Forest Dynamics After Successive Spruce Budworm Outbreaks in Mixedwood Forests

Abstract

In order to assess the long-term spatiotemporal influence of the spruce budworm in sub-boreal mixedwood forests, we studied the effect of three successive outbreaks in a region of western Quebec, Canada. We used dendrochronology to detect past outbreaks in three areas (111-185 ha), based on the recruitment age of balsam fir (Abies balsamea) and on growth patterns of white spruce (Picea glauca), the two main host species of this defoliating insect. We also used a series of aerial photographs taken between 1935 and 2003 to evaluate overstory mortality and post-outbreak succession patterns in these same areas. Individual outbreaks had a spatially homogenous impact on host species throughout the region, but successive outbreaks differed in intensity: the two outbreaks around 1910 and 1980 caused widespread mortality in the overstory, but an outbreak around 1945 had little impact, probably because the forest mosaic had not yet recuperated from the 1910 outbreak. No clear outbreak was detected in the later part of the 19th century. In portions of the study areas where the 1910 outbreak had a major impact, between 36% and 50% of the stands were reoccupied by balsam fir stands in the period up to the 1980 outbreak (cyclic succession), the rest being at least partly replaced by nonhost species such as Betula spp. Changes in forest composition after the 1910 outbreak were mostly associated with upper-slope positions in all study areas. The 1980 outbreak also had a higher impact than earlier outbreaks in lower-slope positions dominated by black spruce (Picea mariana)-balsam fir mixtures. These results suggest that, at the regional scale, the abundance of mature or over-mature balsam fir stands does not determine the outbreak cycle. When an outbreak occurs, however, its impact will be strongly constrained by forest characteristics such as stand composition and structure, which are themselves influenced by previous disturbances and slope position.

Comparing modern and presettlement forest dynamics of a subboreal wilderness: Does spruce budworm enhance fire risk?

“…However, recent research is challenging the idea that insect disturbance leads to enhanced fire activity and severity, on the grounds that changes to fuel architecture in response to insect disturbance are often nonlinear due to the combined effects of increased dead fuel and thinned canopy fuels (Romme et al 2006, Jenkins et al 2008, Lynch and Moorcroft 2008, Simard et al 2008). Long-term consequences of insect disturbance for fire risk are even less understood, since insect disturbance influences successional pathways affecting future forest conditions that have subsequent repercussions for future fire and insect disturbance , Jasinski and Payette 2005, Bouchard et al 2006. Reciprocal interactions between fire, insects, and vegetation change have particular relevance for the mixed Laurentian forests of northeastern Minnesota, USA.…”

Growth Response of Northern White-Cedar (Thuja occidentalis) to Natural Disturbances and Partial Cuts in Mixedwood Stands of Quebec, Canada

“…The exact importance of this outbreak for the region is more difficult to define. According to Bouchard et al [22,25], who studied the region immediately north of our own, this outbreak was of a shorter duration and probably caused little mortality since stands remaining from the previous infestation were young and less vulnerable at that time. However, Blais [24], although providing no quantitative evidence, states that this outbreak may well have been quite severe within the study region.…”

“…In southwestern Quebec, where the study was located, three outbreaks have been documented. The first occurred between 1910 and 1915 [11,22,23]. Although this outbreak cannot be captured by our balsam fir growth series because the sampled trees were too young at that time, important growth responses were noted for both yellow birch and white-cedar in the OR site.…”

“…A second budworm outbreak was reported around 1939-1949 [22,24,25], with the minimum growth of the host species occurring in 1949. The exact importance of this outbreak for the region is more difficult to define.…”

Ecoregional Patterns of Spruce Budworm—Wildfire Interactions in Central Canada’s Forests

“…Consequently, we view our approach to defining a 'likely interaction' as a practical compromise, which is not without difficulties. The opposite interaction (of fire affecting the likelihood of subsequent spruce budworm defoliation) has received more attention [17,[27][28][29].…”