Balsam fir sawfly defoliation effects on survival and growth quantified from permanent plots and dendrochronology.
Balsam fir sawfly (Neodiprion abietis (Harris)) has become a serious pest of young managed balsam fir (Abies balsamea (L.) Mill.) stands in western Newfoundland, Canada. During 1991-2008, a total area of 561 000 hectares was moderate to severely defoliated. We quantified impacts (growth and survival) using data from permanent sample plots (PSPs) and dendrochronology and related these impacts to defoliation severity determined from aerial defoliation data, in order to provide input into a Decision Support System. We analyzed 67 Newfoundland Forest Service PSPs, selected based on severity of defoliation (classes 1-6), stand age and management interventions (pre-commercially thinned vs natural) and measured before and after defoliation (1996-2008). We used Bayesian statistics to combine information from different sources, each having their own limitations and associated uncertainty. No mortality was observed in immature plots 12 years after defoliation, but survival was 54 per cent lower in mature defoliated than in non-defoliated plots. Plots in defoliation class 1 (1 year of moderate, 30-70 per cent, defoliation) showed 22 per cent cumulative growth reduction and complete recovery to pre-defoliation growth increment after 5 years. Plots in defoliation classes 2-6 (one to three consecutive years of severe, 71-100 per cent, defoliation) had mean cumulative growth reductions of 26-40 per cent and did not recover to pre-defoliation levels even 9 years after defoliation ceased. Natural and thinned plots responded similarly to defoliation severity. These results suggest that proactive control measures need to be implemented since impacts are severe, even with only 1 year of severe defoliation.