Fungal decomposition of attached angiosperm twigs I. Decay community development in ash, beech and oak

Initial stages in the development of fungal decay communities were followed in attached ash (Fraxinus excelsior L.) twigs which had been stressed by girdling and defoliation, and in cut lengths from living twigs of ash, oak (Quercus robur L.) and beech (Fagus sylvatica L.) incubated under controlled drying regimes in the laboratory. Community development in artificially stressed ash twigs was similar to that predicted from previous direct observations. There was direct evidence that most primary colonizers in ash, including Phomopsis platanoidis Died, and Fusarium lateritium Nees., were present as endophytes in the bark of healthy twigs and subsequently colonized wood from this inoculum source. By contrast, secondary invaders were non-endophytic fungi, including Peniophora lycii (Pers.) v. Hohn & Litsch and Libertella fraxinea Oganova. Secondary invaders began to colonize twigs after about 11 months; this was associated with a detectable reduction in moisture content. In healthy ash twigs there was evidence of superficial colonization of woody tissues, lying close to the bark, by endophytes which were not commonly isolated from dead ash twigs, including Aureobasidium pullulans (De Bary) Arnaud and Phoma macrostoma Mont. Superficial colonization was seasonal, reaching a maximum during the winter. A range of endophytes was isolated from healthy oak and beech bark, including Phomopsis spp., Cryptosporiopsis spp., Daldinia sp. and Xylaria spp. Some, but not all, of these endophytes were also isolated from the wood of dead attached oak and beech twigs, and some were able to colonize dying twig lengths under laboratory conditions. Whilst, Daldinia sp. and Xylaria spp. developed in dying twigs of all three tree species in the laboratory, neither were found in decaying twigs in the field. This may reflect different rates of drying in the laboratory and field.