Crown defoliation and radial increment among decayed and undecayed Norway spruce trees

Pathogen-caused wood decay is believed to be one of a major cause of decreasing tree vitality and increased susceptibility to external biotic and abiotic damaging agents. Using data collected from twenty decay-affected different maturity Norway spruce (Picea abies (L.) Karst) stands, this study aims to explore the interaction between butt rot and crown defoliation and radial increment as indicators of tree vitality. Study results indicate that vitality decline among butt rot-affected Norway spruce is expressed as a significant reduction in radial increment and considerably greater levels of crown defoliation. Decayed Norway spruce trees were found to have higher crown defoliation than undecayed spruce trees. Additionally, a radial increment reduction in the last five years and the last twenty years of the increment sequence was found when decayed Norway spruce tree growth was compared to undecayed. Severely defoliated trees were observed only among decayed trees, and this made up only 1% of the population, indicating that butt rot acts as a predisposing or inciting factor for the decline of Norway spruce. A stand age-dependent relationship was also found among visually healthy undecayed spruce trees, indicating that spruce trees show signs of increasing defoliation as part of the ageing process. Among decayed Norway spruce, no such relationship was found, indicating changes in the effect-response framework for decayed trees. Naturally, both decayed and undecayed trees showed a negative correlation between the level of crown defoliation experienced and the radial increment for the last five years before increment sequencing.