Mountain pine beetle as an agent of forest disturbance

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The mountain pine beetle as an agent of forest disturbance Philip J. Burton1

Presentation Abstract Outbreaks of mountain pine beetle are evaluated as a generic disturbance agent, and comparisons are made with other forest disturbances such as wildfire, windthrow, and logging. A useful basis for comparison is the degree of disruption to the overstorey, understorey, and forest floor layers. Clear differences are observed in the impacts of bark beetles, fire, and windthrow, but there is overlap with various harvesting systems. Insects are selective in terms of the species or size of tree that is killed; this selectivity varies with stand composition, stand structure, and outbreak stage. The mountain pine beetle functions as part of larger natural disturbance regimes in western North America, which vary with climate and forest type. Outbreaks of many different insects occur throughout western Canada, with the relative role of fire and insects differing among ecoregions and over time. Beetle-killed stands may facilitate extreme fire behaviour and may be more susceptible to future burning. Large expanses of dead or removed trees also result in altered soil water balance and stream flows, disposing some sites to mass movement or flooding. All disturbances generate heterogeneity, with much of the value to biodiversity and ecosystem recovery depending on residual structure and biological legacies. The capacity for unassisted recovery and the value of each stand to timber supply, carbon balance, and habitat needs in a landscape context are relevant when considering salvage logging or forest rehabilitation. The future role of forest pests is expected to fluctuate in response to changes in climate and the altered composition and structure of western forests. keywords: disruption, disturbance agent, disturbance regime, insect selectivity, mountain pine beetle, unassisted recovery.

Contact Information 1 Canadian Forest Service, and University of Northern British Columbia, Ecosystem Science and Management, 3333 University Way, Prince George, BC V2N 4Z9. Email: [email protected]

JEM — Volume 9, Number 3

Published by Forrex Forum for Research and Extension in Natural Resources

Burton, P.J. 2008. The mountain pine beetle as an agent of forest disturbance. In Mountain Pine Beetle: From Lessons Learned to Community-based Solutions Conference Proceedings, June 10–11, 2008. BC Journal of Ecosystems and Management 9(3):9–13. url: http://www.forrex.org/publications/jem/ISS49/vol9_no3_MPBconference.pdf

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Summary

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n ecological disturbance is “ . . . any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment” (White and Pickett 1985:6). As such, natural disturbances are the converse of stand development and succession, renewing forest stands and diversifying landscapes. Every disturbance has dual impacts: inducing mortality but facilitating regeneration; removing biomass but aiding nutrient cycling— essentially serving as an agent of both destruction and renewal. Because of an evolutionary history of repeated exposure to natural disturbances characteristic of a region (e.g., landslides, windthrow, fire, insect outbreaks), all ecosystems are adapted to recover and persist (not unchanged, but within some natural range of variability, nrv) after those disturbances (Figure 1a). Some intermediate level of disturbance frequency and intensity is typically required to maximize biological diversity within a landscape (Reice 2001). Whether due to human

activities, invasive species, or climate change, disturbance can sometimes shift ecosystems beyond their nrv into alternative states from which they cannot readily recover. Considered “degraded” if caused by human activities, these ecosystems may still be functional and stable alternative states (Figure 1b). Typically functioning as a “releasing” disturbance at endemic or background levels, full-scale population explosions by herbivorous forest insects can also serve as agents of wholesale stand mortality and renewal. Outbreaks of the mountain pine beetle can be described and evaluated as a generic disturbance agent, comparable to other forest disturbances such as wildfire, windthrow, and logging. None of these disturbances are homogenous in their effects, even within a single event. As proposed by Roberts (2007), a useful basis for site-to-site comparison is an assessment of the degree of mortality or disruption to the overstorey (mature tree layer), the understorey (tree seedlings and saplings, shrubs, and herbaceous vegetation), and the forest floor (duff and soil layers).

figure 1. Schematic portrayal of ecosystem response to disturbance: (a) Under disturbance regimes to which an

ecosystem is adapted, a disrupted ecosystem will typically return to a condition similar (but not identical) to its original condition, within some envelope we can recognize as the historical or natural range of variability (nrv). (b) Novel or persistent disturbances may push ecosystems into alternative states that do not return to the nrv envelope without assistance (Burton 2005).

