Fungal succession and wood decay in living trees – a seminar report (Part I)

See part II here.

First and foremost, it is absolutely critical for me to extend my deepest thanks to Jon Hartill of Hartill Trädexpert for organising such a superb single-day event (here is an overview of the itinerary) in Sweden. Indeed, bringing together three such critical speakers (Ted Green, Lynne Boddy and Frank Rinn) who, as was expected, but more acutely so in hindsight, worked well as a unit and offered much in the way of vital information and data, was not likely a simple task. Of course, it worked, and my 12 pages of hand-written scrawlings is testament to this. Thus, the purpose of this blog post (or posts, should I say) is to share the information gathered, so that it can be disseminated more widely and stimulate thoughts amongst the internet audience.

Some of the delegates from this seminar. As you’ll see from the below picture, the setting was also rather fitting for a discussion on trees…

Say hello to the town of Kungalv!

Ancient trees – what secrets remain?

The day was opened by Ted Green, the founder of the Ancient Tree Forum. Here, however, his role was not so much to discuss what is known of our ancient trees, but of what we still need to know – what don’t we know? Granted, there’s probably an utterly frightening amount we are yet to understand, though Ted’s talk was not to wallow in such an angst but instead to prompt directed focus towards aspects of ancient trees we really do need to understand next.

Principally, it was important to set the tone of the presentation: hollowing is an entirely natural process, which very few ancient trees escape. In fact, do they even want to escape such a ‘fate’? Arguably, the answer is no. Anyway, at this early stage, Ted made the distinction between the decay of central wood, separating the decay of heartwood (i.e. Quercus) and the decay of ripewood (i.e. Fagus). ‘True’ heartwood forms when phenols and other extractives and toxic substances to fungi are deposited within the non-living (largely) woody tissues, as sapwood becomes redundant. Ripewood, conversely, forms via different mechanics associated with wounding and other events. However, this was a point posited just to illustrate context, more than anyway.

The real ‘meat’ began when Ted began assessing where the notion of hollowing being bad came from: forestry. As an economic practice, it is obvious that hollowing would be seen as destructive, as basal decay hampers return upon the investment of a stand. Despite this, when moving away from forestry, we can observe that hollow trees don’t drop like flies under wind-loading events – as was the case when the 1987 storm (hurricane) hit the UK and hollow trees stood whilst solid neighbours fell. Certainly, hollow trees did fail and when they did it was oft at the point where the internal hollow met sound wood, though the general jist is that hollowing does not necessarily infer a risk of failure beyond that of what a solid tree would be considered to possess. Ted speculated that this hollowing meant that, under wind loads, the main stem could flex and ‘safely’ deform (a bit like a hosepipe when squeased slightly) under tension and torsion, thereby protecting the stem from forces that could overload a solid stem, which cannot deal with such a load in a similar manner (because it is not hollow, to any degree). In oak, for instance, Pseudoinonotus dryadeus (the Eiffel Tower fungus) can actually be seen as a fungus that aids with tree stability, by prompting pronounced buttressing and the creation of wood that over-compensates for the more centralised decay (as was discussed by Frank Rinn later on), whilst also allowing for the oak to deal with wind loading more effectively, given the presence of the hollowing internal to the trunk.

Strong buttressing as caused by Pseudoinonotus dryadeus decay on oak. Does the internal hollowing even impact upon the tree’s structural stability, assuming the buttress roots laid down afford the necessary support? Thus, dooes pruning even have a beneficial impact upon reducing risk, when accepting that pruning damages the tree’s ability to photosynthesise and thus manufacture the sugars demanded for wood formation in these buttress zones?

Indeed, reaction growth comes in forms beyond buttressing – stems also flute; sometimes, quite majorly, in mature and veteran trees. This fluting can, in times of wind loading, afford the tree additional stability, through the over-compensated high wood quality, which allows the tree to deal with loading forces by acting akin to a coiled rope. Certainly, torsional loading against the direction of fluting is going to be a marked issue, though otherwise such fluting can be beneficial for stability – even when there are appreciable central hollows. In fact, this led on to another important point: the extent of hollowing and the resultant residual wall thickness (think Mattheck’s t/R or part of the Wessolly’s Statics Integrated Approach model) means very little, as it assesses a tree in blatant disregard for its wider context (exposure, lean, leaf area, wind drag coefficient, management history, the off-set nature of the hollow, etc). Without appreciating these factors, of which there are numerous, how can one state that hollowing is bad and will increase the risk of tree failure?

