Forests In Colorado Research Articles

Shaping of topography by topographically-controlled vegetation in tropical montane rainforest.

Topography is commonly viewed as a passive backdrop on which vegetation grows. Yet, in certain circumstances, a bidirectional feedback may develop between the control of topography and the spatial distribution of vegetation and landform development, because vegetation modulates the erosion of the land surface. Therefore, if reinforcing feedbacks are established between erosion and land cover distribution over timescales relevant to landform development, then the interactions between vegetation and topography may create distinctive landforms, shaped by vegetation. We expose here a strong correlation between the spatial distribution of vegetation, erosion rates, and topography at a characteristic length scale of 102-103m (mesoscale topography) in the Luquillo Experimental forest (LEF) of Puerto Rico. We use high-resolution LiDAR topography to characterize landforms, satellite images to classify the vegetation into forest types, and in-situ produced cosmogenic 10Be in the quartz extracted from soils and stream sediments to document spatial variations in soil erosion. The data document a strong correlation between forest type and topographic position (hilltop vs. valleys), and a correlation between topographic position and 10Be-derived erosion rates over 103-104 years. Erosion is faster in valleys, which are mostly covered by monocot Palm Forest, and slower on surrounding hills mostly covered by the dicot Palo Colorado Forest. Transition from one forest type to the next occurs across a break-in-slope that separates shallowly convex hilltops from deeply concave valleys (coves). The break-in-slope is the consequence of a longer-lasting erosional imbalance whereby coves erode faster than hills over landscape-shaping timescales. Such a deepening of the coves is usually spurred by external drivers, but such drivers are here absent. This implies that cove erosion is driven by a process originating within the coves themselves. We propose that vegetation is the primary driver of this imbalance, soil erosion being faster under Palm forest than under Palo Colorado forest. Concentration of the Palm forest in the deepening coves is reinforced by the better adaptation of Palm trees to the erosive processes that take place in the coves, once these develop steep slopes. At the current rate of landscape development, we find that the imbalance started within the past 0.1-1.5 My. The initiation of the process could correspond to time of settlement of these mountain slopes by the Palm and Palo Colorado forests.

Wildfire-dependent changes in soil microbiome diversity and function

Forest soil microbiomes have crucial roles in carbon storage, biogeochemical cycling and rhizosphere processes. Wildfire season length, and the frequency and size of severe fires have increased owing to climate change. Fires affect ecosystem recovery and modify soil microbiomes and microbially mediated biogeochemical processes. To study wildfire-dependent changes in soil microbiomes, we characterized functional shifts in the soil microbiota (bacteria, fungi and viruses) across burn severity gradients (low, moderate and high severity) 1 yr post fire in coniferous forests in Colorado and Wyoming, USA. We found severity-dependent increases of Actinobacteria encoding genes for heat resistance, fast growth, and pyrogenic carbon utilization that might enhance post-fire survival. We report that increased burn severity led to the loss of ectomycorrhizal fungi and less tolerant microbial taxa. Viruses remained active in post-fire soils and probably influenced carbon cycling and biogeochemistry via turnover of biomass and ecosystem-relevant auxiliary metabolic genes. Our genome-resolved analyses link post-fire soil microbial taxonomy to functions and reveal the complexity of post-fire soil microbiome activity.

Wildfire and forest thinning shift floral resources and nesting substrates to impact native bee biodiversity in ponderosa pine forests of the Colorado Front Range

In fire-adapted ponderosa pine forests of western North America, fire suppression policies during much of the 19th century gradually resulted in high stem densities undesirable for fire risk management. To restore desirable forest structures, regional ecosystem management efforts often focus on reducing stand densities and re-introducing fire disturbances. However, it remains unclear whether these practices benefit insect species important for ecosystem function, including native bees. Bee community assemblages were sampled across the growing season in 39 ponderosa pine stands for two years in central Colorado to model effects of fire and thinning disturbances on bee populations. Four key findings emerged: (1) overall native bee diversity consisted of 5 families (Andrenidae, Apidae, Colletidae, Halictidae, and Megachilidae), 30 genera, and at least 70 unique bee species. Predominant genera were bumblebees (Bombusspp.), mason bees (Osmiaspp.), and digger bees (Anthophoraspp.), accounting for 50%, 9%, and 5 % of all captures, respectively. (2) Average bee abundance did not vary among burned, thinned, or non-treated control stands, though bee species richness and diversity were highest in stands affected by high severity wildfire. (3) Bee community composition differed among disturbance types, and there was turnover in the ratios of Bombus: Osmia species between control and burned stands. (4) Representation of bee nesting strategies at sample sites also differed due to fire and thinning disturbances, but these effects were moderated by seasonality. We link variation in forest structure to bee assemblages using linear models and show that presence of coarse woody debris has positive effects on overall forest bee abundance, richness, and diversity. Ponderosa pine forests in central Colorado provide habitat for a diverse assemblage of native bee species, and thinning operations are not associated with reductions in bee diversity, and fire disturbances were associated with enhanced bee foraging and nesting habitats. Bee community composition and associated life histories vary across disturbance types and unique bee assemblages are associated with different disturbance histories. Accordingly, maintaining a mosaic of thinning treatments and promoting wildland fire use that results in a gradient of burn severities may increase floral resources and nesting opportunities and can help to maximize overall bee biodiversity at a landscape scale.

