Machine trail effects on tree- and stand-level growth in uneven‑aged Norway spruce forests following single-tree selection harvesting

Abstract Key message Enhanced growth of trees adjacent to machine trails partially compensated for basal area increment lost due to unstocked trail area in uneven-aged Norway spruce stands subjected to single-tree selection harvesting, with the compensatory effect apparently independent of residual stand density within the examined range. Context Machine trail networks are an inherent feature of continuous cover forestry systems, where they occupy a substantial share of stand area and alter local growing conditions. Despite extensive research in even-aged stands, their effects on tree growth and stand-level productivity in uneven-aged forests remain largely unexplored. Aims The study aimed to assess how machine trails influence tree- and stand-level growth in uneven-aged Norway spruce stands, including size- and density-dependent responses and their implications for growth compensation. Methods The study used tree growth data measured over a 12-year period following machine trail establishment in four experimental blocks of uneven-aged Norway spruce, subjected to selection cutting with varying residual stand densities. Generalized additive modeling was applied to quantify the spatial extent of the trail-related growth response by modeling individual-tree basal area increment as a function of distance to machine trail, providing the basis for subsequent stand-level analyses. Stand-level growth compensation was examined by scaling estimated growth differences according to the proportion of trail-affected versus trail-unaffected stand area. Results Individual tree basal area increment decreased with increasing distance to machine trail, with a proximity effect extending up to 4 m into the stand. We found no evidence that the tree-growth response to distance from machine trail varies with tree size or residual stand-level basal area. The increased growth of trail-affected trees compensated for about half of the basal area increment that would have been produced on the unstocked machine trail area over the 12-year study period. Conclusion Growth gains at the edges of machine trails can partially offset productivity losses from the trails themselves, as shown here for uneven-aged stands. Together with evidence from even-aged forests, this suggests that this compensatory effect is primarily driven by increased resource availability at trail edges rather than by specific management systems or stand structures.