Aim: Biotic disturbances (BD, including insects, pathogens and wildlife herbivory) can alter forest structure and the capability of forests to deliver ecosystem services. Impact assessments, however, ...are limited by the lack of reliable and timely disturbance data at large spatial scales. This review synthesizes empirical data on the magnitude and distribution of spatiotemporal impacts of BD. Location: Northern Hemisphere. Methods: Based on large-scale, multi-year BD data sets, covering c. 46% of the global forest, we calculated annual disturbance fractions Df (percentage of forest area affected) and their inter-annual variability at a grid cell resolution of 1°. The impact of BD on forest carbon pools was determined by overlaying Df with data on forest cover and carbon density. Results: Overall, 43.9 million hectares (Mha) (Df=2.6%) of forests were affected annually by BD, particularly by insects (36.5 Mha, Df=2.2%). Our synthesis demonstrates that fractions affected by BD (1) vary greatly over space and time, mainly in response to ephemeral bark beetle and defoliator outbreaks, (2) show temporal trends that are inconsistent across regions, yet are largely increasing over recent decades, and (3) are substantially higher than Df caused by fire and other abiotic disturbances. Tree mortality was estimated over an area of 3.3 Mha year⁻¹ (medium estimate which assumed mortality at 7.5% of the affected area), with associated committed carbon fluxes from living biomass to litter and the atmosphere at 129.9 Mt C year⁻¹. Main conclusions: BD are key drivers of forest dynamics, making a contribution to tree mortality of a similar magnitude to fire. Despite inherent uncertainties, the data reported can be used to improve the representation of BD in global ecosystem models. Our findings call for future forest monitoring approaches to provide accessible, precise and consistent data on the occurrence and severity of BD which are harmonized across jurisdictions.
Outbreaks of aggressive bark beetle species cause widespread tree mortality, affecting timber production, wildlife habitat, wildfire, forest composition and structure, biogeochemical cycling, and ...biogeophysical processes. As a result, agencies responsible for forest management in the United States and British Columbia conduct aerial surveys to map these forest disturbances. Here we combined aerial surveys from British Columbia (2001-2010) and the western conterminous United States (1997-2010), produced 1-km
2
grids of the area of crown mortality from bark beetle attack, and analyzed spatial and temporal patterns. We converted aerial-survey polygon data for each combination of host type and bark beetle species available in the western United States, and for each bark beetle species available in British Columbia. We converted affected area (which includes live and killed trees) to mortality area (crown area of killed trees) using species-specific crown diameters and the number (U.S.) or percentage (British Columbia) of killed trees. In the United States we also produced an upper estimate of mortality area by forcing the mortality area to match that from high-resolution imagery in Idaho, Colorado, and New Mexico. Resulting adjustment factors of 3.7-20.9 illustrate the underestimate of mortality by the U.S. aerial surveys. The upper estimate, which we suggest is more realistic, better matched the spatial patterns and severity of the British Columbia mortality area. Cumulative mortality area from all bark beetles was 5.46 Mha in British Columbia in 2001-2010 and 0.47-5.37 Mha (lower and upper estimate) in the western conterminous United States during 1997-2010. We note that we report year of detection here; studies that consider year of tree mortality should shift the time series back one year. We conclude by discussing uses and limitations of these data in ecological studies, including uncertainties associated with assumptions in the methods, lack of complete coverage by surveys, and the subjective nature of the survey databases.
91 I. 91 II. 92 III. 92 IV. 92 V. 94 VI. 96 96 References 96 SUMMARY: Recently, widespread piñon pine die‐off occurred in the southwestern United States. Here we synthesize observational studies of ...this event and compare findings to expected relationships with biotic and abiotic factors. Agreement exists on the occurrence of drought, presence of bark beetles and increased mortality of larger trees. However, studies disagree about the influences of stem density, elevation and other factors, perhaps related to study design, location and impact of extreme drought. Detailed information about bark beetles is seldom reported and their role is poorly understood. Our analysis reveals substantial limits to our knowledge regarding the processes that produce mortality patterns across space and time, indicating a poor ability to forecast mortality in response to expected increases in future droughts.
Wildfires shape the distribution and structure of vegetation across the inland northwestern United States. However, fire activity is expected to increase given the current rate of climate change, ...with uncertain outcomes. A fire impact that has not been widely addressed is the development of unburned islands; areas within the fire perimeter that do not burn. These areas function as critical ecological refugia for biota during or following wildfires, but they have been largely ignored in methodological studies of remote sensing assessing fire severity under the assumption that they will be detected by algorithms for delineating fire perimeters. Our objective was to develop a model for classifying unburned areas within wildfire perimeters using moderate resolution satellite (i.e., Landsat) and ancillary data. We performed field observations at locations that were unburned or lightly burned within the perimeters of 12 wildfires that burned in 2012 and 2014, and augmented this with field data previously acquired on another seven wildfires across the study region. We used randomForest and classification trees to separate burned from unburned locations with high overall classification accuracy (91.7% and 89.2%, for randomForest and classification tree methods respectively). Classification accuracy was significantly higher than the semi-automated classification products from the Monitoring Trends in Burn Severity (MTBS) program. After application of the most parsimonious and accurate classification tree model, we found that the average unburned proportion of the fires was 20% with high variability between fires (standard deviation: 16.4%). The total area of unburned islands in non-forested areas was significantly higher than the total unburned area in forested areas. Accurate detection and delineation of unburned areas is increasingly critical, as some of these unburned areas contain habitat (i.e., wildfire refugia) that are crucial for maintaining biodiversity and functioning of ecosystems, particularly given observed and projected anthropogenic climate change.
