•Estimated bark beetle-caused tree mortality in the western US from aerial surveys.•A range of results from different methods indicates some uncertainty.•Annual Westwide tree mortality continued to ...be high in recent years.•Multiple severe outbreaks in different locations contributed to mortality.
Bark beetle (Curculionidae, Scolytinae) outbreaks have been extensive and severe across the western United States in recent decades, and assessments of tree mortality are critical for documenting patterns and increasing understanding of drivers and impacts. Aerial surveys have produced a rich data set that includes damage severity, year and location of damage, and beetle and tree species. Here we update a data set of bark beetle-caused tree mortality for the western United States to include five additional years (now 1997–2018) and use these data to characterize recent outbreaks and compare with earlier tree mortality. We estimated “mortality area” (MA), the canopy area of beetle-killed trees and a more accurate representation of outbreak impacts than “affected area”, as well as the number of killed trees. Recently, the US Forest Service changed survey approaches, creating challenges for linking observations from the old and new approaches. We compared four methods to harmonize these approaches to produce consistent time series of tree mortality. General similarity of MA occurred in several methods; however, the range of results indicated some uncertainty. Based on limited analyses and a desire to be conservative in damage estimates, we suggest that the FHPR1-R4 method, which produced intermediate mortality among the methods, is most realistic for representing tree mortality from bark beetles. Using this recommended method, we found that bark beetles caused 4.3 Mha of MA and 3.8x109 killed trees when summed across space, time, and bark beetle/host combination (range among methods: 1.5–7.4 Mha and 1.2–6.3 x 109 killed trees). This total mortality area was 4.7% of forest area in the western United States; 28% of this mortality occurred in 2013–2018. Annual tree mortality remained high recently, with values comparable to earlier years, and was a result of combinations of outbreaks of different beetle species in different regions with different timing. Bark beetles continue to be agents of significant forest disturbance in the western United States. Given a range of mortality area results from the different methods, we encourage further evaluation of estimates using independent observations.
The persistence of wildlife species in fire‐prone ecosystems is under increasing pressure from global change, including alterations in fire regimes caused by climate change. However, unburned islands ...might act to mitigate negative effects of fire on wildlife populations by providing habitat in which species can survive and recolonize burned areas. Nevertheless, the characteristics of unburned islands and their role as potential refugia for the postfire population dynamics of wildlife species remain poorly understood.
We used a newly developed unburned island database of the northwestern United States from 1984 to 2014 to assess the postfire response of the greater sage‐grouse (Centrocercus urophasianus), a large gallinaceous bird inhabiting the sagebrush ecosystems of North America, in which wildfires are common. Specifically, we tested whether prefire and postfire male attendance trends at mating locations (leks) differed between burned and unburned areas, and to what extent postfire habitat composition at multiple scales could explain such trends.
Using time‐series of male counts at leks together with spatially explicit fire history information, we modeled whether male attendance was negatively affected by fire events. Results revealed that burned leks often exhibit sustained decline in male attendance, whereas leks within unburned islands or >1.5 km away from fire perimeters tend to show stable or increasing trends.
Analyses of postfire habitat composition further revealed that sagebrush vegetation height within 0.8 km around leks, as well elevation within 0.8 km, 6.4 km, and 18 km around leks, had a positive effect on male attendance trends. Moreover, the proportion of the landscape with cheatgrass (Bromus tectorum) cover >8% had negative effects on male attendance trends within 0.8 km, 6.4 km, and 18 km of leks, respectively.
Synthesis and applications. Our results indicate that maintaining areas of unburned vegetation within and outside fire perimeters may be crucial for sustaining sage‐grouse populations following wildfire. The role of unburned islands as fire refugia requires more attention in wildlife management and conservation planning because their creation, protection, and maintenance may positively affect wildlife population dynamics in fire‐prone ecosystems.
Unburned islands of vegetation within fire perimeters might mitigate the negative effects of wildfire on wildlife populations in fire‐prone ecosystems, but they remain poorly understood. In this article, we researched and found out that unburned islands allow for the persistence of greater sage‐grouse after fire, thus acting as potential fire refugia. This is important for management because it suggests that creating, protecting, and enhancing unburned islands within fire perimeters may positively impact wildlife populations in these ecosystems.
