Date of Award

Summer 8-2025

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Forest Resources

First Committee Advisor

Michael Premer

Second Committee Member

Aaron Weiskittel

Third Committee Member

Adam Daigneault

Abstract

Forests have been managed by humans to provide materials for timber and wood supplies, recreation, and other cultural activities and utilization of ecosystem processes. One such management technique is density management through thinning. Thinning is an intermediate treatment that removes trees from the stand, reducing competition and reallocating resources such as radiation, water, and nutrients to the residual trees. Research has shown that thinning increases economic value and growth of the trees in the residual stand, while simultaneously, incidentally stimulating natural regeneration in the understory. However, more research is needed to understand the influence of thinning regimes and site-specific environmental factors on natural regeneration patterns, stand density, stand volume, and net present value (NPV). The overarching goal of this research is to illustrate incidental ecological and economic impacts of commercial thinning practices on spruce-fir forests across Maine. Specifically, I aim to analyze how thinning practices as well as site-specific factors influence [1] species diversity and composition, [2] basal area and volume, and [3] long-term trends in net present value utilizing state of the art digital soil maps, climate data, and local estimators of evapotranspiration to infer causal mechanisms of thinning response in spruce-fir forests across Maine. For both objectives, I utilized data collected from the University of Maine’s Cooperative Forestry Research Unit’s long-term Commercial Thinning Research Network (CTRN) study.

For my first objective, I used diversity indices, mixed effects linear models, and quantitative ecological methods including ordination, to assess stand level community composition and regeneration diversity. Results showed that overstory diversity is influenced by temperature, moisture, and density removal, while understory diversity is influenced by certain overstory structural variables such as height, crown competition, and thinning treatment. The species composition of the understory is significantly more variable than the overstory. Additionally, both overstory and understory community composition are influenced by thinning method. Thinning of any method increased diversity of the understory compared to control treatments. Low thinning seems to increase red spruce presence in the understory. These results illustrate that thinning can incidentally increase the diversity of natural regeneration in spruce-fir stands.

For my second objective, local estimates of evapotranspiration were used to derive stand density, yield and associated economic return. A nonlinear mixed effects modeling approach was used to model basal area per hectare (BAPH) and stand volume per hectare (m3ha-1) and a gamma distribution was used to model net present value ($/ha) using water surplus estimates and treatment variables as covariates. For net present value calculations, different pricing was included for red spruce and balsam fir. Water surplus, a metric of site water availability, was included in each model in these analyses to better understand how water availability can enhance estimates of density, and volume. Overall, wetter sites had higher BAPH, volume, and NPV. The use of site water availability estimators such as water surplus provides a unique way of estimating site productivity and carrying capacity which in turn could lead to increases in efficiency and optimization of site-specific silviculture and forest management.

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