Resolving temperature limitation on spring productivity in an evergreen conifer forest using a model–data fusion framework

Abstract. The flow of carbon through terrestrial ecosystems and the response to climate are critical but highly uncertain processes in the global carbon cycle. However, with a rapidly expanding array of in situ and satellite data, there is an opportunity to improve our mechanistic understanding of the carbon (C) cycle's response to land use and climate change. Uncertainty in temperature limitation on productivity poses a significant challenge to predicting the response of ecosystem carbon fluxes to a changing climate. Here we diagnose and quantitatively resolve environmental limitations on the growing-season onset of gross primary production (GPP) using nearly 2 decades of meteorological and C flux data (2000–2018) at a subalpine evergreen forest in Colorado, USA. We implement the CARbon DAta-MOdel fraMework (CARDAMOM) model–data fusion network to resolve the temperature sensitivity of spring GPP. To capture a GPP temperature limitation – a critical component of the integrated sensitivity of GPP to temperature – we introduced a cold-temperature scaling function in CARDAMOM to regulate photosynthetic productivity. We found that GPP was gradually inhibited at temperatures below 6.0 ∘C (±2.6 ∘C) and completely inhibited below −7.1 ∘C (±1.1 ∘C). The addition of this scaling factor improved the model's ability to replicate spring GPP at interannual and decadal timescales (r=0.88), relative to the nominal CARDAMOM configuration (r=0.47), and improved spring GPP model predictability outside of the data assimilation training period (r=0.88). While cold-temperature limitation has an important influence on spring GPP, it does not have a significant impact on integrated growing-season GPP, revealing that other environmental controls, such as precipitation, play a more important role in annual productivity. This study highlights growing-season onset temperature as a key limiting factor for spring growth in winter-dormant evergreen forests, which is critical in understanding future responses to climate change.

Explaining changes in perceived wildfire risk related to the mountain pine beetle outbreak in north central Colorado

Longitudinal studies of risk perception, while growing, remain an understudied area of risk analysis research. Natural resource-based communities provide a key backdrop for analyzing dynamic risk perceptions and related social-ecological processes. Since the mid-1990s, a mountain pine beetle (MPB) outbreak has affected roughly 3.4 million acres of north central Colorado forests. Slow-moving landscape changes have taken place as a result of the outbreak, making it a relevant case study in which to examine wildfire and forest risk perceptions over time. In 2018, we replicated and expanded a 2007 mail survey of residents about local response to the MPB outbreak in nine north central Colorado communities. Using the initial and follow-up survey data from these study communities, we examined the changing wildfire risk perception and its relationships with various aspects of local human–nature interactions. The longitudinal surveys showed that perceptions of all forest risks except for forest fire and falling trees declined across the two study phases. Respondents in the 2018 survey also indicated increases in perceived likelihood of a wildfire, perceived severity of possible wildfire damage, and general concern about wildfire hazard. The analysis suggested that evolving wildfire risk perception was strongly associated with a number of explanatory variables and personal characteristics. Higher levels of perceived tree mortality, community interaction and communication, and individual actions in response to forest risks were consistently related to greater odds of indicating increased wildfire risk perception. In general, community social vulnerability, perceived tree re-growth, satisfaction with local management entities, and age were negatively related to the likelihood of having elevated perceptions of fire risk, whereas males and forest-related occupations were associated with relatively lower odds of reporting increases in perceived wildfire risk. Significant influencing factors for change also varied across the cognitive and affective dimensions of wildfire risk perception. Findings of this research have important implications for conceptual approaches, management and policy, as well as social-ecological indication and analysis in forest risk related areas.

