Thirty-year responses of ecosystem components to stand thinning in lodgepole pine forest: Old-forest attributes, stand structure, and forest-floor small mammals

Biomass production via higher yields of wood fibre in boreal and temperate forests may be enhanced by silvicultural treatments such as stand thinning. Restoration of old-forest structural attributes in second-growth forests is also much needed for conservation of wildlife species and biodiversity. We ask if pre-commercial thinning over a wide range of densities could enhance forest productivity while concurrently developing old-forest structures? We tested the hypotheses (H) that, compared with unmanaged (unthinned and old-growth) stands, large-scale pre-commercial thinning (heavy thinning to ≤ 500 trees/ha) over a 30-year period, would enhance (H₁) structural features (increase diameter and height growth and tree crown volume) of crop trees; (H₂) productivity based on merchantable volume and carbon storage of crop trees; (H₃) development of old-forest structural attributes; and (H₄) total abundance, species richness, and species diversity of forest-floor small mammals. Four replicate blocks of pre-commercially thinned target densities of 250 (very low), 500 (low), 1000 (medium), and 2000 (high) trees/ha, unthinned, and old-growth stands of lodgepole pine (Pinus contorta var. latifolia) were located at three study areas in south-central British Columbia, Canada. Tree and stand measurements were conducted at 30 years post-thinning when stands were 43–47 years old. Small mammals were live-trapped in stands during 2023 and 2024. Mean diameter of crop trees followed the pattern of density with the highest growth in the very low-density stands and lowest in the unthinned stands. Mean tree heights were tallest in the old-growth stands and lowest in the unthinned stands. Very large tree crowns were evident in the three heavily thinned (≤ 1000 trees/ha) stands. These results partly supported H₁. Mean merchantable volume of pine crop trees was highest in the old-growth stands and secondarily in the unthinned and ≥ 1000 trees/ha thinned stands, and hence did not support this part of H₂. Mean carbon storage was similar among stands owing to the very large tree crowns in the heavily thinned stands and provided partial support for H₂. Owing to ingress, total crop tree densities were similar among the four thinned stand densities ranging from 1199 to 1479 trees/ha. Heavily thinned stands had some old forest attributes including large dominant trees with substantial crowns, multi-layered canopies of coniferous trees, canopy gaps, and understory patchiness, and hence supported H₃. Mean total abundance of small mammals was similar among stands in 2023 but higher in the very low-density, unthinned, and old-growth stands than other stands in 2024, and hence partly supported H₄. To our knowledge, this is the first detailed reporting of the 30-year responses of tree growth metrics and stand development in thinned stands of lodgepole pine at a real-world scale and across a wide range of densities. Much of the potential carbon storage lost by thinning was restored in the heavily thinned stands at 30 years post-thinning. Merchantable timber may be achieved in a considerably reduced rotation and may be a viable means to address current and future timber shortfalls. Restored forests, at 30 years after pre-commercial thinning, may be 40–50 years old and be at least partly comparable to uncut old-forest in some respects.