Periodic site index and climatic stress for Norway spruce in Norway

The objective of this study was to demonstrate how height growth recalculated to periodic site index could be used to monitor and identify climatic drivers for growth variations. We used data from Norway's National Forest Inventory (NFI), with attention to Norway spruce in the lowlands (<500 m a.s.l.) of southeastern Norway. We recalculated height growth to periodic site index and extracted a time series with annual values. We supplemented this with climatic data, i.e. monthly mean temperature, precipitation and deMartonne aridity index. The results showed that a characteristic two-peaked time series in volume growth in Norway 1994–2020 corresponded well to a time series of periodic site index for Norway spruce in the specific region mentioned above. Statistical analyses showed that for spruce, the periodic site index was higher in cold and moist summers than in warm and dry. Spruce mortality in this region tripled during 2012–22 when June temperature increased considerably, while periodic site index decreased. This corroborates warm and dry weather in June to be a main stress factor for spruce. In conclusion, periodic site index has a potential for being implemented for monitoring site productivity and for identification of climatic drivers.