In bough and bole: Decay communities in veteran trees
Trunks and functional branches of living trees are a very hostile environment for fungi, yet eventually every tree will be colonised by one fungus or another, and usually by many fungi. Five main, but not mutually exclusive, ways for colonising wood in standing trees have been adopted by fungi: (1) heart rot; (2) wound colonization; (3) active pathogenesis (i.e. killing living cells); (4) endophytes already present hidden within sapwood (latency); and (5) colonization after other fungi have begun the decay process (secondary colonizers). Research began in the mid 1800s, and centred around heart-rot because of loss in the timber trade. Research on heart-rot more or less stopped 50 or 60 years ago, apart from those heart-rotters that enter via pathogenesis e.g. HeterobasidionArmillaria species, and little progress in understanding had been made on non-pathogenic heart-rot fungi. Pathogenesis has always been a priority, although people’s perceptions of what are pathogens are often confused. After early emphasis on heart rot, focus changed to decay following wounding in the 1960’s and 70’s, then in the 1980’s to natural establishment in branches and trunks by fungi latently present in sapwood. Research has now come a full circle, with interest in heart-rot communities increasing in the last 15 or so years.
Ecologically, decay is a good thing, indeed it is essential, because it releases nutrients locked up in dead tissues, making them available again for use by trees. Moreover, this decaying wood provides habitat for thousands of species of invertebrates, birds and other mammals. This includes habitat for endangered species. The initial decay communities are largely determined by abiotic conditions in the wood and, since abiotic environment varies both between tree species and between branches and trunks, so do fungal communities. For example, central trunk tissue - heartwood - sometimes contains compounds that are inhibitory to fungi, whereas sapwood of angiosperm tree branches usually does not. The heartwood of oak (Quercus) is rich in tannins and other polyphenols, has a pH of around 3.5 and relatively few species, e.g. Fistulina hepatica, of wood decay fungi have evolved to cope with such stressful conditions. In contrast, the central tissues of beech (Fagus) trunks are much more benign and many more fungal species can develop. The fungi that develop initially enter in many ways including through large broken limbs, damaged roots, by killing root tissues or as latent endophytes. In branches initial communities usually form from fungi latently present in functional sapwood. Although different angiosperm trees probably have a similar set of latent endophytic propagules in their sapwood, different initial decay communities develop in branches of different tree species due to different microclimatic factors such as rate of drying, temperature and gaseous regime. Over the course of decay, fungal communities change due to antagonistic actions of different fungi and changing environment. This talk will be illustrated largely with examples of fungal communities in attached branches and standing trunks of oak and beech trees.
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