Canadian Forest Service Publications
Historical frequency, intensity and extent of mountain pine beetle disturbance in British Columbia. 2010. Alfaro, R.I.; Campbell, E.; Hawkes, B.C. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, BC. Mountain Pine Beetle Working Paper 2009-30. 52 p.
Available from: Pacific Forestry Centre
Catalog ID: 31405
This project used dendrochronology to reconstruct the history of mountain pine beetle outbreaks over the last two centuries and to describe the relationship of outbreaks to varying climate conditions and fire history. The history of mountain pine beetle outbreaks was reconstructed from increment cores and basal disks collected from 85 stands across British Columbia and Alberta. Using aerial survey data, dendrochronological software was calibrated to detect growth releases that could be attributed to beetle outbreaks. In addition to the outbreak in the 1970s and 1980s, growth release data indicate that beetle outbreaks probably also occurred in the late 1870s and 1900s, and in the 1930s and 1940s. Growth release periods associated with beetle outbreaks lasted up to 20 years, with the magnitude of growth increases averaging 58% to 65%. Based on time intervals between growth release periods, we estimated a return-interval of beetle outbreaks of 30 to 40 years. Although the intensity of past beetle outbreaks appears to be greater in central British Columbia than further north or in Alberta, further investigations are required. A histogram of growth release frequencies among stands suggests outbreaks have become more extensive over the last century. Relationships between beetle outbreaks and climate variability were assessed using instrumental weather data, modelled climate data, and historical reconstructions of climate from tree-ring data. Weather station data indicated that beetle outbreaks occurred during prolonged warm periods that generated drought. Drought indices, as well as fall and spring temperatures, were reconstructed back at least 150 years using tree-ring-width data. Although drought frequency has increased at the edge of the beetles historic geographic range (e.g., Banff National Park), it has been a persistent condition in central British Columbia where outbreaks have occurred for much longer periods. Temperatures reconstructed from tree-rings showed more of an oscillating trend, but those predicted from models indicated a fairly linear increase, with spring temperatures changing most under a climate-change scenario. Drought increased similarly. As drought significantly decreases tree resistance to beetle attack and as warm spring temperatures are critical to larval survival, we expect that climate at the edges of the current beetle outbreak will continue to provide more optimal conditions for beetle outbreaks. Relationships between the occurrence of beetle outbreaks and fire history were undertaken using several approaches in dendrochronology. Dating of fire scars in the Cariboo-Chilcotin Plateau of central British Columbia indicated that fires were much less frequent in the 20th century than they were in the 19th century. This may explain the more extensive outbreaks of the 1930s and 1940s and 1970s and 1980s in this region. Stands in regions characterized by low- or mixed-severity fire regimes had more frequent outbreaks than those in regions where fires are stand-replacing disturbance events. Among stands that originated from stand-replacing fires, we found that the time since fire had an important influence on frequency of beetle outbreaks-outbreaks were most frequent in stands 50 to 100 years after fire, when mature trees are less resistant to beetle attack and thick phloem tissues promote beetle development.
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