Tree Damage by Ice Accumulation in Norway Spruce (Picea abies (L.) Karst.) Stands Regarding Stand Characteristics

Freezing rain is a frequently occurring, but relatively rarely studied disturbance in Europe, although ice accumulation may occasionally cause severe damage for forestry. We aimed to characterize ice-accumulation damage to overstory trees in spruce stands, assess the probability of damage based on the stand and individual tree parameters, and define the most significant parameters that affect the probability of individual tree damage in all stands and in recently thinned stands. Among the studied stands, the proportion of damaged overstory spruce ranged from 1.8% to 60.9% and was higher (p < 0.001) in recently thinned stands (27.8% ± 1.9%) than in the other stands (20.4% ± 1.6%). Stem breakage was the prevalent (98.5% ± 1.1%) damage type. At the stand level, the probability of damage decreased for older, less dense stands with a larger mean diameter. Within stands, overstory trees were more damaged (23.5% ± 1.2%; p < 0.001) than those in the lower stand layers, but, within overstory, trees with larger dimensions and a higher social position (high relative diameter and low slenderness ratio) and a higher proportion of crown were less damaged. The probability of breakage to overstory trees was most accurately predicted using almost the same variables for all stands and recently thinned stands. The site type, tree height, relative diameter, and crown ratio were common for both, with the addition of mean diameter at breast height for all stands and the stand density for recently thinned stands. Our results indicate the importance of the tree and stand characteristics on the resistance of individual tree to ice accumulation and the need for management practices that balance increased growth and the stability of trees throughout the rotation.