Disentangling variables that influence growth response of balsam fir regeneration during a spruce budworm outbreak

Abstract

Height increment of balsam fir (Abies balsamea (L.) Mill.) regeneration during a spruce budworm (Choristoneura fumiferana Clem.) outbreak can be affected by a variety of variables including defoliation, ground vegetation competition, canopy openness, the height of regeneration, and stand type. To disentangle how these variables interact to determine regeneration height increment, we sampled 36 plots in two regions showing contrasting climate, regional vegetation, and budworm outbreak severity. Linear mixed-effects models were used to analyze the effects of these five variables on height increment of balsam fir regeneration. In the Amqui (early defoliation) plots, height increment was significantly affected by a “cumulative defoliation × height of regeneration × hardwood content” interaction. Defoliation significantly reduced height increment when regeneration was >30 cm tall in softwood and mixedwood plots, but regeneration <30 cm tall in softwood and mixedwood plots and all regeneration in hardwood plots had light defoliation, which did not reduce height increment. Results were even more complex for the North Shore (late defoliation) plots, where there were three significant 3-way interactions affecting height increment: “height × hardwood content × ground vegetation cover”, “hardwood content × canopy openness × cumulative defoliation”, and “hardwood content × ground vegetation cover × cumulative defoliation”. Canopy openness was >25% in all softwood plots, and height increment substantially declined with increasing cumulative defoliation; however, in mixedwood and hardwood plots, canopy openness was <25% and height increment increased with defoliation. In softwood plots, height increment declined sharply with increasing cumulative defoliation at all ground vegetation cover levels, but in mixedwood and hardwood plots, height increment increased or decreased slightly with increasing cumulative defoliation. Results showed that the variables affecting height increment have surprisingly complex interactive effects.