Patterns and Severity of Crown Dieback in Young Even-Aged Stands of European Ash (Fraxinus excelsior L.) in Relation to Stand Density, Bud Flushing Phenotype, and Season

The extent and temporal pattern of crown damage (attributed to Hymenoscyphus pseudoalbidus) in even-aged stands of Fraxinus excelsior in relation to bud flushing phenotype, stand density, and season was investigated. Data were collected in 2007 in four statistically designed thinning experiments located in 12-15-years old plantations of ash in Denmark. The study included 21 plots of four contrasting, residual stand densities: (1) 1700-5500 trees/ha (unthinned control plots), (2) 1500 trees/ha, (3) 500 trees/ha, and (4) 100-150 trees/ha. Assessments included estimation of flushing phenotype in May, followed by evaluation of severity of crown damage (percentage of crown killed) in June and September. Simultaneously, for each tree, the presence or absence of crown wilt and dead tops were recorded. The seasonal pattern of disease severity (average crown damage) was similar in all stands, and disregarding stand density the extent of tree crown damage increased significantly towards the end of the growing season (P < 0.005). Disease severity was the worst in unthinned plots, but otherwise unrelated to stand density. Late-flushing trees were most severely affected (P < 0.001). The observed patterns of disease severity are probably associated with ecological features of the pathogen that still remain largely unknown.