Mountain Pine Beetle-caused Tree Mortality Research Articles

Reconstructing historical outbreaks of mountain pine beetle in lodgepole pine forests in the Colorado Front Range

Regional-scale mountain pine beetle (Dendroctonus ponderosae) outbreaks in the first decade of the 2000s affected millions of hectares of lodgepole pine (Pinus contorta) in western North American forests. In Colorado, 1.4 million ha exhibited high mortality. These events prompted questions about whether historical outbreaks reached the scale of this most recent event. We aimed to reconstruct past mortality events in lodgepole pine forests in the northern Colorado Front Range and to determine whether these were of similar extent to the 2000s outbreak. We identified logs of mountain pine beetle-killed trees based on visually identifiable signs of beetle infestation including lower bole breakage, egg galleries, exit holes, and the presence of blue stain. We collected cross-sections, developed tree ring chronologies and determined death dates through tree ring analysis. We detected five mortality events since the 1860s, including widely distributed mortality in the 1910s that was geographically as extensive as the 2000s outbreak in our study area. Trees killed were on average 232 years of age and 36 cm in diameter. In our study area, it takes about 200 years for a lodgepole pine to reach the size suitable for mountain pine beetle attack. We conclude that mountain pine beetle infestation signs remain useful for identifying mountain pine beetle-caused tree mortality for over a century and that well-distributed mountain pine beetle-caused mortality has occurred in the past in the northern Colorado Front Range. Future reconstructions of bark beetle-caused mortality may benefit from integrating the use of beetle symptomatology with growth releases. The inclusion of stand demography and fire history will present a holistic picture of how disturbance interactions create the mosaic of forest landscapes. Awareness of the disturbance histories in forests and the legacies of past events advances understanding of their ecology and will inform researchers and managers in developing management strategies to foster sustainable delivery of ecosystem services and maintain resiliency as climate change manifests.

Using Satellite Imagery to Evaluate Bark Beetle-Caused Tree Mortality Reported in Aerial Surveys in a Mixed Conifer Forest in Northern Idaho, USA

Bark beetles cause significant tree mortality in western North America. The United States Forest Service coordinates annual insect and disease surveys (IDS) by observers in airplanes to map and quantify the tree mortality caused by beetles. The subjective nature of these surveys means that accuracy evaluation is important for characterizing uncertainty. Furthermore, the metric reported for quantifying tree mortality recently changed (2012–2018 depending in region) from killed trees per acre to percent tree mortality within damage polygons, posing challenges for linking older and newer records. Here we evaluated IDS severity estimates in a beetle-affected forest in northern Idaho, USA using fine-resolution satellite imagery, which permitted greater areal coverage than field data. We first used well-established methods to map beetle-caused tree mortality in two WorldView-2 (WV2) images with a high accuracy relative to field observations. Trees-per-acre measurements within collocated IDS polygons were then converted to percent mortality using three methods and evaluated with the WV2 maps. The overall accuracies for the three methods ranged from 35–38% (for methods that used five percent-mortality classes) and 49–56% (three classes). When IDS and WV2 estimates of mortality severity that were within ±15% of each other were considered accurate, overall accuracies were 71–78%. Within the aerial survey damage polygons, the total mortality area tended to be overestimated relative to WV2. WV2 imagery identified ~50% more mortality across the study region compared with the IDS methods, with most of the difference occurring where damage was low severity or in wilderness areas. Severity of Douglas-fir beetle-caused tree mortality was estimated the most accurately, whereas severity of mountain pine beetle-caused tree mortality was estimated the least accurately. Future studies that control for temporal ambiguity between IDS and satellite imagery, as well as IDS spatial error, might provide better assessments of IDS severity accuracy. Our study increases the usefulness of the rich aerial survey database by providing estimates of uncertainty in the IDS database of tree mortality severity.

Interaction between mountain pine beetle-caused tree mortality and fire behavior in subalpine whitebark pine forests, eastern Sierra Nevada, CA; Retrospective observations

The 1351 ha Walker Fire burned in late August 2015 on the eastern Sierra Nevada, California escarpment from sagebrush steppe and pinyon pine woodlands (2150 m) through mid-elevation forests into subalpine whitebark pine (WBP; 3310 m). Fuels were particularly dry due not only to the season but as a result of a record multi-year drought (2012–2015). The fire burned into stands heavily impacted by a mountain pine beetle outbreak (MPB) during a prior drought (2007–2010). Using plot data, we report 100% mortality due to fire of understory vegetation and trees in pinyon pine, aspen, white fir, and lodgepole pine forest types. Fire-caused tree mortality was lower in Jeffrey pine forests (80%), although 100% of the understory was killed by the fire. For WBP plots in areas that had not been affected by MPB, 100% of understory and tree layers also burned. Where the fire burned through plots within the former MPB-outbreak area, 2 of 15 plots had 100% understory vegetation and trees burned by the fire, while the remaining 13 had varying levels of survivorship. In those plots, pines dead from prior beetle-kill as well as live trees remained unburned, as did understory vegetation. Whereas such “gray-stage” stands have been found to reduce fire severity in other regions and forest types, the observed reduction of surface fire was unexpected. In that fires in the subalpine WBP zone and bark beetle-fire interactions have not been studied in California, these preliminary observations suggest that subalpine forests are vulnerable to fire where continuous fuels exist below them. However, in xeric, Mediterranean landscapes, MPB outbreaks in WBP may reduce subsequent burn severity in unexpected ways. These findings may be important given current concerns for WBP’s rangewide vulnerability.

