Effects of selection harvesting on the understorey vegetation in drained Norway spruce peatlands

This study focused on the changes in species abundance after harvest in uneven-aged stands. Selection harvesting was performed at four sites in southern boreal vegetation zones in Finland using two thinning intensities: post-harvest basal area (G) of 17 and 12–13 m2 ha–1. The G pre-harvest and in control plots varied between 19 and 31 m2 ha–1. Vegetation was inventoried before thinning in 2016 and 2 and 6 years after thinning in 2018 and 2022. The effect of thinning intensity was significant for grasses and sedges as groups, Betula pubescens Ehrh. (height < 50 cm), Rubus idaeus L., and Trientalis europaea L., which showed increased abundance after harvest. However, T. europaea abundance turned to decline by 2022. Several species responded to the thinning intensity, showing a greater treatment effect with the more time since harvest. The abundance of Carex globularis L., Dryopteris carthusiana (Vill.) H.P. Fuchs, Epilobium angustifolium L., Vaccinium myrtillus L., V. vitis-idaea L., Linnaea borealis L. and Brachythecium spp. increased, but that of Oxalis acetosella L. decreased. For some species, only the time since harvest was significant. The abundance of Maianthemum bifolium (L.) F.W. Schmidt, Deschampsia flexuosa (L.) Trin. and Plagiothecium spp. increased, whereas that of Sphagnum girgensohnii Russow and S. russowii Warnst. decreased. The thinning intensity did not have a significant effect on the number of species, but the number of species increased slightly on the thinned plots. The effects of logging residues, strip roads, and light availability may be the major drivers of the changes in the species abundance.