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.
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.
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.
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.
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.
Nothing much need be added, in light of who is narrating. As a 20-minute long film, it’s something you can readily watch at any point where you have some time going spare. There’s a few good segments on trees, including on ancient trees, deer, deadwood and wood-decay fungi. Really a fascinating watch!
I have been absurdly busy so haven’t been blessed with the time to get some blogging in, though I have been graced with thirty minutes of time this evening so without any further utterings we’ll delve right into the good stuff – trees and fungi (in my usual frenetic and incoherent manner). Plus, listening to some early Hawkwind has really got me in the mood to do something useful!
The cases are all from an absolutely splendid park down in Maidstone – Mote Park. Honestly, if you live anywhere near there, do pay it a visit and explore as much as you can (it’s massive!). The sheer abundance of mature and veteran trees provides for a magnificent display of fungi and, so I am told, there is a need to record the saproxylic insects on site on the many monoliths and moribund trees.
To kick this post off, I take you to a very interesting case of Pseudoinonotus dryadeus on oak – three of them, all of which are within 8-10m of one another and share a crown. All fruitings of the fungus are historic though its presence on all three trees makes for some tempting considerations – namely, the synchronicity of fruiting (are they similar genotypes?) and the means of colonisation (spore or something else?). Indeed, I can only infer some sort of fungal mysticism or sorcery in positing both aspects for consideration (there is no proof of either, per se), though it did make me think. Perhaps it will make you readers think as well! (!?)
And then…and then…more Pseudoinonotus dryadeus – literally 100 yards down the same path. Oh how Mote Park delivers! This example also really does demonstrate the magnificent buttressing induced by its decay on oak, as you’ll see.
Would you then believe it? Essentially opposite (no joke) were two colossal beech trees fenced-off (as if that ever stopped me??!) that, as anyone who has seen mature or veteran beech buttressing all over the place like egg whites pour out of a broken egg when broken too aggressively (nice analogy? – likely not), drew me in. Was I disappointed? Not at all! Ganoderma australe and Meripilus giganteus all over the option.
To finish up, because I’m getting tired and I am up early tomorrow, here’s something to sit on whilst you ponder the plethora of ultimate questions spewed forth from my mind with little restraint – a dryad saddle. The host? Not sure – lots of ash about though one can never rule out sycamore (unless you’re in the middle of a Douglas fir plantation?). These had actually already over-matured, which means you can see dryad saddle (i.e Cerioporus squamosus – named, prior to that, Polyporus squamosus) out there if you look!
The arthropods are vast in terms of species, and include ants, beetles, butterflies, mites, moths, spiders, and so on. Therefore, covering the entire spectrum of arthropods in this section is impractical, though the general provisioning by trees will be outlined and species will be used to illustrate given examples.
Many arthropods are considered to be saproxylic in nature – they principally utilise dead woody material (both standing and fallen, in both dead and living trees) as habitat, for at least part of their life cycle, though they may also rely upon fungal sporophores associated with the presence of deadwood, as is to be detailed below (Gibb et al., 2006; Harding & Rose, 1986; Komonen et al., 2000). Of all the saproxylic arthropods, beetles are perhaps the most significant in terms of the proportion occupied of total saproxylic species worldwide (Müller et al., 2010), though saproxylic flies also feature in great numerical abundance (Falk, 2014; Harding & Rose, 1986).
