Evaluating the influence of varying levels of spruce budworm defoliation on annualized individual tree growth and mortality in Maine, USA and New Brunswick, Canada

Spruce budworm (SBW; Choristoneura fumiferana (Clem.)) defoliation can cause severe growth reduction and mortality to spruce-fir (Picea-Abies) species in eastern North America. Consequently, it is important to understand and predict how individual trees respond to defoliation in terms of growth and mortality. To address this, species- and region-specific individual-tree models quantifying the effects of cumulative defoliation on annualized diameter and height increment, crown recession, and mortality were developed using data from an extensive network of 424 and 136 permanent sample plots in Maine, USA and New Brunswick, Canada, respectively, during the last SBW outbreak of 1970-1990s. In general, these models fit well with relatively small bias despite large variation observed in the data. The effects of SBW defoliation on diameter increment, crown recession, and mortality were highly significant but relatively moderate depending on species, possibly because overall mean defoliation was rather moderate. Reduction in diameter increment and increase in mortality were always more significant for balsam fir (Abies balsamea L.) than for the spruce species. However, diameter increment returned to higher rates irrespective of species in New Brunswick, where the data were collected towards the end of the SBW outbreak. Overall, the results highlight the variability of tree response to SBW defoliation and the array of factors that may influence it.