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Changes in spatio-temporal patterns of spruce budworm outbreaks during the 20th century in eastern canadian boreal forests
Published on by Lionel Navarro
In scenarios of future climate change, there is a projected increase in the occurrence and
severity of natural disturbances in boreal forests. Spruce budworm (Choristoneura fumiferana)
(SBW) is the main defoliator of conifer trees in the North American boreal forests affecting
large areas and causing marked losses of timber supplies. However, the impact and the
spatiotemporal patterns of SBW dynamics at the landscape scale over the last century remain
poorly known. This is particularly true for northern regions dominated by spruce species.
The main goal of this study is to reconstruct SBW outbreaks during the 20th century at the
landscape scale and to evaluate changes in the associated spatiotemporal patterns in terms of
distribution area, frequency, and severity. We rely on a dendroecological approach from sites
within the eastern Canadian boreal forest and draw from a large dataset of almost 4 000 trees
across a study area of nearly 800 000 km2. Interpolation and analyses of hotspots determined
reductions in tree growth related to insect outbreak periods and identified the spatiotemporal
patterns of SBW activity over the last century. The use of an Ordinary Least Squares model
including regional temperature and precipitation anomalies allows us to assess the impact
of climate variables on trees growth reductions and to compensate for the lack of non host
trees in northern regions . We identified three insect outbreaks having different spatiotemporal
patterns, duration, and severity. The first (1905–1930) affected up to 40% of the studied trees,
initially synchronizing from local infestations and then migrating to northern stands. The
second outbreak (1935–1965) was the longest and the least severe with only up to 30% of
trees affected by SBW activity. The third event (1968–1988) was the shortest, yet it was also
the most severe and extensive, affecting nearly up to 50% of trees and 70% of the study area.
This most recent event was identified for the first time at the limit of the commercial forest
illustrating a northward shift of the SBW distribution area during the 20th century. Overall, this
research confirms that insect outbreaks are a complex and dynamic ecological phenomena,
which makes the understanding of natural disturbance cycles at multiple scales a major priority
especially in the context of future regional climate change.
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