Elevational shifts in thermal suitability for mountain pine beetle population growth in a changing climate

Future forests are being shaped by changing climate and disturbances. Climate change is causing large-scale forest declines globally, in addition to distributional shifts of many tree species. Because environmental cues dictate insect seasonality and population success, climate change is also influencing tree-killing bark beetles. The mountain pine beetle, Dendroctonus ponderosae, is a major disturbance in Pinus forests of the western US. Using a mechanistic, phenology-based demographic model driven by downscaled daily temperature data, we describe recent and future spatial and temporal thermal suitability for mountain pine beetle population growth in a topographically complex region. Trends in model-predicted growth rates among Global Climate Models were similar and suggest that, relative to future trends, mountain pine beetle population growth within the past 60 years was most optimal at middle elevations and least optimal at the lowest and highest elevations. This trend aligns with observed mountain pine beetle-caused tree mortality that was greatest at middle elevations between 1997 and 2013, as estimated from Aerial Detection Surveys. However, thermal suitability for optimal phenological synchrony was predicted to shift in recent years, and by the end of the century, the best thermal habitats for mountain pine beetle will be at the lowest and highest elevations. Mechanistic demographic models are valuable tools for modelling future thermal regimes that may be both beneficial and maladaptive for mountain pine beetle population growth and subsequent tree mortality.

Carbon Dynamics in Central US Rockies Lodgepole Pine Type after Mountain Pine Beetle Outbreaks

Mountain pine beetle-caused tree mortality has substantially changed live tree biomass in lodgepole pine ecosystems in western North America since 2000. We studied how beetle-caused mortality altered ecosystem carbon (C) stocks and productivity using a central US Rockies age sequence of ecosystem recovery after infestation, augmented with growth-and-yield model simulations. Field measurements showed that total C stocks were reduced by beetle outbreaks, although differences compared with undisturbed stands were not easily distinguished. Simulations with outbreaks showed long-term C losses of 1–30%. Field data indicated that aboveground C productivity was reduced among plots infested within 25 years of measurements, but productivity levels of plots with older infestations were indistinguishable from those of undisturbed plots. Simulations indicated that outbreaks reduced C productivity for 20 – 60 years, but rates recovered such that 100-year postoutbreak averages were similar among infested and undisturbed stands. Simulations also showed that C dynamics are affected by outbreak severity and that C productivity might be stimulated by outbreaks relative to that of undisturbed stands. Outbreaks redistributed C from live (sinks) to dead pools (sources), but slow decomposition of snags combined with recovered tree growth resulted in resilience of aboveground C stocks.

Long-term efficacy of diameter-limit cutting to reduce mountain pine beetle-caused tree mortality in a lodgepole pine forest

Mountain pine beetle, Dendroctonus ponderosae Hopkins, is the most significant mortality agent in pine forests of western North America. Silvicultural treatments that reduce the number of susceptible host trees, alter age and size class distributions, and diversify species composition are considered viable, long-term options for reducing stand susceptibility to mountain pine beetle-caused tree mortality. Short-term efficacy of thinning treatments has been evaluated, but long-term efficacy has not. We evaluated mountain pine beetle-caused lodgepole pine mortality in 2008, ∼28 years after diameterlimit cutting from above that removed the largest diameter lodgepole pines in a Wyoming, USA forest. Following extensive recent mountain pine beetle activity, the partially-cut stands had significantly less mountain pine beetle-caused tree mortality compared to untreated reference stands. These results are similar to observations five years post-treatment, albeit using different reference stands because the original controls were lost to timber harvest. The original management objective was reduced mountain pine beetle-caused tree mortality, and this objective was achieved, lasting for up to 28 years. Despite the reduced mortality among partially-cut stands, however, untreated and treated stands had similar densities of residual live mature lodgepole pine and those in untreated stands had larger average diameters.