•Unburned areas can contain important habitat, yet have received little attention.•Field data were used to classify these areas using Landsat and ancillary data.•We found high classification accuracy using classification trees and randomForest.•Average unburned area proportion across the 19 fires was 20%.•Using multi-year post-fire Landsat scenes produced the most accurate models.
Bark beetle epidemics result in tree mortality across millions of hectares in North America. However, few studies have quantified impacts on carbon (C) cycling. In this study, we quantified the ...immediate response and subsequent trajectories of stand-level aboveground tree C stocks and fluxes using field measurements and modeling for a location in central Idaho, USA that experienced an outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins). We measured tree characteristics in lodgepole pine (Pinus contorta) plots spanning a range of structure and mortality conditions. We then initialized the forest vegetation simulator, an individual tree-based model, with these measurements and simulated the response of aboveground production of C fluxes as well as trajectories of C stocks and fluxes in the coming decades. Mountain pine beetles killed up to 52% of the trees within plots, with more larger trees killed. C stocks in lodgepole pine were reduced by 31-83% following the outbreak, and plot-level C fluxes decreased 28-73%. Modeled C stocks increased nearly continuously following the infestation, recovering to preoutbreak levels in 25 years or less. Simulated aboveground tree C fluxes increased following the immediate postoutbreak decrease, then subsequently declined. Substantial variability of C stocks and fluxes among plots resulted from the number and size of killed and surviving trees. Our study illustrates that bark beetle epidemics alter forest C cycling unlike stand-replacement wildfires or clear-cut harvests, due in part to incomplete mortality coupled with the preference by beetles for larger trees. The dependency of postoutbreak C stocks and fluxes on stand structure suggests that C budget models and studies in areas experiencing mountain pine beetle disturbances need to include size distribution of trees for the most accurate results.
Recent bark beetle epidemics have caused regional‐scale tree mortality in many snowmelt‐dominated headwater catchments of western North America. Initial expectations of increased streamflow have not ...been supported by observations, and the basin‐scale response of annual streamflow is largely unknown. Here we quantified annual streamflow responses during the decade following tree die‐off in eight infested catchments in the Colorado River headwaters and one nearby control catchment. We employed three alternative empirical methods: (i) double‐mass comparison between impacted and control catchments, (ii) runoff ratio comparison before and after die‐off, and (iii) time‐trend analysis using climate‐driven linear models. In contrast to streamflow increases predicted by historical paired catchment studies and recent modeling, we did not detect streamflow changes in most basins following die‐off, while one basin consistently showed decreased streamflow. The three analysis methods produced generally consistent results, with time‐trend analysis showing precipitation was the strongest predictor of streamflow variability (R2 = 74–96%). Time‐trend analysis revealed post‐die‐off streamflow decreased in three catchments by 11–29%, with no change in the other five catchments. Although counter to initial expectations, these results are consistent with increased transpiration by surviving vegetation and the growing body of literature documenting increased snow sublimation and evaporation from the subcanopy following die‐off in water‐limited, snow‐dominated forests. The observations presented here challenge the widespread expectation that streamflow will increase following beetle‐induced forest die‐off and highlight the need to better understand the processes driving hydrologic response to forest disturbance.
Key Points:
Streamflow did not increase as predicted
Three empirical methods produced consistent results
Weak, variable streamflow response is consistent with recent process literature
High temperatures and severe drought contributed to extensive tree mortality from fires and bark beetles during the 2000s in parts of the western continental United States. Several states in this ...region have greenhouse gas (GHG) emission targets and would benefit from information on the amount of carbon stored in tree biomass killed by disturbance. We quantified mean annual tree mortality from fires, bark beetles, and timber harvest from 2003-2012 for each state in this region. We estimated tree mortality from fires and beetles using tree aboveground carbon (AGC) stock and disturbance data sets derived largely from remote sensing. We quantified tree mortality from harvest using data from US Forest Service reports. In both cases, we used Monte Carlo analyses to track uncertainty associated with parameter error and temporal variability. Regional tree mortality from harvest, beetles, and fires (MORTH+B+F) together averaged 45.8 ± 16.0 Tg AGC yr−1 (±95% confidence interval), indicating a mortality rate of 1.10 ± 0.38% yr−1. Harvest accounted for the largest percentage of MORTH+B+F (∼50%), followed by beetles (∼32%), and fires (∼18%). Tree mortality from harvest was concentrated in Washington and Oregon, where harvest accounted for ∼80% of MORTH+B+F in each state. Tree mortality from beetles occurred widely at low levels across the region, yet beetles had pronounced impacts in Colorado and Montana, where they accounted for ∼80% of MORTH+B+F. Tree mortality from fires was highest in California, though fires accounted for the largest percentage of MORTH+B+F in Arizona and New Mexico (∼50%). Drought and human activities shaped regional variation in tree mortality, highlighting opportunities and challenges to managing GHG emissions from forests. Rising temperatures and greater risk of drought will likely increase tree mortality from fires and bark beetles during coming decades in this region. Thus, sustained monitoring and mapping of tree mortality is necessary to inform forest and GHG management.