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JEM — Volume 9, Number 3 mountain pine beetle conference proceedings

mountain pine beetle as an agent of forest disturbance

figure 2. Levels of overstorey, understorey, and soil disturbance observed on sites affected by various natural disturbances in the Sub-Boreal Spruce moist cold (sbsmc) subzone, Bulkley and Nadina Forest Districts, west-

central British Columbia.

Clear differences are observed in the impacts of bark beetles, fire, and windthrow, as bark beetles typically have only indirect effects on the understorey and the forest floor (Figure 2). There can be overlap of clearcut logging and alternative (partial cutting) harvesting systems with the range of variability encountered in natural disturbances, though less so in the case of insect outbreaks. By virtue of specific host requirements, insects tend to be very selective in the species (or, in the case of mountain pine beetle, the size) of tree that is attacked and killed. This selectivity varies with stand composition and structure and with outbreak stage (Figure 3). The mountain pine beetle functions as part of larger natural disturbance regimes in western North America, which vary with climate and forest type. Large and small outbreaks of many different insects have been recorded in the forests of central British Columbia, and the relative role of fire differs dramatically among ecoregions and over the past few decades (Figure 4). Beetle-killed stands may facilitate more extreme fire behaviour and may be more susceptible to burning in the future. Large expanses of

dead or removed trees due to any mortality agent also result in altered soil water balance and streamflows, further disposing some sites to seasonal flooding and mass movements (e.g., soil slumping). All disturbances generate heterogeneity in habitat and microsites, with much of the value to biodiversity and ecosystem recovery depending on the residual structure and biological legacies left on site. The carbon in dead trees, fine organic matter, and soils resides in different pools of slow- or fast-decomposing organic matter, typically resulting in net co2 release until the ecosystem recovers. Policies, practices, and guidelines for salvage-logging or forest rehabilitation after natural disturbances may wish to consider the capacity for unassisted post-disturbance recovery on a stand-by-stand basis, as well as the relative value of each stand to current and future timber supply, carbon balance, and habitat needs in a landscape context (Lindenmayer et al. 2008). The role of the mountain pine beetle and other forest pests is expected to fluctuate over the coming decades, in response to changes in climate and the altered composition and structure of western forests.


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a)

b)

figure 3. Sample plot data and linear regression lines fit for percentage of small (< 23 cm dbh) lodgepole pine trees attacked as related to: (a) the abundance of pine in the stand; or (b) variability in stand population structure as indicated by the coefficient of variation in dbh, during two phases of the current mountain pine beetle outbreak. Open circles and solid line illustrate the first, incipient phase, and diamonds and dashed line illustrate the second, full-outbreak phase of attack. Regression slopes are significantly different from zero at p < 0.001.

figure 4. The mean area affected per year (as a percentage of forested area) from 1961 to 2000 by different fire and

insect disturbance agents in different ecozones of British Columbia. Analysis is based on mapped records in the B.C. Natural Disturbance Database (Taylor 2007).

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mountain pine beetle as an agent of forest disturbance

References Burton, P.J. 2005. Ecosystem management and conservation biology. In Forestry handbook for British Columbia, Fifth edition. S.B. Watts and L. Tolland (editors). Faculty of Forestry, University of British Columbia, Vancouver, B.C. pp. 307–322 Lindenmayer, D.B., P.J. Burton, and J.F. Franklin. 2008. Salvage logging and its ecological consequences. Island Press, Washington, D.C.

Taylor, S.W. 2007. British Columbia natural disturbance database. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, B.C. url: http://cfs.nrcan.gc.ca/subsite/disturbance/home-accueil White, P.S. and S.T.A. Pickett. 1985. Natural disturbance and patch dynamics: An introduction. In The ecology of natural disturbance and patch dynamics. S.T.A. Pickett and P.S. White (editors). Academic Press, San Diego, Calif. pp. 3–13

Reice, S.R. 2001. The silver lining: The benefits of natural disasters. Princeton University Press, Princeton, N.J. Roberts, M.R. 2007. A conceptual model to characterize disturbance severity in forest harvests. Forest Ecology and Management 242:58–64.


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