The talk by Ted then actually moved away from hollowing somewhat and onto other aspects of ancient trees. Specifically, the practice of pruning arose, wherein Ted commented that pruning back to the branch collar in older trees is potentially more destructive than leaving stubs – stubs that will afford epicormic growth and the formation of a new crown / part of the wider crown area. Interestingly, he used the example of beavers felling trees not at their base but a little up from the base, from which these trees oft repsrouted and formed natural coppice. The same, he suspected, could be the case for aerial pruning – leave a stub.

This consideration took Ted onto further points, of which the main one was that of where trees fail most routinely. Indeed, aroung 70% of all failures within a tree at at the branch level – of these, many fail not at the collar but out along the branch itself, thereby leaving a stub. Using examples of trees from Windsor and elsewhere that failed in such a way, he demonstrated that new but lower crowns were formed in the years after; even in cases where every single major limb failed on the tree, thereby creating a tree form like what can be seen in shredded trees.

An oak tree that lost almost all of its crown in the years after the 1987 storm. Only one limb actually remains (red square). The other limbs all failed (green arrows) and have since resprouted and formed a new crown. Should we be managing our older trees like this and, to extend this question, should we be managing all of our trees like this, if we are to mimic natural forms of failure?

Further to this, Ted got talking root failure in strong winds. In his experience, following the 1987 storm, some older trees began to decline either partially (in select area of the crown) or wholly in the years following, for no outward apparent reason. Ted’s suspicion is that, in place of aerial failure, structural roots failed and the connectivity to specific connected parts of the crown from these roots thus was severed, triggering localised dieback and retrenchment. Therefore, the question of whether localised root plate damage caused localised aerial dieback in older trees was asked and, assuming the answer was that this does occur, it could actually be beneficial for the longevity of the tree – it can retrench, form a lower crown and thus increase its safety factor. In fact, it would also suggest that the occurrence of fungi such as Meripilus giganteus on older trees, in certain instances, would be as a consequence of saprotrophism, in place or parasitism – the fungus follows the root damage and metabolises the severed roots. I then asked Ted whether he thought that the same phenomenon could occur in grazing ecosystems, in which cows, pigs, sheep or otherwise are grazed amongst wood pasture. His answer was one of grazing also being a cause of retrenchment, wherein grazing pressure damages certain structural roots and this leads to subsequent localised aerial retrenchment.

Can select root damage under wind-loading conditions bring about select crown retrenchment, through the connection of certain roots to certain portions of the crown? If so, can this damage, assuming not all roots are severed, improve the longevity of the tree and increase its stem’s safety factor, in spite of hollowing and major yet probably transient dieback?

Other questions raised within Ted’s talk were as follows: (1) are animal pharmaceuticals, often in the form of de-wormers, harmful to the mycorrhizal networks and the tree’s rhizosphere (i.e. soil biota), when they are excreted by the animal in the vicinity of the tree?, (2) is acid rain the most damaging impact upon our old trees, because of its impact upon the soil?, (3) are earthworms the most crucial soil organism for older trees, given their ability to aerate soil and to recycle nutrients by consuming absiced leaves (including those where over-winter pathogens reside, such as oak mildew), and (4) does stress throughout the tree’s life give it the best chance of reaching the veteran or ancient stage, when noting that slower growth and a more responsible management of energy is more sustainable? To these questions, we do need more research.

Take-away points from this talk are, therefore:

• major hollowing of trees isn’t perhaps an inherently bad thing – notably in older trees,
• we need to understand the species-specific and age-specific impacts of fungal decay upon trees before confidently exclaiming an increased and unallowable risk of failure,
• a tree’s situation and history has a direct and marked impact upon the risk brought about by a hollow
• damage to tree roots on older trees and the possible associated crown effects demands more investigations,
• we need to determine how we should be pruning older trees, if we are concerned with their longevity, and
• the rhizosphere’s importance for the health of older trees is a very viable area of research

I’ll write part II up in the coming times, which will focus on Lynne Boddy’s presentations. The third part will be Frank Rinn’s incredible afternoon talk.