Growth response of Norway spruce trees to selection harvest in continuous cover forestry stands on drained boreal peatlands

Continuous cover forestry (CCF) has less environmental impacts than rotation forestry, but knowledge on growth responses of different tree cohorts to selection harvesting is scarce. In this study, tree-ring data of dominant and suppressed Norway spruce (Picea abies (L.) H.Karst.) trees representing CCF-managed drained peatland stands from north to south boreal forests were analysed with respect to selection harvest to basal area of 12–13 m²/ha using generalized additive mixed models (GAMM). The first statistically significant impacts were seen in suppressed trees one year after thinning. In four out of five sites, the suppressed trees displayed significant impacts one year earlier than the dominant trees. In three sites, the post-thinning ring-widths of the suppressed trees were up to three to four times the predicted widths, three or four years after thinning, while dominant trees showed a 50–100 % increase. However, the growth response of the basal area increment of suppressed trees was smaller, since the suppressed trees were on average smaller than the dominant trees. Thus, the lower growth after selection cutting previously observed at stand-level studies of CCF is likely due to the removal of a large proportion of large productive trees, rather than a small or delayed response of suppressed trees. It seems that the elevated water table in the harvested plots through decreased evapotranspiration did not limit the post-harvest growth release in the case of the selection harvest experiment of the present study. If the elevated water table had any effect on the growth release, this mechanism was probably less counterproductive for suppressed trees which also benefitted from improved light conditions.