Dieback of Fraxinus excelsior in the Baltic Sea Region

This thesis is comprised of three main studies: (1) the wood-inhabiting fungi found in declining European ash (Fraxinus excelsior L.) and their pathogenicity; (2) the relationship between F. excelsior phenology, site density and the susceptibility to the dieback; and (3) the potential of natural regeneration of dieback affected ash stands. The studies that are presented here were conducted in Denmark, Lithuania and Sweden. Combination of different sampling and detection methods revealed a high diversity of fungi in both healthy looking and symptomatic tissues of declining F. excelsior. The most frequently detected fungal taxa were Alternaria alternata, Armillaria cepistipes, Aureobasidium pullulans, Botryosphaeria stevensii, Cladosporium cladosporioides, Cryptococcus foliicola, Epicoccum nigrum, Gibberella avenacea, Hymenoscyphus pseudoalbidus, Lewia sp., Phoma spp. and Phomopsis sp. In pathogenicity tests nine fungal taxa caused symptomatic discoloration of bark and cambium on F. excelsior saplings, though only H. pseudoalbidus infected substantial proportion (50-100%) of tested trees. The seasonal pattern of ash dieback severity, attributed to crown damage of F. excelsior trees, significantly increased towards the end of the investigated growth season. Severity of dieback symptoms was more pronounced in the unthinned stands, but otherwise was not related with stand density. However, susceptibility of F. excelsior to the disease was found to be dependent on the flushing (bud-bursting) phenology of the trees - late-flushing F. excelsior were most severely affected. Our study demonstrated that vigorous natural regeneration of F. excelsior in examined clear-felled sites cannot be expected. Regenerating F. excelsior exhibited abundant dieback symptoms. The species composition in sites with long disease history is likely to shift away from F. excelsior to early successional pioneer species such as Alnus incana, Betula spp., and in some instances Populus tremula