Assessing the Incidence of Butt Rot in Norway Spruce in Southern Finland

The aim of this study was to analyze the occurrence of butt rot damage to Norway spruce in different parts of southern Finland and to quantify the associated loss of quality. The data used in the study are from the 9th National Forest Inventory and consist of 5998 sample plots and 8007 spruce sample trees of saw-timber size. To predict the probability of damage to stands and trees, logistic regression models were constructed. Separate models were made for the whole study area, for the area where the general risk of Heterobasidion root and butt rot damage is high and for the area where the damage frequency is relatively low. In the highrisk area, the probability of damage decreased with increasing elevation and increased with increasing temperature sum. In addition, damage was more common on fertile sites and less common on peatlands; and thick peat layer decreased the risk of damage. The probability of damage was also higher in stands where special or selective cuttings had been carried out. In the sample tree data, the probability of damage increased slightly with increasing diameter and age of the tree. In the low-risk areas, elevation was the only variable that explained the probability of damage to a spruce tree. Site fertility and previous cuttings (more than ten years ago) explained the probability of damage to stands only weakly. For spruce damaged by butt rot, the saw-timber volume was reduced, on average, by 60% both in the high-risk area and in the low-risk area.