Stand development following mountain pine beetle outbreaks in south-central British Columbia
Mountain pine beetles are a natural disturbance of North American forests, and outbreaks play an important role in directing ecological processes and maintaining the biological diversity of forest ecosystems. However, having infested about over 13 million hectares of lodgepole pine forests in BC to date (BC Ministry of Forests and Range, January 2008) the current beetle outbreak seems to be unprecedented in scale. As lodgepole pine comprises about one quarter of the provincial timber supply, socioeconomic impacts of this outbreak are enormous. A variety of silvicultural tools and management strategies can be used to reduce the effects of timber losses, the most important tool being salvage logging. In the short-term, adjusting harvest scheduling to remove standing beetle-killed trees can compensate some of the timber losses. However, due to market, operational, legal, and ecological constraints the proportion of the beetle-killed forests that can be salvage-logged is limited.
Figure 1. Extent of lodgepole pine and areas sampled in central and southern locations in British Columbia.
Throughout much of British Columbia, lodgepole pine forms even-aged stands initiated by stand-replacing fires. Although such dynamics is considered typical for the species, uneven-aged lodgepole pine stands that have historically been maintained by frequent low-intensity surface fires are common in central BC. Forest landscapes of central BC are dominated by lodgepole pine and can generally be described as a mosaic of even-aged and uneven-aged stands (Agee 1993). However, the disturbance ecology of these forests is changing. The frequency of fires has decreased over the last century, mostly due to effective fire suppression programs. When fire is infrequent, outbreaks of mountain pine beetle are likely to have a greater impact on forest structure, composition and dynamics. However, we know surprisingly little about the changes in forest characteristics following beetle outbreaks.
- To understand the current stand structure of even and uneven-aged lodgepole pine stands,
- To reconstruct stand dynamics following beetle outbreaks in even-aged stands in the southern (Logan Lake) and uneven-aged stands in the central interior (Chilcotin Plateau) of BC, and
- To develop conceptual models of stand dynamics that incorporate beetle and fire disturbances.
- We used forest inventory techniques to determine present stand structure and legacies after beetle by taking a number of measurements such as: diameter-at-breast height, tree height, and mountain pine beetle attack status: no attack, green attack (current year), red attack (attacks occurred 2-3 years prior) or grey attack (attacks occurred more than 3 years ago) (Figure 2).
- We used dendrochronology technique to determine past stand structure by collecting tree increment cores and discs from dominant trees, saplings and dead downed trees (coarse woody debris). We then used tree-rings pattern matching to accurately date each ring in every sample to date past beetle episodes, past fire disturbances, year of tree establishment, and year of death (Figures 3 to 5).
Figure 2. Collecting tree inventory data (height, diameter at breast height, mountain pine beetle status) in an even-aged stand in the southern BC.
Figure 3. Collecting a disc from dead and downed tree (coarse woody debris) to date year of death and extend stand chronologies back in time, southern BC.
Figure 4. Collecting an increment core from a lodgepole pine in an uneven-aged stand in the Chilcotin Plateau, BC.
Figure 5. Collecting a disc from a dead lodgepole pine that has multiple scars (both mountain pine beetle and fire scars) from the Chilcotin Plateau, BC.
Reconstructing forest history
Some wildfires and mountain pine beetle attacks leave permanent records in the form of scars on the tree bole. After careful dating of the tree-rings we identified the year in which a fire or mountain pine beetle attacks occurred (Figure 6). Based on tree-ring dating we found that in even-aged stands forests were initiated by a stand replacing event (crown fires) in the late 1800s or the early 1900s. In the uneven-aged stands we found that many less severe fires occurred, leaving multiple scars on surviving trees but not necessarily eliminating the entire stand.
Figure 6. Trees maintain a record of past canopy disturbances in the stand. This 290 year old lodgepole pine lived through a fire in 1776 and 1869. It was scarred by mountain pine beetle in 1842 and finally succumbed to MPB outbreak and died in 2005.