Wildfire behavior depends on the type, quantity, and condition of fuels, and the effect that bark beetle outbreaks have on fuels is a topic of current research and debate. Remote sensing can provide ...estimates of fuels across landscapes, although few studies have estimated surface fuels from remote sensing data. Here we predicted and mapped field-measured canopy and surface fuels from light detection and ranging (lidar) and Landsat time series explanatory variables via random forest (RF) modeling across a coniferous montane forest in Colorado, USA, which was affected by mountain pine beetles (Dendroctonus ponderosae Hopkins) approximately six years prior. We examined relationships between mapped fuels and the severity of tree mortality with correlation tests. RF models explained 59%, 48%, 35%, and 70% of the variation in available canopy fuel, canopy bulk density, canopy base height, and canopy height, respectively (percent root-mean-square error (%RMSE) = 12–54%). Surface fuels were predicted less accurately, with models explaining 24%, 28%, 32%, and 30% of the variation in litter and duff, 1 to 100-h, 1000-h, and total surface fuels, respectively (%RMSE = 37–98%). Fuel metrics were negatively correlated with the severity of tree mortality, except canopy base height, which increased with greater tree mortality. Our results showed how bark beetle-caused tree mortality significantly reduced canopy fuels in our study area. We demonstrated that lidar and Landsat time series data contain substantial information about canopy and surface fuels and can be used for large-scale efforts to monitor and map fuel loads for fire behavior modeling at a landscape scale.
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•Small (1 m2 ≤ area ≤ 900 m2) unburned patches occupied 5% of the area burned.•Areas with all fire severity classifications contained small unburned patches.•Tree survival was higher ...when trees were rooted in an unburned patch.•Small tree survival was higher towards the interior of unburned patches.
Fire refugia – the unburned areas within fire perimeters – are important to the survival of many taxa through fire events and the revegetation of post-fire landscapes. Previous work has shown that species use and benefit from small-scale fire refugia (1–1000 m2), but our understanding of where and how fire refugia form is largely limited to the scale of remotely sensed data (i.e., 900 m2 Landsat pixels). To examine the causes and consequences of small fire refugia, we field-mapped all unburned patches ≥1 m2 within a contiguous 25.6 ha forest plot that burned at generally low-to-moderate severity in the 2013 Yosemite Rim Fire, California, USA. Within the Yosemite Forest Dynamics Plot (YFDP), there were 685 unburned patches ≥1 m2, covering a total unburned area of 12,597 m2 (4.9%). Small refugia occurred in all fire severity classifications. Random forest models showed that the proportion of unburned area of 100 m2 grid cells corresponded to pre-fire density and basal area of trees, distance to the nearest stream, and immediate fire mortality, but the relationships were complex and model accuracy was variable. From a pre-fire population of 34,061 total trees ≥1 cm diameter at breast height (1.37 m; DBH) within the plot (1330 trees ha−1), trees of all five of the most common species and those DBH < 30 cm had higher immediate survival rates if their boles were wholly or partially within an unburned patch (P ≤ 0.001). Trees 1 cm ≤ DBH < 10 cm that survived were located closer to the center of the unburned patch than the edge (mean 1.1 m versus 0.6 m; ANOVA; P ≤ 0.001). Four-year survival rates for trees 1 cm ≤ DBH < 10 cm were 58.8% within small refugia and 2.7% in burned areas (P ≤ 0.001). Species richness and the Shannon Diversity Index were associated with unburned quadrats in NMDS ordinations 3 years post-fire. Burn heterogeneity in mixed-conifer forests likely exists at all scales and small refugia contribute to diversity of forest species and structures. Thus, managers may wish to consider scales from 1-m2 to the landscape when designing fuel reduction prescriptions. The partial predictability of refugia location suggests that further work may lead to predictive models of refugial presence that have considerable potential to preserve ecological function or human habitation in fire-frequent forests.
Extensive bark beetle outbreaks have recently occurred in western North American forests, resulting in overstory tree mortality across millions of hectares. Annual aerial surveys are currently used ...to operationally monitor bark beetle induced tree mortality, though this method is subjective and can exclude some forest areas. Daily Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data offer a potential alternative means to develop regional tree mortality maps. Accurate methods using such data could aid natural resource managers in surveys of forests with frequent overstory mortality, helping to prioritize forest treatment and restoration activities. This paper discusses a study to test the potential of using MODIS data to detect tree mortality. We developed and tested an approach to use 250-m resolution MODIS Normalized Difference Vegetation Index (NDVI) data products collected during a mountain pine beetle (MPB) outbreak and related tree mortality event in the northern Rocky Mountains of Colorado, USA. The 94 km2 study area is predominantly lodgepole pine forest with most of the MPB-caused mortality occurring between 2003 and 2008. We used a 2.4-m forest conditions map from 2008 aerial multispectral imagery to calculate percentage of mortality within 240-m pixels for use as reference data. Using either daily or 16-day products, MODIS NDVI change products were calculated for 2008 versus either 2000 or 2003 baselines. MODIS change products were used as predictors in linear regression analysis to assess correlation between MODIS data and the aerial percent forest mortality map. Depending on the MODIS product, linear regression analyses yielded r2 values ranging from 0.362 to 0.544 without outliers removed and from 0.406 to 0.570 with extreme outliers removed. Daily MODIS NDVI products from 2003 and 2008 were used with exponential regression to improve the r2 to 0.593. The project showed some MODIS NDVI data potential for mapping percent tree mortality in forests subjected to regional bark beetle outbreaks and severe drought.