Forest Roads and Operational Wildfire Response Planning

Supporting wildfire management activities is frequently identified as a benefit of forest roads. As such, there is a growing body of research into forest road planning, construction, and maintenance to improve fire surveillance, prevention, access, and control operations. Of interest here is how road networks directly support fire control operations, and how managers incorporate that information into pre-season assessment and planning. In this communication we briefly review and illustrate how forest roads relate to recent advances in operationally focused wildfire decision support. We focus on two interrelated products used on the National Forest System and adjacent lands throughout the western USA: potential wildland fire operational delineations (PODs) and potential control locations (PCLs). We use real-world examples from the Arapaho-Roosevelt National Forest in Colorado, USA to contextualize these concepts and illustrate how fire analytics and local fire managers both identified roads as primary control features. Specifically, distance to road was identified as the most important predictor variable in the PCL boosted regression model, and 82% of manager-identified POD boundaries aligned with roads. Lastly, we discuss recommendations for future research, emphasizing roles for enhanced decision support and empirical analysis.

Singing Inside a Tree

Singing Inside a Tree Anne Haven McDonnell (bio) Many regions are projected to experience an increase in the probability of compound events with higher global warming (high confidence). In particular, concurrent heatwaves and droughts are likely to become more frequent. —Valérie Masson-Delmotte and Panmao Zhai, “Regional trends in extreme events in the IPCC 2021 report” The Celts believed a tree’s presence could be felt more keenly at night or after a heavy rain, and that certain people were more attuned to trees and better able to perceive them. There is a special word for this recognition of sentience, mothaitheacht. It was described as a feeling in the upper chest of some kind of energy or sound passing through you. It’s possible that mothaitheacht is an ancient expression of a concept that is relatively new to science: infrasound or “silent” sound. These are sounds pitched below the range of human hearing, which travel great distances by means of long, loping waves. They are produced by large animals, such as elephants, and by volcanoes. And these waves have been measured as they emanate from large trees. —Diana Beresford-Kroeger, To Speak for the Trees It is the balance between the rational and imaginative that will ultimately solve the most serious problems that threaten us. —David Dunn There’s a tree I see in my mind when I need to find shelter. This Douglas fir tree lives on an island in the northwest, and simultaneously this tree lives inside me. It’s one of the few remaining old-growth trees on the island. I’ve sat inside the burned-out cave at its base where lightning struck. I’ve watched light glitter across strands of spider silk spanning its entrance, breathing cool air and smelling damp earth, bark, and wood. Inside the tree, I am young and old and animal and human. I am held inside the slow language of xylem, [End Page 44] phloem, cambium, sap, and centuries of survival. When I approach the tree as I walk on the duff trail around the border of the lake, I feel the tree’s voice move like water through my chest. The voice of the tree is a hush, a swell, a silent rolling wave like the slow rising of a whale. There’s another kind of silence in my chest when I walk through the beetle-killed forests of northern New Mexico and Colorado. This silence is brittle. It spreads from too much sky, cracks along barkless gray branches, unsheltered, unsheltering. I’ve watched forest after forest that I love succumb to the mountain pine beetles. Where my mom has a cabin in the high Fraser Valley in Colorado, I’ve watched whole mountainsides of lodgepole pines erupt with sap balls along the trunk as they try to expunge the beetles. And then I’ve watched as rusty red clusters of dead needles spread like dull fire, until all the needles are rust. It’s almost the color of fall. It’s almost beautiful. Eventually the needles drop and leave skeletons of gray. What is called silence is usually the absence of human sounds. Inside the cave in my tree on the island, surrounded by Douglas fir, western red cedar, and hemlock forests, I hear the upward spiral of a russet-backed thrush, a loon calling from nearby on the lake, and the high tinkling glassy sounds of a Pacific wren. The thick layers of moss and duff and bark cushion each voice, as my body is held inside the body of the tree. Inside what used to be a living forest in the Rocky Mountains, inside what is now a tangle of fallen and standing dead lodgepole pine, I hear the far-off hammering of a woodpecker. Other than the woodpecker, all I hear is wind. I hear the way dead branches creak and carve the wind. What are the sounds of drought and climate unravelling in a forest? If you insert a homemade microphone inside the phloem and cambium of a drought-stressed piñon tree, as artist, composer, and acoustic ecologist David Dunn did, you’ll hear a symphony of sounds. I called David...

