Tree-level responses to commercial thinning in spruce-fir forests across northern Maine, USA

Despite the widespread use of pre-commercial (PCT) and commercial thinning (CT) in spruce-fir (Picea-Abies) forests of North America, critical knowledge gaps exist on their long-term influences on individual tree growth and development. In this study, we used extensive repeated measurements from replicated experimental research sites across Maine (n = 96090 obs. from 7159 trees in 103 plots at 15 locations) to quantify the tree-level response of two shade-tolerant conifers: balsam fir (Abies balsamea (L.) Mill.; BF) and red spruce (Picea rubens Sarg.; RS) to contrasting thinning treatments in spruce-fir stands with and without a prior PCT (NoPCT). Treatments at the nine PCT sites included a combination of CT entry timings (immediate, 5-, and 10-year delay) and removal intensities (0, 33, and 50% relative density reduction). In contrast, 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 results showed that compared to the unthinned control, BF in thinned NoPCT stands exhibited substantial increases in annual growth for basal area, merchantable volume, and aboveground carbon, ranging from 99 to 280%, 75 to 214%, and 104 to 312%, respectively. In comparison, RS showed more moderate increases of 36 to 121%, 6 to 81%, and 32 to 135%, respectively. Similarly, in thinned PCT stands, BF displayed annual growth increases of 11 to 139%, 28 to 87%, and 15 to 145% in basal area, merchantable volume, and aboveground carbon, respectively, while RS exhibited similar increases of 50 to 120%, 35 to 96%, and 51 to 122%, respectively. In addition, CT treatments in both PCT and NoPCT stands effectively reduced mean height diameter ratios, while preserving live crown ratios and increasing tree-level growth efficiency. Overall, BF had a higher tree-level growth response than RS in NoPCT stands, while such differences were less pronounced in PCT stands. While stand-level growth and financial and operational factors should also be considered, our findings provide valuable insights into the long-term influences of both PCT and CT on individual tree growth, stability, vigor, and carbon sequestration potential in spruce-fir forests.