Date of Award

Fall 12-21-2019

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Doctor of Philosophy (PhD)


Forest Resources


Aaron Weiskittel

Second Committee Member

Mohammad Bataineh

Third Committee Member

David MacLean

Additional Committee Members

Mindy Crandall

Erin Simons-Legaard


Spruce budworm (SBW; Choristoneura fumiferana (Clem.)) is the primary forest defoliator in North America. SBW defoliation has affected tens of millions ha of forests during its periodic outbreaks and caused severe growth reduction and mortality of spruce-fir (Picea-Abies) species. Evaluating these damaging effects of SBW defoliation requires understandings of the variation and dynamics of defoliation, as well as trees' variable responses to defoliation in consideration of various tree- stand-, and site-level factors. In this dissertation, we developed statistical models to 1) evaluate influences of SBW defoliation on spruce-fir stand dynamics of annual volume net growth, mortality, and ingrowth, 2) quantify effects of SBW defoliation on annualized diameter and height increment, crown recession, and mortality, and 3) assess patterns and temporal development of SBW defoliation on individual trees. Measurements of individual trees and their defoliation collected from 560 permanent sample plots in Maine and New Brunswick during the last SBW outbreak in the 1970s-1980s, which covered > 40 000 km2 as well as 10 years of varying ranges of defoliation and forest conditions were used in the analyses. Our results strongly demonstrated that 1) even relatively low levels of cumulative defoliation were significantly related to stand-level mortality and ingrowth, while net growth was more competition driven, 2) effects of defoliation on diameter increment, crown recession, and mortality were highly significant but relatively moderate depending on species, and 3) variation in individual tree defoliation was predominantly dependent on species. In addition, defoliation of each host species analyzed developed towards their respective converged trajectories. Based on these findings, we developed annualized modifiers for the Acadian variant of the Forest Vegetation Simulator (FVS-ACD) to account for effects of SBW defoliation on forest development, which consistently had smaller biases and prediction errors than FVS-ACD refined by STAMAN (a Canadian growth model) SBW modifiers. Overall, our findings highlight the high variability in SBW defoliation and trees' responses to defoliation, which were consistent between Maine and New Brunswick despite varying forest management history and species composition. We believe the developed modeling framework should also be extendable to analyzing other forms of defoliation in broader regions.