Simulations of the effects of changes in mean fire return intervals on balsam fir abundance, and implications for spruce budworm outbreaks

In boreal forests of eastern Canada, the end of the little ice age (ca. 1850) coincided with a lengthening of mean fire return intervals, which has been hypothesized to increase the abundance of late-successional forests dominated by balsam fir. This increase could have generated unusually severe eastern spruce budworm (Choristoneura fumiferana) outbreaks from 1910 onward. The aim of this paper is to simulate the effect of various changes of fire return intervals on regional-scale balsam fir abundance, and to examine potential effect on spruce budworm outbreaks. We developed a regional-scale succession model based on empirical information on fire return intervals, fire sizes (using a reverse Weibull function) and post-fire successional trends (using a logistic growth function), based on an extensive dataset collected in a 65,000km2 section of the boreal forest located in eastern Quebec. The simulations indicate that lenghtening the fire return intervals from 300 to 500 years, or from 100 to 300 years, can increase mean balsam fir basal area by 1.3–5.7m2/ha at the regional scale, or increase the proportion of stands dominated by balsam fir by 3.5% to 25%. However this increase takes place very gradually, corresponding with the time needed for the forest age-class structure to reach a new equilibrium following the change in fire rate. Overall, we estimate that it is unlikely that an increase in balsam fir abundance following a change in fire return intervals around 1850 was sufficient to explain the change in outbreak patterns observed in the early 20th century.