Does stump removal reduce pine weevil and other damage in Norway spruce regenerations? – Results of a 12-year monitoring period

Abstract

Although impacts of intensive forest harvesting practices such as removing logging residue and stumps for bioenergy on forest ecosystem has been extensively studied, no information is available about the long-term effects of stump harvesting on the health status of young Norway spruce stands. In the present study, we surveyed damage caused by the large pine weevil (Hylobius abietis L.) and other causal agents as well as the height growth of the Norway spruce seedlings for 12 years in stump-harvested and non-harvested experimental plots. The overall effect of stump harvesting combined with mounding on the health and survival of spruces was positive: 70% of spruce seedlings in stump-harvested plots were spared any damage, while 58% of seedlings remained undamaged in undisturbed control plots. The positive effect of stump removal and mounding on tree health was mainly due to reduced pine weevil damage. In all, 14.8 and 24.6% of spruce seedlings were gnawed by pine weevil in stump-harvested and control plots, respectively. In stump-harvested plots, pine weevil damage was slighter and lasted for a shorter time than in control plots. In addition, damage caused by Hylastes spp., black root beetles, was significantly less on stump-harvested plots. By contrast, vole damage was most frequent in stump-harvested plots during the population peak when the vole density was exceptionally high. Irrespective of whether stumps were harvested or not, stump roots provided a source of Heterobasidion inoculum in the next spruce regeneration. During this early stage of stand development only a few spruce seedlings were killed by Heterobasidion root rot and there were neither temporal nor quantitative treatment-related differences between the treatments. From the third year onwards the height growth of undamaged seedlings was more rapid in stump-harvested than non-harvested plots. Our results suggest that the overall risk of damage in young Norway spruce stands can be reduced by stump harvesting. Thus, stump harvesting and mounding seem to decrease the need for supplementary planting, which reduces regeneration costs that represent a considerable financial investment for a forest owner.