Dynamics of beetle-killed snags following mountain pine beetle outbreaks in lodgepole pine forests

Snags (standing dead trees) are important components of forest ecosystems that, among other benefits, provide critical habitat for many species of wildlife, but also represent important safety concerns to firefighters, forest workers, and the public. We identified factors that influence the fall rates of lodgepole pines, Pinus contorta Dougl. ex Loud., killed by bark beetles during a severe regional-scale outbreak of mountain pine beetle, Dendroctonus ponderosae Hopkins, that occurred in 2004–2012. Data were obtained during annual assessments (2010–2019) of a network of 107 0.081-ha circular plots installed in P. contorta forests in Colorado, Idaho, Montana, Utah and Wyoming, U.S. A total of 3789 P. contorta snags were monitored, and in each case we recorded, among other variables, the year the snag was created and the year the snag fell to the forest floor. Among snag age classes (i.e., number of years since tree death), the highest number of snags (1046) were 12 years since death (YSD), and those 13 and 14 YSD exhibited the lowest fall rates to date (<10%) despite being the oldest in our study. Snags 13 and 14 YSD were among the largest in diameter. Snag fall occurred in every snag age class from 1 to 14 YSD, with the greatest proportion of snag fall events occurring 4–8 YSD. By 2019, 24.7% (937) of snags fell to the forest floor. We modeled snag fall using a Cox’s proportional-hazards model that included six covariates of interest: elevation (m), slope aspect (categorical, cardinal direction), slope (%), canopy cover (%), snag height (m), and snag height:dbh (m, diameter at 1.37 m in height). Slope aspect had the strongest influence on fall rates. Northern aspects, increased canopy cover, and taller snag heights decreased the probability of snag fall. Conversely, southern aspects and increased height:dbh ratios (i.e., taller, skinner snags) increased the probability of snag fall. The predicted half-life (i.e., the amount of time since death required for 50% of the snag population to fall to the forest floor) was ~16 YSD after which the function predicted a linear, ~0.04/year decline in snag survival probability 15–30 YSD. The observed longevity of P. contorta snags confers important ecological benefits for some wildlife, and may offer opportunities for extended periods of salvage. However, checking (cracks) rapidly occurred, and many snags 6 YSD and older had at least one check at 1.37 m and 5.5 m in height, which can negatively impact lumber recovery. The relatively slow snag fall rates observed in our study also lengthen concerns regarding hazard trees, human safety, and protection of critical infrastructure. The implications of these and other results to management of P. contorta forests are discussed.

Revisiting particle dry deposition and its role in radiative effect estimates

Wet and dry deposition remove aerosols from the atmosphere, and these processes control aerosol lifetime and thus impact climate and air quality. Dry deposition is a significant source of aerosol uncertainty in global chemical transport and climate models. Dry deposition parameterizations in most global models were developed when few particle deposition measurements were available. However, new measurement techniques have enabled more size-resolved particle flux observations. We combined literature measurements with data that we collected over a grassland in Oklahoma and a pine forest in Colorado to develop a dry deposition parameterization. We find that relative to observations, previous parameterizations overestimated deposition of the accumulation and Aitken mode particles, and underestimated in the coarse mode. These systematic differences in observed and modeled accumulation mode particle deposition velocities are as large as an order of magnitude over terrestrial ecosystems. As accumulation mode particles form most of the cloud condensation nuclei (CCN) that influence the indirect radiative effect, this model-measurement discrepancy in dry deposition alters modeled CCN and radiative forcing. We present a revised observationally driven parameterization for regional and global aerosol models. Using this revised dry deposition scheme in the Goddard Earth Observing System (GEOS)-Chem chemical transport model, we find that global surface accumulation-mode number concentrations increase by 62% and enhance the global combined anthropogenic and natural aerosol indirect effect by -0.63 W m-2 Our observationally constrained approach should reduce the uncertainty of particle dry deposition in global chemical transport models.

Changes in understory vegetation including invasive weeds following mountain pine beetle outbreaks

Bark beetle outbreaks alter forests in many ways including stand structure, fuels and fire behavior, wildlife habitat, and aesthetic value. Less understood are the effects outbreaks have on understory vegetation, despite the importance for overstory succession, nutrient cycling, water quality, soil erosion, and wildlife. Beetle outbreaks also change forests in ways that could promote invasion by nonnative weeds, but this is rarely studied. We assessed changes in cover of understory vegetation and presence/abundance of weeds in lodgepole pine forests in Colorado, Idaho, Montana, Utah and Wyoming, USA following recent mountain pine beetle outbreaks. Incipient beetle populations began in 2004, peaked around 2008, and returned to endemic levels by 2012. Understory vegetation was sampled in 2010, 2012, 2014 and 2018. Total understory cover and cover of shrubs and graminoids remained unchanged, while cover of forbs generally increased. Forb cover was negatively correlated with shrub and canopy cover. Approximately 20% of plots contained weeds and weed abundance increased over time. Presence of weeds was negatively correlated with graminoid cover and positively correlated with tree mortality and snag fall. By 2018, weed abundance was only positively correlated with snag fall. Localized soil disturbance created when snags uproot and fall appears to act as sites of weed invasion and proliferation. Lastly, we provide a global review of the effects of bark beetle outbreaks on understory vegetation which reveals a prevailing short-term increase following outbreaks, but the magnitude of response and cover type affected varies with beetle species/forest type and time since outbreak.