Mountain pine beetle outbreaks history
Mountain pine beetle, under epidemic conditions preferentially kills the largest trees in the stand. Under intense beetle pressure this will result in the mortality of all or almost all of the mature trees within a given stand. The trees that are killed by mountain pine beetle turn red (around 2 years after initial attack), the needles fall off leaving a denuded tree which has a grey appearance (3 to 6 years). Dead trees eventually fall to the ground and become coarse woody debris. The removal of the tree foliage allows much more light to enter lower portions of the canopy, and trees that are not killed by beetle experience an accelerated period of growth (referred to as a growth release) which is enhanced by reduced competition.
We reconstructed past beetle outbreaks by looking for sustained (greater than 5 years) periods of growth release in the tree-ring record. We quantified the growth release using an increased growth factor, for example a 50% increase in mean radial growth, recorded by a percentage of trees sampled in the stand. In both even and uneven-aged stands residual trees experienced a 50% increase in annual growth during the 1930s, 1960s and the 1980s, which all coincide with known periods of beetle outbreak (Forest Insect and Disease Surveys).
Examination of the diameter at breast height (DBH) and height distributions of the sample stands revealed the prevalence of different forest layers, or cohorts, in both the even and uneven aged forests.
In the "even-aged" stands the largest and oldest trees were very large Douglas-fir trees, or veterans, which survived stand replacing fires in the late 1800s and early 1900s. The dominant canopy, which initiated after stand replacing fires, contained the oldest pine in the stand. Below this layer was a lodgepole pine dominated sub-canopy, which was initiated after the 1930s beetle outbreak thinned the dominant canopy. The advanced regeneration dates to the 1980s and was initiated after the 1980s outbreak. Some seedlings were present, initiating around 2004-2005, but there were very infrequent due to very low light conditions at the forest floor (Figure 7).
Figure 7. Schematic diagram of stand structure in "even-aged" stands sampled in the southern-interior, BC.
In the uneven-aged stands there were no Douglas-fir veterans present, and so the oldest trees in the dominant canopy were lodgepole pines. Because of the mixed-severity fire regime of this area, many pine trees survived ground fires and as a result did not establish in one simple pulse (as in the even-aged stand). Also present in the dominant canopy were trembling aspen, which likely established after ground fire events. The next layer was the sub-canopy which contained both pine and aspen which were both largely initiated after beetle outbreaks of the 1930s and 1960s. Similar to the "even-aged" stands the advanced regeneration layer dates to the 1980s and was initiated after the 1980s outbreak. Unlike the "even-aged" stands numerous seedlings were present, initiating around 2004-2005, as there was ample light in these stands due to the open canopy with few live mature pine trees (Figure 8).
Figure 8. Schematic diagram of stand structure in uneven-aged stands sampled in the central-interior, BC.
Figure 9. Seedlings established in response to increased light reaching the ground after canopy trees died due to mountain pine beetle attack in the Chilcotin Plateau, BC.
Stand development after beetle
Mountain pine beetle is a natural thinning agent that promotes increased growth among the surviving trees and allows for establishment of seedlings in understory (Figure 9). In all sampled stands the sub-canopy, which consisted of a well developed cohort in both even and uneven-aged stands, was mainly free of mountain pine beetle attack and will endure after the dominant-canopy has been killed and fallen to the ground. The advanced regeneration layer was present and along with the sub-canopy forms an important secondary structure, meaning that the entire forest was not all dead.
Putting it all together
Forest disturbances such as wildfire and mountain pine beetle outbreaks are natural processes that operate on the stand to landscape level. We were able to develop a conceptual model of stand dynamics in lodgepole pine ecosystems in BC (Figure 10). This cycle diagram shows that in the absence of fire mountain pine beetle plays a more important and frequent role in determining forest structure and species composition in both the southern and central interior regions of British Columbia.
Figure 10. Conceptual model of stand dynamics of lodgepole pine ecosystems, showing the cyclical nature of forest disturbances and how disturbance regimes change forest structure in the southern and central interior of BC. (Source: Forest Ecology and Management 257 (2009) 1874-1882).
- In the absence of fire disturbance the MPB plays a more frequent role in directing stand dynamics and structure in the lodgepole pine stands of BC.
- Stand replacing fires initiate even-aged lodgepole pine stands; multiple MPB disturbances create stands that have variable canopy and cohort structure.
