Seasonal dynamics of the radial increment of Scots pine and Norway spruce in the southern and middle boreal zones in Finland

In the Finnish National Forest Inventory thousands of trees are cored every year. Based on the increment cores we assessed the progress of the radial increment in Scots pine ( Pinus sylvestris L.) and Norway spruce ( Picea abies (L.) Karst.) during the growing season. Data for 18 and 15 years were available from the southern and middle boreal zones, respectively. No major differences were found in the progress of the radial increment between either tree species or vegetation zones. In both zones, the radial increment began at approximately the same time, in late May or early June. On average, half of the annual radial increment was completed in early July, and ceased in late July or early August. However, there was large variation among years in the progress of the radial increment. On average, the radial-increment period (10%–90% of the total increment) was 41–59 days depending on tree species and region. The increment period was slightly shorter in the middle boreal zone, as increment ended 5–11 days later in the southern boreal zone. The variation of annual radial-increment indices was related to the dates of increment onset and cessation. However, increment-period length was more closely related to the differences in increment indices.