Ecological niches of tree species drive variability in conifer regeneration abundance following fuels treatments

One goal of fuels treatments is to limit potential fire behavior by reducing overstory tree density, but this may precipitate regeneration, which contributes to increasing potential fire behavior over time. To understand factors that influence tree regeneration in treated stands, we compared abundance of advance and post treatment regeneration in 5–14-year-old thinning and mastication treatments covering approximately 2600 ha within two National Forests in Colorado, U.S.A. The study sites were dominated by two species with complimentary regeneration niches. Ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) is less shade tolerant, but more fire and drought resistant than Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). We considered three critical features of the post treatment environment: species composition, forest structure (especially density), and site characteristics. Regeneration densities at the plot level varied considerably: 37% of the plots had no regeneration, while 15% had more than double the average density. High-density areas tended to occur in moist sites, mostly on north aspects that were dominated by Douglas-fir in the overstory. These areas are where wildfire mitigation benefits will likely deteriorate most rapidly. The vast majority (69%) of all regeneration was Douglas-fir. 31% of all regeneration occurred post treatment. In this group, ponderosa pine abundance was positively related to time since treatment but Douglas-fir was not, suggesting a stronger positive effect of treatment for ponderosa pine, although Douglas-fir was still more abundant. This is likely because treatments reduced the seed source for Douglas-fir more than for ponderosa pine while reducing total overstory density to create conditions that met the regeneration requirements of this shade intolerant species. Advanced regeneration was common throughout the study area, consisting of nearly 80% Douglas-fir and only 13% ponderosa pine. Although the abundance of advance regeneration decreased over time since treatment, likely due to mortality given that we encountered few sapling-sized trees, surviving trees reduce treatment longevity and have the potential for subsequent growth release and contribution to fuel hazard development. Conifer regeneration did not vary between mastication and thinning treatments. The impact of regeneration on treatment longevity was highly variable at smaller-than-stand scales. On the Colorado Front Range, moist sites with low overstory density and mature Douglas-fir to provide a seed source are where treatment effectiveness is likely to degrade most rapidly. These areas with abundant regeneration may be best left untreated, or managers should anticipate the need to re-treat them more frequently.

Soil C storage following salvage logging and residue management in bark beetle-infested lodgepole pine forests

Bark beetle outbreaks have altered carbon (C) dynamics in lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) forests across western North America. The sensitivity of soil C to post-beetle management operations remains unknown in these forests. We quantified total C stocks in the O horizon and upper mineral soil (0–10 cm) 6–7 years after salvage logging in lodgepole pine forests of northern Colorado and compared biomass retention or removal residue management to uncut beetle-infested forests. We evaluated the quantity and chemical composition of C in physical fractions of the O horizon and upper mineral soil layers to better understand how management impacts the quality of C with implications for soil C persistence. Post-harvest residue retention increased O horizon C storage compared to the removal treatment and uncut stands. Total soil C (O horizon and upper 0–10 cm mineral soil) was an average of 11.4 (±5.6) Mg C ha−1 higher in salvage logged plots compared to uncut forests and the increase was primarily in particulate organic matter. Salvage logging did not alter the chemical composition of the soil C fractions analyzed. The C:N ratios, Fourier-transform mid-infrared spectra, and 13C values analyses each showed that the C composition of the organic and upper mineral soil layers were distinct. This indicates that physical transfer of fresh or decomposed O horizon material may not be the primary source of particulate C, and we suggest that belowground inputs are likely to be an important contributor to these mineral soils. Our finding that logging residue removal did not reduce post-harvest upper mineral soil C stocks has implications for biomass harvesting in these forests, though the long-term impacts of post-bark beetle management on C storage in these ecosystems depend on understory dynamics, tree regeneration and forest recovery that will progress over the course of decades. This work suggests that greater understanding of post-logging organic matter inputs and C dynamics in O horizon and mineral SOM fractions can help inform forest management aimed at sustaining soil C.