•Unburned areas in fires are essential for allowing fire-sensitive species to persist.•Landcover and suitability of unburned areas for wildlife vary considerably.•We present a method to assess the ...relative quality of unburned areas for wildlife.•Higher quality unburned areas were identified for conservation planning.
Wildfires have important effects on wildlife habitat. Within fire perimeters, ‘fire refugia’ are unburned or less severely burned areas that contain residual pre-fire habitat structures and facilitate species persistence following fire. However, fire refugia may vary in their relative quality. The ability to evaluate the relative quality of fire refugia for wildlife habitat may help efficiently allocate resources for post-fire conservation planning, especially for at-risk species. Using spatial overlays of habitat models and burn severity data, we developed the refugia index, a flexible multi-scale, multi-criteria fuzzy logic model to assess the value of unburned areas for wildlife habitat affected by wildfires and other disturbances. We conducted a case study of the northern spotted owl (Strix occidentalis caurina) and applied our method to 4278 unburned areas in 15 fires that each burned > 400 ha of spotted owl nesting/roosting habitat in the eastern Cascade Mountains, U.S.A. In our study area, unburned areas represented ~ 1.7% of the area within the fire perimeters. Most fire refugia were low quality, because unburned areas were small relative to the desired nest patch size of spotted owls and contained little suitable nesting/roosting habitat. However, each fire contained a few high-quality fire refugia, because they co-occurred with suitable nesting/roosting habitat and were surrounded by desirable habitat characteristics for spotted owls. Our method illustrates how land managers can use readily available spatial data and analysis tools to understand fire effects on species’ habitat (e.g., habitat lost) and prioritize areas for post-fire conservation and management.
Temperatures in arctic-boreal regions are increasing rapidly and pose significant challenges to moose (
), a heat-sensitive large-bodied mammal. Moose act as ecosystem engineers, by regulating forest ...carbon and structure, below ground nitrogen cycling processes, and predator-prey dynamics. Previous studies showed that during hotter periods, moose displayed stronger selection for wetland habitats, taller and denser forest canopies, and minimized exposure to solar radiation. However, previous studies regarding moose behavioral thermoregulation occurred in Europe or southern moose range in North America. Understanding whether ambient temperature elicits a behavioral response in high-northern latitude moose populations in North America may be increasingly important as these arctic-boreal systems have been warming at a rate two to three times the global mean.
We assessed how Alaska moose habitat selection changed as a function of ambient temperature using a step-selection function approach to identify habitat features important for behavioral thermoregulation in summer (June-August). We used Global Positioning System telemetry locations from four populations of Alaska moose (
= 169) from 2008 to 2016. We assessed model fit using the quasi-likelihood under independence criterion and conduction a leave-one-out cross validation.
Both male and female moose in all populations increasingly, and nonlinearly, selected for denser canopy cover as ambient temperature increased during summer, where initial increases in the conditional probability of selection were initially sharper then leveled out as canopy density increased above ~ 50%. However, the magnitude of selection response varied by population and sex. In two of the three populations containing both sexes, females demonstrated a stronger selection response for denser canopy at higher temperatures than males. We also observed a stronger selection response in the most southerly and northerly populations compared to populations in the west and central Alaska.
The impacts of climate change in arctic-boreal regions increase landscape heterogeneity through processes such as increased wildfire intensity and annual area burned, which may significantly alter the thermal environment available to an animal. Understanding habitat selection related to behavioral thermoregulation is a first step toward identifying areas capable of providing thermal relief for moose and other species impacted by climate change in arctic-boreal regions.
