosystems. In western North America, ponderosa pine and dry mixed‐conifer forest types appear increasingly vulnerable to uncharacteristically extensive, high‐severity wildfire. However, unburned or only lightly impacted forest stands that persist within burn mosaics—termed fire refugia—may serve as tree seed sources and promote landscape recovery. We sampled tree regeneration along gradients of fire refugia proximity and density at 686 sites within the perimeters of 12 large wildfires that occurred between 2000 and 2005 in the interior western United States. We used generalized linear mixed‐effects models to elucidate statistical relationships between tree regeneration and refugia pattern, including a new metric that incorporates patch proximity and proportional abundance. These relationships were then used to develop a spatially explicit landscape simulation model. We found that regeneration by ponderosa pine and obligate‐seeding mixed‐conifer tree species assemblages was strongly and positively predicted by refugia proximity and density. Simulation models revealed that for any given proportion of the landscape occupied by refugia, small patches produced greater landscape recovery than large patches. These results highlight the disproportionate importance of small, isolated islands of surviving trees, which may not be detectable with coarse‐scale satellite imagery. Findings also illustrate the interplay between patch‐scale resistance and landscape‐scale resilience: Disturbance‐resistant settings (fire refugia) can entrain resilience (forest regeneration) across the burn matrix. Implications and applications for land managers and conservation practitioners include strategies for the promotion and maintenance of fire refugia as components of resilient forest landscapes.

Natural resource managers are increasingly applying tree reduction treatments to piñon–juniper woodlands to meet a range of ecological, social, and economic goals. However, treatment effects on woodland-obligate bird species are not well understood. We measured multiscale avian occupancy on 29 paired (control/treatment) sites in piñon–juniper woodlands in central Colorado, USA. We conducted point counts at 232 stations, 3 times each season in 2014 and 2015. We used hierarchical multiscale modeling to obtain unbiased estimates of landscape and local occupancy (i.e. probability of use) in treated and untreated sites for 31 species. Treatments reduced the occupancy of conifer obligates, including Mountain Chickadee (Poecile gambeli), Clark’s Nutcracker (Nucifraga columbiana), and White-breasted Nuthatch (Sitta carolinensis), and increased occupancy of Lark Sparrow (Chondestes grammacus) and Mountain Bluebird (Sialia currucoides). Occupancy of Virginia’s Warbler (Oreothylpis virginiae) and Gray Flycatcher (Empidonax wrightii), two piñon–juniper specialists, decreased at the landscape scale in treated sites, and Pinyon Jay (Gymnorhinus cyanocephalus) occupancy decreased at the local scale. Tree reduction treatments in piñon–juniper woodlands have the potential to reduce habitat quality for a suite of bird species of conservation concern. We suggest that treatments designed to retain higher tree density and basal area will benefit conifer-obligate and piñon–juniper specialist bird species.

Extensive high‐severity wildfires have driven major losses of ponderosa pine and mixed‐conifer forests in the southwestern United States, in some settings catalyzing enduring conversions to non‐forested vegetation types. Management interventions to reduce the probability of stand‐replacing wildfire have included mechanical fuel treatments, prescribed fire, and wildfire managed for resource benefit. In 2011, the Las Conchas fire in northern New Mexico burned forested areas not exposed to fire for >100 yr, but also reburned numerous prescribed fire units and/or areas previously burned by wildfire. At some sites, the combination of recent prescribed fire and wildfire approximated known pre‐settlement fire frequency, with two or three exposures to fire between 1977 and 2007. We analyzed gridded remotely sensed burn severity data (differenced normalized burn ratio), pre‐ and post‐fire field vegetation samples, and pre‐ and post‐fire measures of surface fuels to assess relationships and interactions between prescribed fire, prior wildfire, fuels, subsequent burn severity, and patterns of post‐fire forest retention vs. conversion to non‐forest. We found that Las Conchas burn severity was lowest, and tree survival was highest, in sites that had experienced both prescribed fire and prior wildfire. Sites that had experienced only prescribed or prior wildfire exhibited moderate burn severity and intermediate levels of forest retention. Sites lacking any recent prior fire burned at the highest severity and were overwhelmingly converted to non‐forested vegetation including grassland, oak scrub, and weedy, herbaceous‐dominated types. Burn severity in the Las Conchas fire was closely linked to surface woody fuel loads, which were reduced by prior wildfire and prescribed fire. Our results support the restoration of fire regimes via prescribed fire and resource benefit wildfire to promote the resiliency of forest types vulnerable to fire‐mediated type conversion. The application of prescribed fire to reduce surface fuels following wildfire may reduce forest loss during subsequent fire under more extreme conditions. These findings are especially relevant given likely increases in vulnerability associated with climate change impacts to wildfire and forest dynamics.