Subalpine fir mortality in Colorado is associated with stand density, warming climates and interactions among fungal diseases and the western balsam bark beetle

Subalpine fir mortality complex has caused significant damage to high elevation forests within Colorado. For the past two decades the climate of spruce-fir forests has trended towards being warmer and drier, which indirectly affects subalpine fir mortality. We examined potential links among abiotic (i.e. deviations in temperature and precipitation), biotic [Armillaria root disease (Armillaria spp.), Western balsam bark beetle (Dryocoetes confusus), a black stain fungus (Ophiostoma dryocoetidis)] and forest structure factors with mortality in subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) dominated forests of Colorado. Sampling was performed in two separate field surveys: 1. Statewide characterization plots and 2. Stand health monitoring plots. The statewide characterization plots were compared to aerial surveys and to establish suitable locations for stand health plots. Stand health monitoring plots collected direct measurements of abiotic and biotic factors affecting forests, with an emphasis on identifying the presence of D. confusus and Armillaria spp. The objectives of this study were to determine: (1) Is spruce-fir morality observed at ground level similar to that estimated through aerial surveys? (2) Is the presence dead subalpine fir and Englemann spruce on a plot associated with site and stand characteristics?, (3) What factors are associated with the incidence of Armillaria spp., Dr. confusus, and Dendroctonus rufipennis?, and (4) Do trends in warming temperatures and reductions in precipitation in the past five years influence subalpine fir mortality and/or the presence of biotic agents? Our results suggested that the presence of biotic agents (Dr. confusus, De. rufipennis, Armillaria spp., and O. dryocoetidis) and stand density are associated with spruce-fir mortality, whereas climatic factors had a direct association to the presence of Armillaria and were indirectly associated with mortality via Dr. confusus. Increasing maximum summer temperatures were found to be associated with the presence of Armillaria spp. The climatic variables investigated in this study did not significantly link to incidence of Dr. confusus, yet stand density was associated with increasing prevalence of Dr. confusus. The presence of Armillaria spp. and Dr. confusus were significantly related to tree decline, but several other factors are also associated with subalpine fir mortality, suggesting complex interactions of abiotic and biotic factors are likely involved. We hypothesized that increasing stand density, drought and the presence of Dr. confusus and Armillaria spp. are threatening Colorado spruce-fir forests. Understanding the factors that are involved in subalpine fir mortality are important for management.

Avian relationships with bark beetle outbreaks and underlying mechanisms in lodgepole pine and spruce-fir forests of Colorado

Bark beetle (Dendroctonus spp.) outbreaks have historically shaped the structure and function of western North American conifer forests by contributing to heterogeneous conditions needed to support various wildlife species. Previous studies of beetle impacts have primarily focused on pine-dominated systems within 1–6 years of outbreak, limiting our knowledge for informing wildlife habitat management to a relatively short timeframe and narrow range of forest types. Increases in extent and severity of outbreaks since 1900, caused in part by anthropogenic climate warming and forest management, elevates the value of understanding how bark beetle outbreaks impact wildlife populations. Our objectives were (1) to evaluate species and community relationships with outbreak severity (percent conifer mortality) and years since outbreak, (2) to evaluate potential environmental mechanisms underlying these relationships, and (3) to compare patterns across the two forest types for improved general knowledge. We studied avian species occupancy and richness in relation to outbreak conditions using two 18-year chronosequence datasets collected in 2013 and 2014 representing lodgepole forests (predominantly Pinus contorta) and spruce-fir forests (co-dominated by Picea engelmannii and Abies lasiocarpa) throughout Colorado. We employed hierarchical models to account for imperfect detection and spatial dependencies when analyzing population and community patterns apparent in data representing 73 bird species. We found various relationships and potential underlying mechanisms largely but not entirely consistent with a priori hypotheses based on species life histories and previous study. As expected, understory-associated birds related positively with outbreak conditions apparently following understory vegetative release. Consistent with our hypotheses, aerial insectivores and snag-associated species also related positively with outbreak conditions, although our data highlighted understory vegetation more so than canopy structure or snags as potential mechanistic factors. Contrary to our overall hypothesis for canopy-associated species, we did not observe many negative outbreak relationships for this group. Overall, bird species richness increased with years since outbreak, with clear increases in lodgepole forest. In contrast, the data from spruce-fir forest provided statistical support for fewer patterns (i.e., fewer with 90% credible intervals excluding zero), and they supported primarily negative relationships with outbreak severity. Our results suggest potential differences in ecological significance of bark beetle outbreaks in different forest types. At least in lodgepole forest, however, observed patterns were apparently consistent with the purported historical value of bark beetle outbreaks for promoting biodiversity (represented here by birds) despite recent increases in extent and severity.

