Date of Award


Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)


Forest Resources


Aaron Weiskittel

Second Committee Member

Robert G. Wagner

Third Committee Member

Robert Seymour


Pre-commercial (PCT) and commercial thinning (CT) are important silvicultural treatments in northern Maine. However, their influence on stand - growth and mortality of balsam fir (Abies balsamea) and red spruce (Picea rubens) forests have not been well quantified. To address this issue, the University of Maine Cooperative Forestry Research Unit (CFRU) established the Commercial Thinning Research Network (CTRN) in 2001 to measure the effects of PCT and CT on spruce - fir stands throughout the state. The PCT experiment is testing the timing of CT (thin now, wait 5 years, wait 10 years) and relative density RD reduction (33% and 50%) on stands with a history of PCT. The NoPCT experiment is testing the method of CT (dominant, crown, low) and RD reduction of older stands without a history of PCT. Repeated observations of more than 12,000 individual trees over the last 10 years in 12 stands (6 per experiment) with 7 plots per stand have created an extensive database. This analysis used generalized linear mixed models (GLMM) to quantify the effects on timing, method, and intensity of CT on both cumulative and annual mortality, as well as damage, at both the stand - and individual-tree - level. An analysis of variance (ANOVA) was used to evaluate the treatment and RD reduction effects on 10-year cumulative mortality rates for the PCT and NoPCT experiments separately. At the stand - level, a two stage approach was used to (l) predicted the annual probability of mortality occurring and (2) the extent of mortality as a percent of basal area per hectare (BA) when it occurs. The percentage of dead trees in a particular group (snag, brash failure, blowdown, splintered) and the average QMD of dead trees was also evaluated. In general, the PCT sites had lower cumulative mortality (0 - 32%) than the NoPCT sites (5 - 94%). We found that the timing of CT and RD reduction had no significant effect on the cumulative rate or annual probability of mortality on sites receiving PCT. However, waiting to apply a CT can increase mortality following the treatment as stands approach the average stand trajectory for self thinning (Wilson et al. 1999). Within the NoPCT experiment, crown and dominant CT had the highest cumulative mortality rates (11.4 - 72.1% and 28.5 - 94.3%) and were significantly higher than the Low CT (8.1 - 31.7%). At the individual-tree - level, the final model used included tree size, species, social status, treatment, and removal. All variables and most interactions were significant at a = 0.05. In general, balsam fir had a higher probability of mortality than red spruce amongst treatments and years since treatment. Our results lead us to recommend: (1) apply PCT when possible to minimize mortality; (2) thinning from below can significantly reduce mortality rates, particularly for stands with no history of PCT; (3) increase the proportion of red spruce when possible to decrease the probability of mortality occurrences.