Does butt rot affect the crown condition of Norway spruce trees?

KEY MESSAGE: Due to the effect of butt rot, Norway spruce crown defoliation has increased, and the proportion of healthy trees has decreased; butt rot acts as a predisposing factor for the decline of spruce. The exact contribution of butt rot in influencing the health and vitality of Norway spruce is still unclear. Previous studies have mostly evaluated the effect of butt rot on the radial increment in Norway spruce, as an indicator of the productivity and vitality of the tree. To our knowledge, no study has evaluated the influence of butt rot on the crown condition of Norway spruce; previous studies mainly determined the incidence and frequency of butt rot based on external signs (crown parameters). In this paper, we performed a comparison of the average crown defoliation among decayed and non-decayed Norway spruce trees within three single Norway spruce stands. We addressed the following questions: did the average Norway spruce crown defoliation differ among decayed and non-decayed trees? Did butt rot predispose, initiate, or contribute to the Norway spruce decline process? Results revealed that the average crown defoliation was higher among decayed trees in all three assessed stands. Among the decayed Norway spruce trees, the proportion of trees with defoliation class 0 (crown defoliation from 0 to 10%) was considerably low and that of trees with defoliation class 2 (crown defoliation from 26 to 60%) was much greater compared to that in non-decayed trees. The study was able to verify the assumption that as per Manion’s tree decline concept, butt rot acts as a predisposing factor for the decline of Norway spruce because of a non-lethal effect. This study provides meaningful knowledge about the influence of butt rot on the crown condition of Norway spruce and contributes to a better understanding of the impact of disease on spruce vitality.