Storm and snow damage in a Norway spruce thinning experiment in southern Sweden

To assess uncertainties regarding the effects of thinning on risks of storm and snow damage, three thinning treatments (control, normal and heavy: 0, 30 and 61 per cent basal area removal, respectively) with four repli-cations were applied in a 33-year-old Norway spruce plantation on a fertile site in south-west Sweden. Amajor storm event occurred three growing seasons after thinning followed by snow damage 2 months later and another storm 2 years later. There was a near-linear relationship between thinning intensity and damage caused by the first storm (adj R2 0.94): 7, 42 and 74 per cent of the standing basal area was damaged in the control, normally and heavily thinned plots, respectively. Corresponding percentages after both the storm and snow damage were also approximately linearly correlated with thinning intensity: 8, 53 and 89 per cent, respectively (adj R2 0.91). Damage caused by the second storm was related to both previous damage levels and standing basal area in the control and normally thinned plots. Stem taper between breast height and 6 m stem height was negatively correlated with risk for storm and snow damage in the nor-mally thinned plots. Of the trees categorized as damaged (up-rooted, broken or heavily leaning), the frequency of stem breakage after the first stormwas almost negligible in the normally thinned plots, (,5 per cent), while 24–50per cent of the damaged stemswere broken (mean, 36 per cent) in theheavily thinned plots. The relative diameter growth of the remaining trees in the control plots, for the first and second growing seasons following the storm and snow damage, were significantly decreased compared with the three preceding years. The results confirm previous findings that increasing thinning intensity increases risks of windfall and highlight the need for new Norway spruce management programmes if climate change results in more frequent storm events