Date of Award


Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)


Ecology and Environmental Sciences


Mike Day

Second Committee Member

Brian Roth

Third Committee Member

Laura Kenefic


The eastern spruce budworm (Choristoneura fumiferana (Clem.); SBW) has been present in spruce-fir forest ecosystems for centuries, periodically causing widespread defoliation, reductions in growth and increases in mortality. In response to this type of stress, trees have developed two strategies: tolerance and defense. Survival depends on the appropriate balance between the two. Nonstructural carbohydrate (NSC) reserves provide the energy needed to support metabolic functions when photosynthesis may be limited. This serves an important role in trees’ ability to tolerate a variety of stresses, including defoliation. Capacity for recovery after consecutive years of defoliation (i.e., tolerance) is linked to the pool of NSC available to stimulate growth of latent buds and recover leaf area. While thinning treatments have the potential to increase NSC pools thereby increasing tolerance to defoliation, studies conducted during past outbreaks have shown that thinning can have a complicated effect on outbreak severity and duration. Additionally, trees’ ability to resist defoliation (i.e., defense) is determined by the presence of defensive compounds, such as tannins, which protect against feeding by the SBW and other herbivores, and have been shown to negatively impact SBW development.

This study evaluates the eco-physiological responses of red spruce (Picea rubens) and balsam fir (Abies balsamea) to stand-level thinning and mechanical defoliation treatments across three experimental locations in the state of Maine. Chapter 1 presents a comprehensive literature review of spruce-fir forests in Maine, including a brief history of land use and SBW outbreaks, stand vulnerability, silviculture and forest management, spruce-fir ecology and eco-physiology.

Chapter 2 compares NSC storage allocation patterns at the species level, between red spruce and balsam fir, and evaluates the effects of stand-level commercial thinning treatments on storage reserves. Contrary to expectations, NSC concentrations were higher in balsam fir than red spruce. This may support the high degree of environmental plasticity typical of balsam fir. In contrast to carbohydrate allocation theory, the results from the stand-level thinning treatment suggest that commercial thinning lowers NSC concentrations within large branch storage pools. This is likely due to increased resource availability within thinned stands and a shift in allocation away from storage, towards increased growth. However, lower concentrations of NSC reserves may reduce tolerance to defoliation during a SBW outbreak.

Chapter 3 investigates the effects of defoliation on the production of secondary defensive compounds. Mechanical defoliation treatments, with two levels of intensity, were applied to red spruce and balsam fir trees. Concentrations of soluble phenolic compounds within foliage were measured as tannic acid equivalence (TAE). Surprisingly, concentrations of foliar TAE were twice as high in balsam fir than red spruce. This should mean that balsam fir foliage is better protected against defoliation caused by the SBW. No effect of the defoliation treatment was observed on the concentrations of TAE for either species, which suggests TAE act as a constitutive defense mechanism and are not induced in response to defoliation.