The effects of whole-tree harvesting on three sites in upland Britain on the growth of Sitka spruce over ten years

Summary Three experiments were established in the 1990s to examine the impact of complete residue (brash) and above-ground biomass removal (i.e. ‘whole-tree harvesting’, WTH) at clearfelling on the subsequent growth and yield of replanted Sitka spruce (Picea sitchensis). The sites were of varying fertility; two would now be considered to be of ‘medium’ risk for brash removal, while one would be a ‘high’-risk site. The interactions between brash removal and regular remedial fertilizer applications and weed control regimes were also investigated at each site. After 10 years, trees had been successfully established at all sites, and in most cases, the treatments were close to canopy closure. The main effects observed at all sites were due to brash retention and fertilizer application. The benefits from the former were not evident until the last stages of the establishment period, whereas benefits from fertilizer application were evident once the trees had reached 5–6 years of age. The impacts of weed control were inconsistent, providing temporary benefits on the more fertile sites, and having a negative effect on the poorest site, primarily because the herbicide regime favoured the development of ericaceous vegetation which competed with the planted trees for nutrients. After 10 years at the two medium-risk sites, the difference in growth between plots with brash retained and those where brash was removed was 5–9 per cent for height growth and 5–7 per cent for diameter. However, at the poorest site, the equivalent differences were ~9 per cent and 19 per cent. The results show that the impact of brash removal due to WTH are significantly affected by site type and soil fertility and also that it may take nearly a decade before the impacts of such practices are evident.