Snagfall the first decade after severe bark beetle infestation of high-elevation forests in Colorado, USA.

The persistence and fall rate of snags (standing dead trees) generated during bark beetle outbreaks have consequences for the behavior, effects, and suppression of potential wildfires, hazard tree and timber salvage operations, wildlife habitat, and numerous ecosystem processes. However, post-beetle snagfall dynamics are poorly understood in most forest types. We tagged standing live and dead lodgepole pine (Pinus contorta), subalpine fir (Abies lasiocarpa), and Engelmann spruce (Picea engelmannii), including beetle-killed pine snags following the peak of a recent mountain pine bark beetle outbreak in watersheds at the Fraser Experimental Forest in northcentral Colorado and sampled snagfall 10 and 12 years later. Bark beetle attacks began in 2003, peaked by 2006, and killed 78% of overstory lodgepole pine in 133 plots distributed across a range of stand and site conditions. Of those snags, only 17% fell between 2007 and 2018. Most snags broke at ground level, due to butt rot, and were oriented downhill. In contrast, snags that tipped up or snapped off above the ground were oriented with the prevailing winds. Equal numbers of snags fell singly and in multiple-tree groups, and equal numbers remained elevated rather than in contact with the ground. Lodgepole pine snagfall was 1.6-times higher on steep slopes (>40%) where dead pine density was higher, compared to flatter sites. Based on our findings and previous research, we estimate that one-half the beetle-killed lodgepole pine in high-elevation forests such as those at Fraser may fall within 15-20yr of beetle infestation, but that some pine snags are likely to persist for decades longer. Post-outbreak snagfall dynamics create a multiple-decade legacy of bark beetle outbreaks that will persist longer in high-elevation compared to lower-elevation forests.

Using conjoint constitution to understand responses to slow-moving environmental change: the case of mountain pine beetle in north-central Colorado

ABSTRACTA growing body of work exists on the social implications of forest insect disturbances. However, although increasingly explored, there remains a need to further understand social responses to large-scale forest insect disturbances over time. This requires longitudinal investigations into people’s conceptualizations of their changing local environments, associated risk perceptions, and forest management preferences in the context of forest insect disturbance-related changes to the landscape. Within the context of the mountain pine beetle outbreak that affected approximately 3.4 million acres of Colorado forests, this study is a ten-year follow-up to a 2006–2007 study that explored local responses to the outbreak. Using qualitative interviews with 54 informants from nine north-central Colorado communities affected by the outbreak, this paper incorporates a social construction approach known as ‘conjoint constitution’ to examine the interplay between slow-moving biophysical changes to the landscape as a result of the outbreak, communities’ understandings and risk perceptions of these changes, as well as shifts in locals’ thinking about forest management.

Resistance and Representation in a Wildland–Urban Interface Fuels Treatment Conflict: The Case of the Forsythe II Project in the Arapaho-Roosevelt National Forest

Land treatments in wildland–urban interface (WUI) areas are highly visible and subject to public scrutiny and possible opposition. This study examines a contested vegetation treatment—Forsythe II—in a WUI area of the Arapaho-Roosevelt National Forest in Colorado. An initial phase of the research found vocal opposition to Forsythe II. The purpose of the present study was to understand how well the resistance narrative represented the broader community in the WUI area affected by the Forsythe II treatments. More than one third (36%) of households responded to a census survey focused on Forsythe II, demographics, wildfire risk perceptions, and variables associated with generic land management activities and place attachment. Overall, while public opposition to Forsythe II has resulted in a nearly 25% reduction in the project’s size, the survey data demonstrate that just over a quarter of respondents (27%) opposed or strongly opposed the Forsythe II project, and the majority of survey respondents reported broad support for forest management approaches similar to those detailed in the project plans. Notably, a similar portion (28%) did not report an opinion on the project. Results include a systematic comparison of opinion/no opinion respondents.

Limitations to recovery following wildfire in dry forests of southern Colorado and northern New Mexico, USA.

