Thinning response to weather variations in Norway spruce

Sammanfattning: Climate change is likely to affect the prerequisites for forest management. Therefore, it is imperative to investigate how silviculture practices can be adjusted to changing conditions. Norway spruce (Picea abies) is one of the most common species in the Swedish forests, with high economic and ecological values. The species is relatively sensitive to drought, and climate change is likely to entail more frequent drought periods. Thinning is considered to decrease the competition of resources between the trees in a forest stand. Thus, it might be possible to decrease the negative effect of drought by conducting thinnings. The purpose of this study was to investigate if weather conditions affect the individual tree growth response to thinning in spruce stands, and if some thinning methods made the stands more resilient to weather conditions. The analysis was conducted with tree cores sampled from the thinning and fertilization experiments, a large field experiment with sites all over the country. Four sites in southern Sweden were selected. One control treatment and two thinning methods were chosen from each site: one heavy thinning (treatment c) and 2 – 6 thinnings from below (treatment A/E). The ring widths were measured in a dendrochronology laboratory. The results showed a clear difference in growth between the heavy thinning and the other treatments. After a heavy thinning had been conducted, the Basal area increment of the remaining trees increased. The same pattern could to some extent be observed for the other treatments as well, however the increase was not as great. For the years where the Basal Area Increment was unusually low in the control plot, the treatments seemed to do slightly better, however the result was not significant. The weather-growth relationship was analysed with the R package treeclim, however precipitation and growth only seemed to correlate before thinning for the A/E treatment. The sample used in this study was relatively small and larger experiments including more samples and perhaps other tree species could give a better picture of how Norway spruce should be managed in the future. Thinning is a powerful forest management tool that could play a key role when adapting forests to climate change. Thus, the application of thinnings in forest management needs to be further investigated.