Laetiporus sulphureus – a brown rot
A Basiomycetes, L. sulphureus causes stem decay and fracture. It occurs widely across the globe and affects a broad range of species (Sinclair et al. 1987).
L. sulphureus is an important fungus as it is found to be a common cause of failure in trees. It was reported as the most common cause of decay of failed trees after the great storm of 1987 (Gibbs and Greig, 1990). As a brown rot, this fungus essentially favours cellulose and hemicellulose leaving the slightly altered lignin intact (Rayner and Boddy 1988). In simple terms, enzymes act to split the cellulose molecules and therefore the microfibrils into shorter lengths. The removal of the cellulose (carbohydrate) is what causes the loss of tensile strength and gives the wood a brittle consistency (Schwartz et al, 2013). This embrittlement, under stress such as loading or wind, causes tensile stem fractures as seen in figure 3. Schwarz has found consistently in experiments that just a small amount of weight loss (as a result of fungal decay from L. sulphureus) led to a ‘drastic reduction in strength’ (Schwarze and Fink 1994; Schwarze 1995).
Furthermore, the ‘very frequent failure of infected hosts is presumably closely connected with the preferred occurrence on tree species with heartwood. In the stem of Robinia or Oak, a heart-rot caused by L. sulphureus can result in the bulk of the stem cross section being destroyed, without the host reacting in an active manner to the decay’ (Schwarze et al, 2013). Therefore this fungus can be particularly problematic in tree hazard assessment as the symptoms may not be apparent due to the lack of reaction wood or it only becoming evident at a late stage. The fruiting bodies of the fungus are also seasonal (July to October).
Ganoderma applanatum – a white rot
Ganoderma applanatum is a Basiomycetes which causes root and butt decay. It occurs widely throughout the world affecting a wide range of broadleaved and some conifer species (Sinclair et al, 1987).
A white rot, this fungus differs in its mode of decay by degrading the lignin from wood. It is generally considered to be a selective delignifer meaning enzymes breakdown lignin leaving the cellulose strands intact. This leads to wood with reduced stiffness but a high tensile strength. One of the key contrasting features of white rot to brown rot is that the loss of lignin and greater flex means more movement in the tree – and the response is the development of reaction wood to counter that movement. Therefore potential concerns are more apparent.
A common effect is ‘bottle butt’ seen at the base of trees where the base is decaying or roots rising from the ground where the underside of roots are decaying. The effect is due to ‘increased cambial activity as a response to the increased flexure of the stem in the area affected, leading to the local formation of unusually wide annual rings.’ (Schwartz et al 1997, p.12). Also G. applanatum, like other Ganoderma spp., differ to brown rots such as L sulphureus as the brackets are perennial.
So considering the implications for the structural integrity of trees, because of the more obvious signs of biomechanical issues and infection, whole tree failure is less of an immediate concern (providing the inspection schedule is right and thorough). Also this type of decay is thought to develop slowly, as Schwarze points out, ‘selective delignification is now regarded as being of less concern for tree safety than other kinds of decay, having been shown in recent studies to cause only a slow decrease in strength’ (Schwarze, 2000 cited in Schwarze 2003 p. 60).
So while this may mean a high value tree with G. applanatum can be potentially retained for longer, perhaps even decades, it must be noted that cellulose degradation can follow in later stages of the decay (Schwartz, 1997, p.13) and so infected trees are prone to fail, just later. Ganoderma spp. were found to be another significant fungus responsible for whole tree failure following the 1987 storm (Gibbs and Greig, 1990) although this may be subject to misinterpretation as the study refers to Ganoderma spp. as one entity, rather than individual species, and as we believe now, different species within this group have varying levels of invasiveness.
Schwartz, again, has found differences in experiments where G. adspersum and G. resinaceum frequently breached the reaction zones but G. applanatum did not as it was not able to modify the defensive compounds in wood (Schwarze et al, 2003) suggesting some tree species can more effectively compartmentalise G.applanatum.
Most practical arboricultural guidance concerning decay fungus generalise the strategies of fungus types – that is how brown and white rots affect trees – but there is a growing body of research that indicates some fungus to be capable of different modes of decay or producing a decay showing elements of both white and brown rot. Major advancements in genetic analysis of fungi is producing research is demonstrating the inadequacy of the white rot/ brown rot paradigm. The two decay modes are distinguished by the presence of ligninolytic peroxidases and /or the level of enzymes affecting crystalline cellulose (Riley et al, 2014). Riley analysed 33 fungal genomes and found ‘some species lack [peroxidases], and thus resemble brown-rot fungi, but possess the cellulose-degrading apparatus typical of white-rot fungi…Our results indicate that the prevailing paradigm of white rot vs. brown rot does not capture the diversity of fungal wood decay mechanisms’ (Riley et al, 2014, p9923). Schwarz et al (2013) has found several fungal species, such as Inonotus hispidus, able to switch between decay modes based on the host species. Therefore, if decay types are operating more on a spectrum rather than a dichotomy and that fungal species behave differently depending on species, with greater of lesser effects on structural integrity, this has implications for tree hazard assessment.
Tree Risk Management Portfolio - M140849 