Climate warming is contributing to increases in wildfire activity throughout the western United States, leading to potentially long-lasting shifts in vegetation. The response of forest ecosystems to wildfire is thus a crucial indicator of future vegetation trajectories, and these responses are contingent upon factors such as seed availability, interannual climate variability, average climate, and other components of the physical environment. To better understand variation in resilience to wildfire across vulnerable dry forests, we surveyed conifer seedling densities in 15 recent (1988-2010) wildfires and characterized temporal variation in seed cone production and seedling establishment. We then predicted postfire seedling densities at a 30-m resolution within each fire perimeter using downscaled climate data, monthly water balance models, and maps of surviving forest cover. Widespread ponderosa pine (Pinus ponderosa) seed cone production occurred at least twice following each fire surveyed, and pulses of conifer seedling establishment coincided with years of above-average moisture availability. Ponderosa pine and Douglas-fir (Pseudotsuga menziesii) seedling densities were higher on more mesic sites and adjacent to surviving trees, though there were also important interspecific differences, likely attributable to drought and shade tolerance. We estimated that postfire seedling densities in 42% (for ponderosa pine) and 69% (for Douglas-fir) of the total burned area were below the lowest reported historical tree densities in these forests. Spatial models demonstrated that an absence of mature conifers (particularly in the interior of large, high-severity patches) limited seedling densities in many areas, but 30-yr average actual evapotranspiration and climatic water deficit limited densities on marginal sites. A better understanding of the limitations to postfire forest recovery will refine models of vegetation dynamics and will help to improve strategies of adaptation to a warming climate and shifting fire activity.

Comprehensive comparative economic evaluation of woody biomass energy from silvicultural fuel treatments

Fuel treatments are used in overstocked, fire-prone forests to alter wildfire behavior and reduce fire risk. Some of the benefits they provide are not captured in markets, and therefore represent unaccounted environmental externalities that can lead to inefficient decision making. This study uses a replicable method to integrate market and nonmarket economic values into a comprehensive economic evaluation of fuel treatment and bioenergy production using a case study of ponderosa pine and mixed-conifer forests in Colorado's wildland-urban interface. Treatment costs and people's willingness to pay for better forest health, lower likelihood of wildfire, improved air quality, and expanded renewable energy production are incorporated into techno-economic analysis of biopower production. Results show that fuel treatments are likely to be undervalued when evaluated strictly on a financial basis. Under the standard practice of disposing of treatment residues through pile-burning, net present value (NPV) of fuel treatment on 138,034 ha over 20 years is -$275 million, without consideration of nonmarket benefits. If nonmarket benefits associated with forest health, wildfire likelihood and air quality are included, NPV improves to -$116 million. Without the consideration of nonmarket benefits, when treatment resides are used for biopower production, NPV is -$178 million, with net cost savings compared to pile burning attributable to reduced biomass disposal costs and electricity revenue. Accounting for additional air quality benefits and nonmarket value associated with renewable energy, the bioenergy scenario improves NPV to -$25 million, with 27.7% of outcomes having positive NPV. The impact of additional nonmarket values and potential revenues from timber harvest are discussed, and are likely to make mean NPV positive for this scenario.

Economic and Environmental Optimization of the Forest Supply Chain for Timber and Bioenergy Production from Beetle-Killed Forests in Northern Colorado

Harvesting mountain pine beetle-infested forest stands in the northern Colorado Rocky Mountains provides an opportunity to utilize otherwise wasted resources, generate net revenues, and minimize greenhouse gas (GHG) emissions. Timber and bioenergy production are commonly managed separately, and their integration is seldom considered. Yet, degraded wood and logging residues can provide a feedstock for bioenergy, while the sound wood from beetle-killed stands can still be used for traditional timber products. In addition, beneficial greenhouse gas emission (GHG) savings are often realized only by compromising net revenues during salvage harvest where beetle-killed wood has a relatively low market value and high harvesting cost. In this study we compared Sequential and Integrated decision-making scenarios for managing the supply chain from beetle-killed forest salvage operations. In the Sequential scenario, timber and bioenergy production was managed sequentially in two separate processes, where salvage harvest was conducted without considering influences on or from bioenergy production. Biomass availability was assessed next as an outcome from timber production managed to produce bioenergy products. In the Integrated scenario, timber and bioenergy production were managed jointly, where collective decisions were made regarding tree salvage harvest, residue treatment, and bioenergy product selection and production. We applied a multi-objective optimization approach to integrate the economic and environmental objectives of producing timber and bioenergy, and measured results by total net revenues and total net GHG emission savings, respectively. The optimization model results show that distinctively different decisions are made in selecting the harvesting system and residue treatment under the two scenarios. When the optimization is fully economic-oriented, 49.6% more forest areas are harvested under the Integrated scenario than the Sequential scenario, generating 12.3% more net revenues and 50.5% more net GHG emission savings. Comparison of modelled Pareto fronts also indicate the Integrated decision scenario provides more efficient trade-offs between the two objectives and performs better than the Sequential scenario in both objectives.