Wildfire’s resistance to control in mountain pine beetle-attacked lodgepole pine forests

Concerns about the impacts of mountain pine beetle (Dendroctonus ponderosae Hopkins)-caused tree mortality on wildfire potential in lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) forests have to date largely focused on the potential for extreme fire behaviour, including the development and spread of crown fires. Given that the wildland fire environment in which fire managers and firefighters work is composed of many interacting physical and human factors, viewing crown fire behaviour as the only or even the most important outcome of the tree mortality associated with a mountain pine beetle outbreak is questionable. Proper assessment of wildfire potential entails a broader approach, which requires expanding the concept of wildfire resistance to control to include an analysis of all relevant factors and their interactions. In this paper we describe a holistic concept of analyzing the impacts of mountain pine beetle-caused tree mortality on wildfire potential in lodgepole pine forests on the basis of fire behaviour characteristics, fire suppression operations, and firefighter safety considerations within the framework of three recognizable stages of the approximate time since the initiation of an outbreak (i.e., “red” ∼1 to 5 years, “gray” ∼5 to 15 years, and post-epidemic ∼15+ years).

Surface Fire Intensity Influences Simulated Crown Fire Behavior in Lodgepole Pine Forests with Recent Mountain Pine Beetle-Caused Tree Mortality

Recent bark beetle outbreaks have had a significant impact on forests throughout western North America and have generated concerns about interactions and feedbacks between beetle attacks and fire. However, research has been hindered by a lack of experimental studies and the use of fire behavior models incapable of accounting for the heterogeneous fuel complexes. We populated the Wildland-Urban Interface Fire Dynamics Simulator with data from 11 field sites to investigate the effect of mountain pine beetle (MPB)-caused tree mortality on simulated crown fire behavior across a range of surface fire intensities. Simulations addressed fire behavior during a 1- to 2-year period after the initiation of the outbreak in which some proportion of the trees have been killed but no foliage has yet fallen. The effect of MPB-caused tree mortality on simulated crown fire behavior significantly changed as a function of surface fire intensity. The largest effects of mortality on crown fire behavior occurred at moderate levels of surface fire intensity, whereas diminished effects occurred at low and high levels of surface fire intensities. Our results suggest that increased crown fire potential immediately after bark beetle infestations is dependent on the fire intensity generated by the preoutbreak surface fuels complex. FOR .S CI. 59(4):390 -399.

Impact of Forest Fragmentation on Patterns of Mountain Pine Beetle-Caused Tree Mortality

The current outbreak of mountain pine beetle, Dendroctonus ponderosae Hopkins, has led to extensive tree mortality in British Columbia and the western United States. While the greatest impacts of the outbreak have been in British Columbia, ongoing impacts are expected as the outbreak continues to spread eastward towards Canada’s boreal and eastern pine forests. Successful mitigation of this outbreak is dependent on understanding how the beetle’s host selection behaviour is influenced by the patchwork of tree mortality across the landscape. While several studies have shown that selective mechanisms operate at the individual tree level, less attention has been given to beetles’ preference for variation in spatial forest patterns, namely forest fragmentation, and if such preference changes with changing population conditions. The objective of this study is to explore the influence of fragmentation on the location of mountain pine beetle caused mortality. Using a negative binomial regression model, we tested the significance of a fragmentation measure called the Aggregation Index for predicting beetle-caused tree mortality in the central interior of British Columbia, Canada in 2000 and 2005. The results explain that mountain pine beetle OPEN ACCESS Forests 2013, 4 280 exhibit a density-dependent dynamic behaviour related to forest patterns, with fragmented forests experiencing greater tree mortality when beetle populations are low (2000). Conversely, more contiguous forests are preferred when populations reach epidemic levels (2005). These results reinforce existing findings that bark beetles exhibit a strong host configuration preference at low population levels and that such pressures are relaxed when beetle densities are high.

Dispersal of Warren Root Collar Weevils (Coleoptera: Curculionidae) in Three Types of Habitat

Warren root collar weevil, Hylobius warreni Wood, is a native, flightless insect distributed throughout the boreal forest of North America. It is an emerging problem in young plantings of lodgepole pine, Pinus contorta variety latifolia, in western Canada, where larval feeding can kill young trees by girdling the root collar. Susceptible plantings are becoming more abundant following salvage harvesting and replanting activities in the wake of an ongoing epidemic of mountain pine beetle, Dendroctonus ponderosae (Hopkins). Previous studies using mark-trap-recapture methods found that movement rates of adult H. warreni were elevated in areas with high numbers of dead trees, consistent with a hypothesis that the insects immigrate from stands with high mountain pine beetle-caused tree mortality to young plantings in search of live hosts. Sampling methods were necessarily biased to insects captured in traps; however, potentially missing individuals that had died, left the study area, or simply remained stationary. Here, we used harmonic radar to examine weevil movement in three different habitats: open field, forest edge, and within a forest. We were able to reliably monitor all but two of 36 insects initially released, over 96 h (4 d). Weevils released in the open field had the highest rates of movement, followed by weevils released at the forest edge, then weevils released within the forest. Movement declined with decreasing ambient air temperature. Our results suggest that weevils tend to be relatively stationary in areas of live hosts, and hence may concentrate in a suitable area once such habitat is found.