- Mixed severity fires create complex structures in uneven-aged lodgepole pine stands; multiple MPB disturbances maintain complex stand structures and contributes towards the succession of non-pine tree species (e.g., trembling aspen).
- Frequency and severity of MPB outbreaks determines the structure and composition of the residual stand.
We wish to thank the Forest Investment Account-Forest Sciences Program of British Columbia for funding this two-year project (project number: M08-6044) and the following Pacific Forestry Staff for their assistance and contributions: Andrew Copeland, Lara van Akker, Sarah Buddingh, Victoria Alfred, Kevin Pellow, Thandi Gurp, and Terry Holmes.
Publications, reports and presentations
Alfaro, R.; Axelson, J.; Hawkes, B.; Copeland, A. 2008. Stand dynamics following mountain pine beetle outbreaks in central British Columbia: a synthesis for decision-support [45 kb PDF]. Final Report FIA-FSP Project # M08-6044. Pacific Forestry Centre, Victoria, BC. 46 pp.
Hawkes, B. 2003. Impacts of Mountain Pine Beetle attack on ecosystem dynamics. [930 kb PDF]
Hawkes, B.; Watt, R.A.; Taylor, S.; Stockdale, C.; Shore, T. 2003. Impacts of mountain pine beetle attack on stand and ecosystem dynamics in south-eastern British Columbia. Canadian Forest Service Canadian Forest Service. FII Project R2003-111.
Impacts of Mountain Pine Beetle Attack on Stand and Ecosystem Dynamics in South Eastern British Columbia. [5.4 Mb PDF]
Waterton Lakes National Park, Forest insect assessment report. [3.48 Mb PDF]
Waterton Lakes National Park, Forest damage assessment. [3.48 Mb PDF]
Hawkes, B.; Taylor, S.; Stockdale, C.; Shore, T.L.; Unger, L.; Dalrymple, G.N.; Beukema, S.J.; Robinson, D.C.E. 2004. Impact of mountain pine beetle on stand dynamics in British Columbia, Imprint: Victoria, B.C.: Pacific Forestry Centre.
Predicting mountain pine beetle impacts on lodgepole pine stands and woody debris characteristics in a mixed severity fire regime using PrognosisBC and the fire and fuels extension. [1.6 Mb PDF]
Long-term impacts of mountain pine beetle on stand dynamics in Waterton and Kootenay National Parks. [77 Mb PDF]
Alfaro, R.; Axelson, J.; Hawkes, B. 2008. Role of the mountain pine beetle in maintaining the complexity of lodgepole pine stands in British Columbia, Canada. Symposium on Silviculture of Complex Stands. October 24-27, 2008, Shizuoka, Japan.
Axelson, J.; Alfaro, R.; Hawkes, B. 2008. What stand reconstructions can tell us about the stand dynamics following mountain pine beetle outbreaks in south-central British Columbia, Canada. AmeriDendro, June 23-27 2008, Vancouver, BC.
Axelson, J.; Alfaro, R.; Hawkes, B. 2008. Stand dynamics following mountain pine beetle outbreaks in British Columbia, Canada. American Association of Geographers Annual Meeting, April 14-19, 2008, Boston, MA.
Axelson, J.; Alfaro, R.; Hawkes, B. 2008. Stand dynamics following past mountain pine beetle outbreaks in south-central British Columbia [129 kb PDF]. Timber Growth and Value Conference. February 6-7, 2008, Smithers, BC.
Axelson, J.; Alfaro, R.; Hawkes, B. Shore, T. 2007. Stand dynamics following mountain pine beetle outbreaks in central British Columbia [7.68 Mb PDF]. North American Forest Ecology Workshop, June 18-20 2007, Vancouver, BC.
Alfaro, R.I.; Campbell, R.A.; Vera, P.; Hawkes, B.C.; Shore, T.L. 2004. Dendroecological reconstruction of mountain pine beetle outbreaks in the Chilcotin Plateau of British Columbia. Pages 245-256 in T.L. Shore, J.E. Brooks, and J.E. Stone, editors. Mountain Pine Beetle Symposium: Challenges and Solutions, October 30-31, 2003, Kelowna, British Columbia, Canada. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia, Information Report BC-X-399. 298 p.
