Fagus sylvatica, once felled (or following death), can quickly become riddled with decay pockets. It is suspected that fungal endophyte species latently present within the tree’s system, which cause no disease / decay symptoms during the host’s life, are responsible for the rapid manifestation of decay following death. The exact entry of these endophytes in Fagus sylvatica is not largely understood, though it is anticipated that leaf scars, bud scale scars, the thin periderm, and lenticels are the main means of entry.
Laboratory experiments in the past have given weighting to such an assertion, when freshly-cut and healthy branches of Fagus sylvatica were incubated under varying drying regimes. After a period of 14 days, active mycelium could already be observed on the branches, indicating that latent endophytic fungi present at the time of the healthy branches being cut were the species now developing mycelial networks, and it is thought that the reduction in water content and increase of oxygen availability lead to the development of the mycelium (basically initiating the switch from latent to active).
Past studies had not shown that Fomes fomentarius, a very common white rot of Fagus sylvatica, could persist within healthy Fagus sylvatica for a period of time before the host’s death (or detachment of a part of the tree), though the decay strategy adopted by Fomes fomentarius (in Fungal Strategies of Wood Decay in Trees, the authors here allege, though I cannot find a specific direct reference to this in the book asides from comments regarding colonisation of the xylem in living hosts) suggests that the species should be able to exist as an endophyte prior to the right conditions manifesting within the host. The reason for past studies not showing Fomes fomentarius as an endophyte may perhaps be because past studies looked only at branch wood, whereas Fomes fomentarius is known to principally colonise the main stem and larger limbs of its host.
In this study, therefore, samples were taken in February from the ‘lower’ canopy (diameter: 5-10cm), ‘upper’ canopy (diameter: 10-25cm), and main stem (diameter: 20-50cm), in order to test whether Fomes fomentarius does indeed exist within healthy wood. 360 main samples were taken from ten (outwardly) healthy Fagus sylvatica, all of which were 70-85 years old, once they had been felled, and then the samples were transported to a laboratory and processed within 24 hours into 2,160 smaller samples.
Once the 2,160 samples had been processed and incubated for the duration of the study (either 8, 16, or 24 weeks), mycelium observed to have grown from the samples were analysed, identified, and recorded. Of the 2,160 samples, 61 had mycelial growth of Fomes fomentarius. The table below shows the breakdown of how long it took for the mycelium to begin developing, and in what samples they were found (lower canopy, upper canopy, or stem).
What can be ascertained from these readings is that, whilst not a common endophyte (in these samples), Fomes fomentarius can indeed exist latently within a healthy Fagus sylvatica. Analysis of the locations of the samples also suggests that Fomes fomentarius will exist most frequently within the stem, much less frequently within the upper canopy, though not at all in the lower canopy. The data also indicates that Fomes fomentarius will not immediately begin its active mycelial phase following host death, but instead manifest after 16-24 weeks (or even longer, though this study ended after a 24 week period). A period of incubation is thus necessary, and active growth is likely initiated by a change in wood moisture content and the increase of oxygen (which may be the result of cracks associated with the drying wood).
The study did also address concerns regarding whether Fomes fomentarius presence was simply a result of contamination post-branch removal, and the authors suggest that because the samples were taken in February, before the sporulating season of the fungus (starting in March across Central Europe, where the study was done), contamination was unlikely. Additionally, because isolates were found growing within heartwood of the main stem and ‘upper’ canopy (where diameters of samples were over 10cm in diameter, and up to 50cm), it is unlikely that contamination would have caused this – if isolates were found in the xylem of the ‘lower’ canopy samples that were 5-10cm in diameter, contamination may have been possible. Furthermore, the location of the isolates correlates with the position of Fomes fomentarius sporophores, which are typically found on the main stem of the host. Lastly, all isolates of Fomes fomentarius were genetically different, which means contamination to such extents would have been very unlikely (particularly given the time of year).
Interestingly, the study also mentions that there has been evidence recorded by Danby, Boddy, & Lonsdale (but not published, it seems) of Fomes fomentarius being latently present within Betula pendula as well.
Source: Baum, S., Sieber, T., Schwarze, F. and Fink, S. (2003) Latent infections of Fomes fomentarius in the xylem of European beech (Fagus sylvatica). Mycological Progress. 2 (2). p141-148.
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