Insect outbreaks are major forest disturbances, causing tree mortality across millions of ha in North America. Resultant spatial and temporal patterns of tree mortality can profoundly affect ...ecosystem structure and function. In this study, we evaluated the classification accuracy of multispectral imagery at different spatial resolutions. We used four-band digital aerial imagery (30-cm spatial resolution and aggregated to coarser resolutions) acquired over lodgepole pine-dominated stands in central Colorado recently attacked by mountain pine beetle. Classes of interest included green trees and multiple stages of post-insect attack tree mortality, including dead trees with red needles (“red-attack”), dead trees without needles (“gray-attack”), and non-forest. The 30-cm resolution image facilitated delineation of trees located in the field, which were used in image classification. We employed a maximum likelihood classifier using the green band, Red–Green Index (RGI), and Normalized Difference Vegetation Index (NDVI). Pixel-level classification accuracies using this imagery were good (overall accuracy of 87%, kappa
=
0.84), although misclassification occurred between a) sunlit crowns of live (green) trees and herbaceous vegetation, and b) sunlit crowns of gray- and red-attack trees and bare soil. We explored the capability of coarser resolution imagery, aggregated from the 30-cm resolution to 1.2, 2.4, and 4.2
m, to improve classification accuracy. We found the highest accuracy at the 2.4-m resolution, where reduction in omission and commission errors and increases in overall accuracy (90%) and kappa (0.88) were achieved, and visual inspection indicated improved mapping. Pixels at this resolution included more shadow in forested regions than pixels in finer resolution imagery, thereby reducing forest canopy reflectance and allowing improved separation between forest and non-forest classes, yet were fine enough to resolve individual tree crowns better than the 4.2-m imagery. Our results illustrate that a classification of an image with a spatial resolution similar to the area of a tree crown outperforms that of finer and coarser resolution imagery for mapping tree mortality and non-forest classes. We also demonstrate that multispectral imagery can be used to separate multiple postoutbreak attack stages (i.e., red-attack and gray-attack) from other classes in the image.
► We evaluated methods for mapping multiple stages of insect-caused tree mortality. ► We classified four-band aerial multispectral imagery (30-cm spatial resolution). ► The imagery was also classified using aggregated imagery (1.2
m, 2.4
m, and 4.2
m). ► We find that the 2.4-m resolution classification outperformed other classifications. ► High accuracies were achieved for green, red, gray tree stages and nonforest classes.
Recent, large-scale outbreaks of bark beetle infestations have affected millions of hectares of forest in western North America, covering an area similar in size to that impacted by fire. Bark ...beetles kill host trees in affected areas, thereby altering water supply, carbon storage, and nutrient cycling in forests; for example, the timing and amount of snow melt may be substantially modified following bark beetle infestation, which impacts water resources for many western US states. The quality of water from infested forests may also be diminished as a result of increased nutrient export. Understanding the impacts of bark beetle outbreaks on forest ecosystems is therefore important for resource management. Here, we develop a conceptual framework of the impacts on coupled biogeophysical and biogeochemical processes following a mountain pine beetle (
Dendroctonus ponderosae
) outbreak in lodgepole pine (
Pinus contorta
Douglas var
latifolia
) forests in the weeks to decades after an infestation, and highlight future research needs and management implications of this widespread disturbance event.
Snow covers Arctic and boreal regions (ABRs) for approximately 9 months of the year, thus snowscapes dominate the form and function of tundra and boreal ecosystems. In recent decades, Arctic warming ...has changed the snowcover's spatial extent and distribution, as well as its seasonal timing and duration, while also altering the physical characteristics of the snowpack. Understanding the little studied effects of changing snowscapes on its wildlife communities is critical. The goal of this paper is to demonstrate the urgent need for, and suggest an approach for developing, an improved suite of temporally evolving, spatially distributed snow products to help understand how dynamics in snowscape properties impact wildlife, with a specific focus on Alaska and northwestern Canada. Via consideration of existing knowledge of wildlife-snow interactions, currently available snow products for focus region, and results of three case studies, we conclude that improving snow science in the ABR will be best achieved by focusing efforts on developing data-model fusion approaches to produce fit-for-purpose snow products that include, but are not limited to, wildlife ecology. The relative wealth of coordinated in situ measurements, airborne and satellite remote sensing data, and modeling tools being collected and developed as part of NASA's Arctic Boreal Vulnerability Experiment and SnowEx campaigns, for example, provide a data rich environment for developing and testing new remote sensing algorithms and retrievals of snowscape properties.
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