Alfaro, R.I.; Campbell, E.; Hawkes, B.C. 2010. Historical frequency, intensity and extent of mountain pine beetle disturbance in British Columbia. B.C. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, BC. Mountain Pine Beetle Working Paper 2009-30. 52 p.
Alfaro, R.; Axelson, J.; Hawkes, B. 2009. Mountain pine beetle increases the complexity of fire-origin lodgepole pine stands in British Columbia, Canada. In Mountain pine beetle: From lesson learned to community-based solutions conference proceedings [4.14 Mb PDF], June 10-11, 2008. BC Journal of Ecosystems and Management 9(3):85-89.
Axelson, J.N.; Alfaro, R.I.; Hawkes, B.C. 2010. Changes in stand structure in uneven-aged lodgepole pine stands impacted by mountain pine beetle epidemics and fires in central British Columbia. The Forestry Chronicle 86(1):87-99.
Axelson, J.N.; Alfaro, R.I.; Hawkes, B.C. 2009. Influence of fire and mountain pine beetle on the dynamics of lodgepole pine stands in British Columbia, Canada. Forest Ecology and Management 257(9):1874-1882.
Campbell, E.; Alfaro, R.I.; Hawkes, B.C. 2007. Spatial distribution of mountain pine beetle outbreaks in relation to climate and stand characteristics: a dendroecological analysis. Journal of Integrative Plant Biology 49(2):168-178.
Hawkes, B.C.; Taylor, S.W.; Stockdale, C.; Shore, T.L.; Alfaro, R.I.; Campbell, R.A.; Vera, P. 2004. Impact of mountain pine beetle on stand dynamics in British Columbia. Pages 177-199 in T.L. Shore, J.E. Brooks, and J.E. Stone, editors. Mountain Pine Beetle Symposium: Challenges and Solutions, October 30-31, 2003, Kelowna, British Columbia, Canada. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia, Information Report BC-X-399. 298 p.
Hawkes, B.C.; Taylor, S.W.; Stockdale, C.; Shore, T.L.; Beukema, S.J.; Robinson, D. 2005. Predicting mountain pine beetle impacts on lodgepole pine stands and woody debris characteristics in a mixed severity fire regime using PrognosisBC and the fire and fuels extension. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, BC. Mountain Pine Beetle Initiative Working Paper 2005-22. 17 p.
Kaufmann, M.R.; Aplet, G.H.; Babler, M.G.; Baker, W.L.; Bentz, B.; Harrington, M.; Hawkes, B.C.; Huckaby, L.S.; Jenkins, M.J.; Kashian, D.M.; Keane, R.E.; Kulakowski, D.; McCaughey, W.; McHugh, C.; Negron, J.; Popp, J.; Romme, W.H.; Schoennagel, T.; Shepperd, W.; Smith, F.W.; Sutherland, E.K.; Tinker, D.; Veblen, T.T. 2009. The Status of Our Scientific Understanding of Lodgepole Pine and Mountain Pine Beetles – A Focus on Forest Ecology and Fire Behavior. The Nature Conservancy. GFI Technical Report 2008-2, Arlington, VA.
Li, C.; Barclay, H.J.; Hawkes, B.C.; Taylor, S.W. 2005. Lodgepole pine forest age class dynamics and susceptibility to mountain pine beetle attack. Ecological Complexity 2(3):232-239.
Shore, T.L.; Safranyik, L.; Hawkes, B.C.; Taylor, S.W. 2006. Effects of the mountain pine beetle on lodgepole pine stand structure and dynamics [208 kb PDF]. Pages 95-114 (Chapter 3) in L. Safranyik and W.R. Wilson, editors. The mountain pine beetle: a synthesis of biology, management, and impacts on lodgepole pine. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia. 304 p.
Stockdale, C.; Taylor, S.W.; Hawkes, B.C. 2004. Incorporating mountain pine beetle impacts on stand dynamics in stand and landscape models: a problem analysis [435 kb PDF]. Pages 200-209 in T.L. Shore, J.E. Brooks, and J.E. Stone, editors. Mountain Pine Beetle Symposium: Challenges and Solutions, October 30-31, 2003, Kelowna, British Columbia, Canada. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, British Columbia, Information Report BC-X-399. 298 p.