Long-term reduction in the diameter growth of butt rot affected Norway spruce, Picea abies

Abstract The long-term effect of butt rot on the diameter increment of Norway spruce, Picea abies Karst., was studied by analysing data collected by the Swedish National Forest Inventory. The 15 698 Norway spruce trees were classified as healthy or infected based on bore cores taken at breast height. Healthy and infected trees, similar in terms of age, diameter at breast height in 1954, increment from 1954 to 1963, and from sites of the same productivity class in the same region were paired. Owing to restrictive criteria used, only 329 pairs were chosen for further analysis. These trees were analysed groupwise as well as pairwise. In the period 1954–1958, trees classified as infected grew faster than the healthy controls; however, within 5 years the relationship had reversed. The degree to which the growth of infected trees was suppressed increased with time, and was 8.9% and 8.2% respectively during the last two 5 year periods. Differences in growth between healthy and infected trees within pairs were highly significant. The diameter growth of the infected trees was reduced by an average of 1.4 mm, or 9.4%, during the last 5 years, corresponding to a 9.6% reduction in the basal area growth.