These examples show some common cases (other than perhaps the tree graft) that tree risk managers may face and indicates how some of the developing theories and evidence have relevance in assessing tree defects. There are major steps being made in understanding the complexities of colonisation especially with the advancement of genetic analysis and this area is likely to continue to give us greater insights which have the potential to guide us in tree risk management in terms of rates and relevance of decay.
However, existing guidance, especially in regard to fungal decay, is still currently heavily based on anecdotal evidence and observations of industry leaders, rather than empirical evidence. On the ground, this means the likelihood of condemning trees is higher as human nature is to err on the side of caution. I suspect we are losing significantly more older trees than we need to. We need to gain more knowledge on rates of colonisation and understand better the factors affecting this such as tree species, ability to access nitrogen, ability to overcome fungitoxic compounds, and the interaction between fungal species or even fungal genets in the same species.
An interesting area affecting rates of colonisation is the infection source. Great weight has traditionally been placed on the impact of inflicted wounds (i.e. in the British Standards) but research (i.e. Boddy 1994, Parfitt et al 2010) is indicating the existence of latent fungus to be much more relevant than previously thought. Parfitt et al believe wood decay fungi are latently present in all angiosperm species and its development is only constricted by environmental factors (Parfitt et al, 2010). This useful in terms of justifying the importance of tree protection measures at development sites, as it underlines the high likelihood of a tree becoming diseased (i.e. the fungi is already present rather than it might become affected by airborne propagules).
What we also need as risk managers, are more tools and guidance especially in terms of thresholds to support decision making in more relevant areas. For example, as Slater (2018a) points out there has been an enormous level of research in areas such as hollowing (for example Mattheck’s T/R ratios) which are useful but hollowing is not a major cause of failure (like uprooting or bark included junctions). While there has been extensive new research into bark included junctions and natural bracing (Slater, 2018b), there is virtually nothing concerning root plates, and neither tools available to assist in identifying structural issues in root plates.
On the same theme, models such as CODIT (Shigo, 1977), considered the back bone of arboriculture, there are now researchers challenging the validity of such models. Not that these models can no longer be relied upon, but a more open, inquiring mind is needed in tree risk assessment. One good example is how some fungus may be more likely to cause axial cracking, combined with aspects of the tree’s natural anatomy (or inflicted through ill-considered pruning). And cracks, especially axial cracks, present ideal conditions for the spread of disease and is an example of where models of compartmentalisation do not apply.
Tree Risk Management Portfolio - M140849 