Long-term influence of commercial thinning on stand structure and yield with/without pre-commercial thinning of spruce-fir in northern Maine, USA

Pre-commercial (PCT) and commercial thinning (CT) are important silvicultural tools applied to spruce-fir (Picea-Abies) forests, a key forest type in the northeastern portion of North America. However, the long-term influences of CT, particularly when combined with PCT, are relatively unknown, except for a few specific locations in the region. Utilizing the repeated measurements from replicated experimental research sites (n = 15) initiated in the early 2000s throughout Maine, we quantified the influence of contrasting thinning treatments on spruce-fir stands with prior PCT and without a prior PCT (NoPCT). Thinning treatments at the nine sites with a prior PCT were a combination of multiple entry timings (immediate, 5-, and 10-year delay) and removal intensities (0, 33, and 50 % relative density reduction). At the six NoPCT sites, the CT treatments were a combination of thinning methods (dominant, crown, and low) and removal intensities (0, 33, and 50 %). The most effective CT in terms of large tree response, sawlog volume, and stand value were immediate CT rather than delayed treatments after PCT, and low thinning in NoPCT stands. Dominant thinning in NoPCT showed detrimental effects on residual stand conditions leading to the lowest yield and generated product values. In general, the earlier CT entry in PCT stands led to greater long-term benefits of the treatment in terms of tree size, merchantable volume, and financial value of the stand. No thinning treatment significantly enhanced cumulative total volume or merchantable volume or financial value compared to unthinned controls because even though thinning enhanced sawlog production, unthinned stands produced more pulpwood and studwood. Although we did not find significant economic benefits of CT in stands with or without PCT, light low thinning in NoPCT, and light crown thinning without delay in PCT could be an optimal strategy to maximize the average merchantable stem size without compromising the total stand value, while providing additional benefits to stand composition and generating mid-rotation revenue. Overall, the findings highlight some complexities and challenges with effective thinning regimes in highly shade-tolerant conifer species.