Aim Studies of fire activity along environmental gradients have been undertaken, but the results of such studies have yet to be integrated with fire-regime analysis. We characterize fire-regime components along climate gradients and a gradient of human influence. Location We focus on a climatically diverse region of north-western North America extending from northern British Columbia, Canada, to northern Utah and Colorado, USA. Methods We used a multivariate framework to collapse 12 climatic variables into two major climate gradients and binned them into 73 discrete climate domains. We examined variation in fire-regime components (frequency, size, severity, seasonality and cause) across climate domains. Fire-regime attributes were compiled from existing databases and Landsat imagery for 1897 large fires. Relationships among the fire-regime components, climate gradients and human influence were examined through bivariate regressions. The unique contribution of human influence was also assessed. Results A primary climate gradient of temperature and summer precipitation and a secondary gradient of continentality and winter precipitation in the study area were identified. Fire occupied a distinct central region of such climate space, within which fire-regime components varied considerably. We identified significant interrelations between fire-regime components of fire size, frequency, burn severity and cause. The influence of humans was apparent in patterns of burn severity and ignition cause. Main conclusions Wildfire activity is highest where thermal and moisture gradients converge to promote fuel production, flammability and ignitions. Having linked fire-regime components to large-scale climate gradients, we show that fire regimes – like the climate that controls them – are a part of a continuum, expanding on models of varying constraints on fire activity. The observed relationships between fire-regime components, together with the distinct role of climatic and human influences, generate variation in biotic communities. Thus, future changes to climate may lead to ecological changes through altered fire regimes.

Montane and subalpine grasslands are prominent, but poorly understood, features of the Rocky Mountains. These communities frequently occur below reversed tree lines on valley floors, where nightly cold air accumulation is spatially coupled with fine soil texture. We used field experiments to assess the roles of minimum temperature, soil texture, grass competition, and ungulate browsing on the growth, photosynthetic performance, and survival of transplanted ponderosa pine (Pinus ponderosa) seedlings at 32 sites straddling such reversed tree lines in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains, New Mexico (USA). Seedling growth increased most strongly with increasing nighttime minimum temperatures away from the valley bottoms; seedlings experiencing the coldest temperatures on the caldera floor exhibited stunted needles and often no measurable height growth. Based on the chlorophyll fluorescence ratios PhiPSII and Fv/Fm, we found that low minimum temperatures, low soil moisture, and fine soil texture all contributed to photoinhibition. Neighboring herbs had only minor negative effects on seedlings. We found no effect of ungulates, but golden-mantled ground squirrels (Spermophilus lateralis) caused substantial seedling mortality. Second-year seedling survival was highest on sandy soils, and third-year survival was highest at sites with higher minimum temperatures. We conclude that differential tree seedling establishment driven by low minimum temperatures in the valley bottoms is the primary factor maintaining montane grasslands of the VCNP, although this process probably operated historically in combination with frequent surface fire to set the position of the tree line ecotone. As at alpine tree lines, reversed tree lines bordering montane and subalpine grasslands can represent temperature-sensitive boundaries of the tree life form.

Aim Recent forest encroachment into montane and subalpine grasslands has occurred in the Rocky Mountains and many other mountain ranges globally. The timing, rate, and extent of tree invasion can depend on interactions among topography, positive spatial feedbacks, and temporally variable factors (especially climate, grazing, and fire). Here we examine spatial and temporal patterns of tree invasion in the Valles Caldera of the Jemez Mountains. Location This study was conducted in the Valles Caldera (35°50′–36°00′ N; 106°24′–106°37′ W), a 24-km-wide volcanic basin in northern New Mexico, USA. Grasslands in this otherwise forested region occur in broad valley bottoms of the caldera floor between 2575 and 2700 m, and on south-facing slopes and mountain tops up to 3300 m. Methods We used a GIS analysis of orthorectified aerial photos taken in 1935 and 1996, covering a 40,000-ha study area, to quantify the extent of tree invasion and to assess its relationship to spatial factors. We obtained dates of establishment from 299 increment cores and basal disks from 50 sites in the Valles Caldera National Preserve (VCNP) to reconstruct temporal patterns of tree invasion. Results The area of grasslands in our study area declined from 11,747 to 9336 ha (nearly 18%) between 1935 and 1996. Tree invasion increased with slope, elevation, and proximity to the previous tree line, but showed no relationship to aspect. Tree invasion was more rapid and continuous on upper mountain slopes, while the invasion of valley-bottom grasslands below reversed tree lines was more episodic, and appeared to track mean summer minimum temperatures. Main conclusions The rapid and continuous invasion of steep, high-elevation slopes suggests that frequent fire was the single most important factor in maintaining grassy communities in these sites. The slower, episodic invasion of valley-bottom grasslands, and the apparent relationship between increased invasion and years of higher summer minimum temperatures are consistent with the hypothesis that these grasslands have been maintained by low temperatures or frosts damaging to tree seedlings. We encourage prescribed fire to restore and maintain grasslands in the VCNP, especially small patches on steep, high-elevation slopes.

We observed black bears (Ursus americanus) foraging on aggregations of army cutworm moths (Euxoa auxiliaris) (Lepidoptera: Noctuidae) in subalpine felsenmeers (block fields) in the Jemez Mountains of New Mexico. Moth aggregations serve as food for grizzly bears (Ursus arctos horribilis) in the northern Rocky Mountains of Wyoming and Montana. However, black bears have not been reported to use these aggregations, nor have such aggregations been documented to occur this far south in the Rocky Mountains.