A warming climate, fire exclusion, and land cover changes are altering the conditions that produced historical fire regimes and facilitating increased recent wildfire activity in the northwestern ...United States. Understanding the impacts of changing fire regimes on forest recruitment and succession, species distributions, carbon cycling, and ecosystem services is critical, but challenging across broad spatial scales. One important and understudied aspect of fire regimes is the unburned area within fire perimeters; these areas can function as fire refugia across the landscape during and after wildfire by providing habitat and seed sources. With increasing fire activity, there is speculation that fire intensity and combustion completeness are also increasing, which we hypothesized would yield smaller unburned proportions and changes in fire refugia patterns. We sought to determine (1) whether the unburned proportion of wildfires decreased across the northwestern United States from 1984 to 2014 and (2) whether patterns of unburned patches were significantly different across ecoregions, land cover type, and land ownership. We utilized a Landsat‐derived geospatial database of unburned islands within 2298 fires across the inland northwestern USA (including eastern Washington, eastern Oregon, and Idaho) from 1984 to 2014. We evaluated patterns of the total unburned proportion and spatial patterns of unburned patches of the fires across different ecoregions, land cover types, and land ownership. We found that unburned area proportion exhibited no change over the three decades, suggesting that recent trends in area burned and overall severity have not affected fire refugia, important to post‐fire ecosystem recovery. There were ecoregional differences in mean unburned proportion, patch area, and patch density, suggesting influences of vegetation and topography on the formation of unburned area. These foundation findings suggest that complex drivers control unburned island formation, and yield insights to locate potential important fire refugia across the inland northwest.
•Avian richness changes little after beetle outbreaks in lodgepole pine forests.•Species richness does not fully describe avian community change after outbreaks.•Cavity nesting birds became more ...common following beetle outbreaks.•Shrub nesting species had consistently lower occurrence rates following outbreaks.•Consider snag density and slash retainment in post-outbreak management of forests.
Disturbance is an integral component of forest ecosystems. Therefore, contemporary changes to the frequency and severity of disturbances in forests may have lasting ecological consequences. In recent decades, widespread mountain pine beetle (MPB; Dendroctonus ponderosae) outbreaks have contributed more to total tree mortality than other disturbances such as wildfire. Past studies have used a broad range of measurements to evaluate the local effects of beetle outbreaks on birds (e.g., dead trees per acre versus percent mortality) and this has led to some challenges and inconsistencies in generalizing results across larger spatial scales. We sought to address these inconsistencies by using a single data source estimating outbreak-related tree mortality across the western United States to better understand the effect of beetle outbreaks on avian communities in lodgepole pine forests. This study uses five years of avian monitoring data (2008–2012) from the Integrated Monitoring in Bird Conservation Regions program, collected across Colorado, Idaho, Montana, and Wyoming. We developed a hierarchical multispecies occupancy model to quantify changes in the forest bird community following beetle outbreaks while accounting for the imperfect detection of species. Mean species richness (N) did not differ between MPB outbreak sites (N = 19.35, 95%CRI = 17.00, 22.01) and sites with no outbreak (N = 20.23, 95%CRI = 16.83, 24.25). Furthermore, we found weak evidence for a community-level response to increasing outbreak severity and time since outbreak, due in part to the variability among species within each of the four nesting guilds analyzed in this study (canopy, cavity, shrub, and ground nesting birds). However, we did find evidence of turnover in community composition with just over 50% of species (28 of 55) having statistically different occurrence rates following MPB outbreaks. Our study underscores the notion that species richness alone does not fully express changes in the forest bird community. Our results also suggest many species, particularly cavity nesters, utilize beetle-killed forests and persist at higher occurrence rates for up to 10 years post-outbreak. This period includes the timeframe when beetle-killed trees are most readily salvageable and emphasizes the need for continued evaluation of snag-retention policies prior to the implementation of post-outbreak management activities.
Forest insect outbreaks have caused and will continue to cause extensive tree mortality worldwide, affecting ecosystem services provided by forests. Remote sensing is an effective tool for detecting ...and mapping tree mortality caused by forest insect outbreaks. In this study, we map insect-caused tree mortality across three coniferous forests in the Western United States for the years 1984 to 2018. First, we mapped mortality at the tree level using field observations and high-resolution multispectral imagery collected in 2010, 2011, and 2018. Using these high-resolution maps of tree mortality as reference images, we then classified moderate-resolution Landsat imagery as disturbed or undisturbed and for disturbed pixels, predicted percent tree mortality with random forest (RF) models. The classification approach and RF models were then applied to time series of Landsat imagery generated with Google Earth Engine (GEE) to create annual maps of percent tree mortality. We separated disturbed from undisturbed forest with overall accuracies of 74% to 80%. Cross-validated RF models explained 61% to 68% of the variation in percent tree mortality within disturbed 30-m pixels. Landsat-derived maps of tree mortality were comparable to vector aerial survey data for a variety of insect agents, in terms of spatial patterns of mortality and annual estimates of total mortality area. However, low-level tree mortality was not always detected. We conclude that our methodology has the potential to generate reasonable estimates of annual tree mortality across large extents.
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