Beetles may be either generalist or specialist in nature (on either broadleaved or coniferous hosts), and they will normally require a host with an abundance of deadwood (or large sections of coarse woody debris) usually over 7.5cm in diameter that resides within an area typically not heavily shaded (Müller et al., 2010; Siitonen & Ranius, 2015). This may be, in part, due to many beetle species (in their adult stage) requiring nectar from herbaceous plants, which would be lacking in woodland with significant canopy closure (Falk, 2014; Siitonen & Ranius, 2015). This means that veteran trees amongst wood pasture and parklands (including in urban areas) may be particularly suitable (Bergmeier & Roellig, 2014; Harding & Rose, 1986; Ramírez-Hernández et al., 2014; Jonsell, 2012; Jørgensen & Quelch, 2014), though this is not at all a steadfast rule as species may also be found abundantly in (perhaps more open) woodland, and particularly where there are large amounts of veteran trees and deadwood – around 60 cubic metres per hectare, according to Müller et al. (2010). Granted, they are found particularly in older (mature to veteran) trees, including within cavities that possess wood mould, water-filled rot holes, dead bark, exposed wood, sap flows, fruiting bodies (of fungi and slime moulds), mycelia of fungi, dead branches, and dead roots (Carpaneto et al., 2010; Falk, 2014; Harding & Rose, 1986; Siitonen & Ranius, 2015; Stokland et al., 2012). Beetle species may also not necessarily associate preferentially with a species (or group of species), but with the conditions aforementioned that are present within a tree (Harding & Rose, 1986; Jonsell, 2012). At times, preferable conditions may be an infrequent as one veteran tree in every hundred (Harding & Rose, 1986).
Despite this, species preference is observed. For broadleaved obligates, heavier shade may be more necessary, and in such instances there is a closer affinity of the beetles with fungal mycelium. Because fungi tend to produce more mycelium in cooler and more humid conditions (though this does, of course, vary with the species), the broadleaved obligates may therefore be found normally in greater abundance where conditions are more suited to fungal growth, and their presence may thus be associated with a canopy openness of as little as 20% (Bässler et al., 2010; Müller et al., 2010). This is, of course, not a steadfast rule, and many open wood pastures may support a great abundance of saproxylic beetles (Harding & Rose, 1986).
It is also important to recognise that many species of saproxylic beetle are reliant upon particular stages of the wood decay process. For instance, species that require fresh phloem tissue will only be able to colonise briefly in the first few summers following on from the death of the phloem tissue (Falk, 2014). Other species require significantly-decayed wood in a particular micro-climate, and even of a particular tree species (Harding & Rose, 1986). There also exist intricate associations between species of fungi and saproxylic insects. Inonotus hispidus, which is usually found upon ash, is the habitat for Triplax russica and Orchesia micans, whilst the coal fungus (Daldinia concentrica), also oft found upon the deadwood of ash (Fraxinus excelsior), is the main provider of habitat for Platyrhinus resinosus (Falk, 2014). The birch polypore (Fomitopsis betulina) is also host to numerous species of Coleoptera (Harding & Rose, 1986); as is the polypore Fomitopsis pinicola (Jonsson & Nordlander, 2006; Komonen, 2003; Komonen et al., 2000). This means that these species may be found where there is a suitable population of the fungus’ host species, where sporophores are present and will likely fruit again in the future, across numerous trees, and for many years. Most beetle species rely on oak more so than other tree species however, as oak generally lives for much longer and thus provides a wider array of different micro-habitats, and possesses increased compositional complexity as a result (Harding & Rose, 1986; Siitonen & Ranius, 2015).
Therefore, the loss of suitable habitat through active management programmes (including logging, and felling trees for safety reasons in urban areas) will have a very adverse impact upon saproxylic beetles, though also certain species of moth, and even species associated with saproxylic insects, including parasitic wasps, solitary wasps (which use beetle bore holes for habitat), and predatory Coleoptera (Harding & Rose, 1986; Komonen et al., 2000). Curiously, research by Carpaneto et al. (2010) concluded that trees that were ranked as the most evidently ‘hazardous’ were host to the most saproxylic beetle species, and their removal would therefore have a drastic impact upon local populations. Similarly, fragmentation of woodland patches suitable for saproxylic populations has led to a decline in the meta-populations (Grove, 2002; Komonen et al., 2000), as has deadwood removal in a managed site itself (Gibb et al., 2006). Interestingly, though not surprisingly, ‘deadwood fragmentation’ also has an adverse impact upon saproxylic insect populations (Schiegg, 2000).
