Norway spruce forest management in the Czech Republic is linked to the solar cycle under conditions of climate change - from tree rings to salvage harvesting

Norway spruce (Picea abies [L.] Karst.) is the most important economic tree species in Central Europe. It has been affected by extensive windstorms and bark beetle disturbances, which have intensified in recent years. Spruce stands are subject to regular reoccurring disturbances with different intensities. This study examines timber harvests in the Czech Republic and the tree-ring series of Norway spruce from six permanent research plots across the country. The timber harvest shows a cyclical connection with the sunspot number, and of these, the percentage of spruce logging shows a substantial link with the sunspot number while the percentage of salvage logging indicates a relation with Total Solar Irradiance (TSI) and seasonal temperature. The seasonal North Atlantic Oscillation (NAO) indicated a link with logging types as well. However, the connection between TSI and SunSpot Number (SSN) with logging types was statistically significant, while the tree rings were more related to seasonal NAO than timber harvests. The tree Ring Width Index (RWI) series shows reduced growth, which precedes the peak in the timber harvests by 1 year. Timber harvests peak one year after minimum RWI, and they coincide with solar minimum years. In summary, average tree ring series reach predominantly their lowest values one year before the solar minimum, coinciding with increased spruce and salvage logging. Conversely, during the solar maximum, harvests are lower, and tree ring series remain relatively stable. In the spectral analysis, tree-ring series and all types of timber harvests show a quasi-11-year cycle. Spruce trees indicate higher tree-ring growth and lower timber harvests during solar maximum compared to solar minimum and this may be associated with specific meteorological conditions that may be affected by solar variability. European forestry is unfamiliar with the effects of the solar cycle; nevertheless, this phenomenon is present in both spruce tree rings and the aggregate of timber harvests. Further research on this issue will be necessary, but it is already apparent that regularly recurring calamities in spruce stands are likely to be reflected in all of Central Europe.