Trees located within the urban environment are, by default, more of a risk to public and property. This is because beneath many such trees are permanent (or more frequently temporary) target zones, which would not necessarily be found to such a degree in a rural setting. Having said that however, many urban trees have a very low level of risk, and are thus deemed to hold an acceptable level of risk. Granted, some urban trees present more of a risk, and the manifestation of a greater level of risk can occur through a variety of different means. One significant contributing factor to the increaed risk of an urban tree, and which is the focus of this post, is fungal decay. Where trees have been wounded, are stressed, or may even be largely in good condition, fungi can enter into and establish within the structure of a tree, break down the lignin and / or (hemi-)cellulose within the wood, and thereby reduce the tree’s structural integrity. In turn, the associated hazards increase. Therefore, understanding more about fungal decay is important, and the authors of the study discussed below pursue a greater undestanding of fungi in our urban trees.
By using some of the trees in Helsinki, Finland, the authors sought to gain more of an understanding about the abundance and patterns of decay of certain common wood-decay fungi found in the city. In total, 194 trees (76 of the genus Tilia, 60 of the genus Acer, and 58 of the genus Betula) in both parks and streets were assessed within the study, of which all were showing signs of health decline, or were outwardly hazardous. During the inspections to determine that such trees were indeed hazardous or in decline, information was gathered in relation to the external symptoms (fungal brackets, cracks, cavities, etc) and their exact locations (including height upon the structure) upon the trees. Subsequently, all were felled over a period of time between 2001-2004, and from there the research began into assessing what fungal species caused the decay (at times, DNA sequencing was needed, and notably for differentiating Ganoderma species from one another), and how extensive horizontal decay was within the tree’s structure (by taking cross-sections around areas of dysfunction).
Following on from the investigations post felling, the authors identified 13 fungal species and genera within the host Acer spp., Betula spp., and Tilia spp. Pholiota sp. was most frequently found to have colonised all three tree genera, and often other fungal species were found alongside. Other fungal species, such as Piptoporus betulinus, were only found upon trees of the Betula genus, though of course the host preferences (not all fungi are generalists) of different fungal species will determine, from the limited range of tree species assessed, what fungi could possibly be present on each of the three genera, and how often (Rigidoporus populinus, for example, was predominantly – but not exclusively – found upon Acer spp.).
Interestingly, Piptoporus betulinus was also never found alone within its host, which therefore suggests other fungi will also have contributed to decay within the Betula spp. featuring in this study (this may align with its strategy of specialised opportunism, where the fungal spores wait for the host Betula sp. to become stressed before attacking). In fact, the only three fungal species isolated from the trees of this study that occurred alone around 50% of the time were Ganoderma applanatum (syn: G. lipsiense), Hypholoma sp., and Phellinus igniarius. This perhaps suggests they they are either far more competitive and defend their ‘patch’ aggressively, they enter into trees and take advantage of substrate conditions other fungi cannot similarly exploit, or succeed into substrates other fungi had already decayed (and then exited). Below, the table (pardon the small size) outlines all fungal species isolated, and from which hosts.
Additionally, only certain fungal species could actually be readily identified by their sporophores (including Rigidoporus populinus and Ganoderma applanatum), because not all fungi had produced sporophores at the time of inspection and felling (and if they did, they may have been few and far between – Hypholomoa sp. never produced fruiting bodies, and no symptoms were externally evident to suggest it was there). However, the authors do note that sporophores were found on the trees where decay was most extensive in the radial direction, and this may indeed make sense when one considers that fungi will not produce a sporophore (an exit strategy) unless there is a need to do so (such as running out, locally, of substrate – indicating wider radial spread in the direction of the sporophore). Not only this, but even where fungal sporophores were not evident, crown dieback could be observed as a result of decay by Piptopirus betulinus and Inonotus obliquus, for example. The table beneath further outlines on how some fungi, whilst present in the tree (either outwardly or via laboratory analysis), did not cause crown symptoms to show. For many fungal species, the crown symptom rate is actually very low.
In relation to the extent of decay, most trees were either hollow (57%) or were significantly decayed (35%), and particular fungal species were only found in hollowed trees (including Armillaria spp., Phellinus igniarius, and Pholiota spp.). Conversely, species such as Ustulina deusta (syn. K. deusta) and Ganoderma applanatum (the former, exclusively) were found where the tree was in advanced stages of decay, and variation existed between how frequent their presence was (the latter was more routinely found as a primary decay agent, whereas the former was not found in discoloured wood but only where wood was already very decayed). Of the 8% of trees where wood was only discoloured, Piptoporus betulinus and Ganoderma applanatum were two examples of fungal species that could be found. From this data, one can recognise how different fungi will occupy different stages of the decay process, and therefore a succession of sorts may perhaps occur amongst individual species.
Building upon the above, the authors also recognised how different fungal species would create different radial decay patterns (see the below table). Species including Cerrena unicolor and Ganoderma applanatum were observed to more readily create decay cross-sections of greater radial spread, for instance (and also invade the vascular cambium). However, most species were found to extend out to across more than half of the cross-sectional area of a tree’s main stem, which means the critical t/R value of 0.3 (Mattheck’s hollow tree failure theory) is quite significantly encroached upon, and for 5-6 of the fungal species here, surpassed. In terms of the location of decay, it was found that Ganoderma applanatum was usually found in the lowest 1m of the main stem, which means entire tree failure is very possible when decay is very extensive. Other fungal species were found in different locations, including branch forks (Rigidoporus populinus). In such an instance, decay may only induce failure further up the tree’s structure.
Looking further at the table above, a few additional observations can be made. Firstly, as even stated by the authors, Pholiota spp. may be pathogenic within the rooting system of a tree, and because this study only assessed the above-ground structure, exact extents of decay within host trees may not have been fully understood. Armillaria spp. is also a root pathogen, and the same comments apply to that genus as well (thought A. mellea and A. ostoyae were not isolated in this study, so Armillaria spp. may be under-represented for severity). However, in general, the table does demonstrate that fungi may very well facilitate failure in the stem or branching area, and particularly for those that extend over the 70% threshold – Ganoderma applanatum may very well cause basal failure, whilst Rigidoporus populinus may cause failure in the lower crown (at a branch junction).
In recognising all of this, we can begin to appreciate how fungal strategies (some are notably pathogenic, whilst others are not so) have significant impacts upon how we manage urban trees, and therefore we must understand specific species’ strategies and cater management to meet the ‘needs’ of the tree. There is little use in not discriminating between different fungi, as some (Cerrena unocolor, Ganoderma applanatum, and Inonotus obliquus) certainly have the potential to be more hazardous than others (Pleurotus spp.). Additionally, as some fungal species (Hypholoma spp.) will more typically (or exclusively) succeed into trees already extensively decayed or hollowed (see the below table), recognising their presence may be important for understanding what condition the host may be in. Granted, this is likely common knowledge, though it is still important to recognise this over and over again, as it really is a main crux of tree management for safety reasons in urban areas.
It is necessary to note, however, that only three tree genera were studied in this situaton. Therefore, caution should be exercised in forming absolute conclusions, as many urban locations consist of tree species and genera over and above what was assessed in this case. Nonetheless, credit where credit is due – a fantastic study!
Source: Terho, M. & Hallaksela, A. (2007) Occurrence and decay patterns of common wood-decay fungi in hazardous trees felled in the Helsinki City. Forest Pathology. 37. p420-432.
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