Transformation of Sitka spruce to continuous cover forestry: Results of a thinning trial

Almost all forests in Ireland are planted and are managed under the clearcut rotation system. In these forests the predominant tree species is the exotic conifer Sitka spruce ( Picea sitchensis (Bong.) Carr.). There is increasing interest in adopting continuous cover forestry (CCF) management as a means to increase resilience to climate change. This study reports the results of a long-term trial, established in 2010, addressing the transformation of two Sitka spruce stands to CCF. It focusses on the early stages of the transformation process. The aim of the trial was to compare how two thinning regimes (crown thinning and graduated density thinning (GDT)) deliver on key considerations in transformation relative to the industry norm of low thinning. Between 2010 and 2023, the two forest stands had four thinning interventions applied. In the first three thinning interventions larger trees, on average, were removed in the crown thinned plots than in the low thinned plots. In the second and subsequent thinnings, the average tree size removed from the GDT plots was similar to that in the crown thinned plots and larger proportions of sawlog and pallet were removed in the GDT plots compared to the low thinned plots. The values for Hegyi and Gini indices have increased in the crown thinning and GDT treatments, suggesting increasing structural diversity, yet remain far from would be expected in a CCF stand. Stability is a key concern in the transformation process, and the trees in the crown thinned and GDT plots had the highest slenderness on average, with crown thinning having the highest slenderness. Although plot basal area values were significantly lower in GDT plots, they are high across all treatments and increase rapidly following thinning reaching close to 55 m 2 ha −1 in some plots. Despite 14 years of CCF management, there is no evidence of natural regeneration to date under any of the treatment regimes. This suggests that the management strategy adopted may be too conservative and that thinning may need to commence earlier and occur more frequently if the basal area thresholds for natural regeneration of Sitka spruce are to be achieved within the first four interventions. This is especially the case in sites and stands where the basal area recovers very rapidly following thinning. Further research is required to more fully understand interactions between site factors (productivity, soil properties, exposure and wind risk) and thinning interventions (type, cycle and intensity) so that management protocols can be developed for the transformation of upland conifer plantations to CCF in Ireland.