Date of Award

Summer 8-18-2023

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forest Resources

Advisor

Aaron R. Weiskittel

Second Committee Member

Anil Raj Kizha

Third Committee Member

John-Pascal Berrill

Additional Committee Members

Anthony W. D'Amato

David Marshall

Abstract

This study investigates long-term influences of pre-commercial thinning (PCT) and commercial thinning (CT) treatments in spruce-fir forests, a key forest type in northeastern North America. Utilizing repeated measurements from replicated experimental research sites (n=15) in Maine, both stand- and tree-level responses to contrasting CT with and without prior PCT (NoPCT) have been quantified. In addition, an in-depth economic analysis, exploring the impact of different thinning treatments, associated costs, merchantability specifications, varying timber and carbon prices, and discount rates, on net present value (NPV) has been carried out using both measured and projected data. Thinning treatments at nine PCT sites combined various CT entry timings (immediate, 5-, and 10-year delay) and removal intensities (0, 33, and 50%), while the six NoPCT sites tested various thinning methods (low, crown, and dominant) and intensities. Stand-level analysis revealed the most effective CT to be immediate (i.e., no delay) in PCT stands, and low thinning in NoPCT stands, enhancing tree response, sawlog volume, and stand value. Tree-level analysis showed significant growth increases in basal area, merchantable volume, and aboveground carbon in thinned NoPCT stands for balsam fir (BF), while red spruce (RS) showed more moderate increases. Similarly, thinned PCT stands resulted in growth increases for both BF and RS. In addition, CTs were effective in reducing height-diameter ratio which is expected to enhance tree stability, while also preserving live crown ratio, and increasing growth efficiency. Economic analysis highlighted the strong influence on NPV of discount rates and price paid for sawlogs, studwood and carbon offsets. For example, a stand with no PCT history, a 40-50% CT generated the highest peak NPV at 2% discount rate without carbon offsets, while the NPV was maximized by doing no thinning when an offset revenue of $25 (MT CO2e)-1 was included. In PCT stands, earlier CT exhibited higher NPV than delayed CT. Driven by different tree- and stand-level responses to PCT and/or CT, decision-making should consider multiple factors due to complex interactions among costs, prices, interest rates, and market conditions. This comprehensive analysis supports decision-making by revealing how different thinning treatments affect growth, yield, and economics in spruce-fir forests.

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