Both ants and termites also benefit from the presence of deadwood. With regards to both, nests will usually form at the base of a tree or at an area where there is at least moderate decay – enough to support a viable population (Jones et al., 2003; Shigo, 1986; Stokland et al., 2012). Ants and termites both follow CODIT (compartmentalisation of damage in trees) patterns in relation to how their nests progress, and thus their territory will increase as fungal decay propagates further into the host. Ants will not feed on the decaying wood of the host however, and will simply use the decaying site as a nesting area. Conversely, termites will feast upon decayed wood and essentially control (perhaps by slowing down) the spread of fungal decay in a manner that provides as much longevity of the host as possible for a viable nesting site (Shigo, 1986). In tropical rainforests, termites are in fact considered to be one of the principal means of wood decomposition (Mori et al., 2014), and thus the provisioning of deadwood habitat is absolutely critical. Without decaying wood within trees therefore, ants and particularly termites will lack a potential habitat, and thus where a stand is actively managed populations may be markedly reduced (Donovan et al., 2007; Eggleton et al., 1995). Of course, termites are not necessarily to be desired when they are invading the wood structure of a property, and therefore deadwood is not universally beneficial (Esenther & Beal, 1979; Morales-Ramos & Rojas, 2001) – at least, when human properties are involved.
The presence of deadwood may also be beneficial for ground-nesting and leaf-litter dwelling spiders, which can utilise downed woody debris (particularly pieces with only slight decay) for both nesting and foraging (Varady-Szabo & Buddle, 2006). In fact, research by Buddle (2001) suggested that such spiders may more routinely utilise downed woody material when compared to elevated woody material (dead branches and telephone poles) because of the greater array of associated micro-habitats, and particularly at certain life stages – such as during egg-laying, for females (Koch et al., 2010). Furthermore, as fallen woody debris can help to retain leaf litter (or even facilitate in the build-up leaf litter), spider populations are more abundant and more diverse in sites where such woody debris is present (Castro & Wise, 2010). Therefore, where woodlands are managed and areas are clear-cut, spider populations may be markedly reduced in terms of the diversity of species. However, generalist species may benefit from the amount of cut stumps (Pearce et al., 2004). Curiously, Koch et al. (2010) suggest that spiders may perhaps benefit from woodland clearance, because the vigorous re-growth of trees and the higher light availability to the woodland floor (promoting herbaceous plant growth) increases the abundance of potential prey. Despite this, old-growth species will suffer (Buddle & Shorthouse, 2008), and thus the population structure of spider populations may dramatically change.
Soil mites are a further group that benefit from coarse woody debris, though also from hollows and holes throughout the basal region of a tree (including water-filled cavities), and from fungal sporophores and hyphae associated with wood decay (Fashing, 1998; Johnston & Crossley, 1993). Typically, termites will use fungi and insects found within the wood as a food source, and the wood structure itself will provide for an array of niche micro-habitats that are critical at different life stages of a mite. Certain mite species are obligates that associate with coarse woody debris exclusively, and may in fact only be associated with certain species’ woody debris. Additionally, mites may utilise woody debris and hollows within trees to parasitise upon other species using the ‘resource’, with both lizards and snakes being parasitised by mites following their frequenting of such resources. Beetles may also be parasitised, though the mite in such an instance may use the beetle as a means of entry into woody debris (Norton, 1980).
It is not just deadwood that arthropods will utilise, however. Foliage, both alive and abscised, is also of use (Falk, 2014). For example, the ermine moth (Yponomeutidae) will rely upon the living foliage of a host tree as a food source, and the bird cherry ermine moth (Yponomeuta evonymella) is one example of this. During late spring, larvae will fully defoliate their host Prunus padus, before pupating, emerging, and then laying eggs upon the shoots ready for the following year (Leather & Bland, 1999). Many other moth species will, during their larval stage, also behave in such a manner and thus defoliate their host – either entirely, or in part (Herrick & Gansner, 1987). Other species may alternatively have larvae mine into the leaf and feed upon the tissues within (Thalmann et al., 2003), such as horse chestnut leaf miner (Cameraria ohridella). Flies, including the holly leaf-miner (Phytomyza ilicis), will also mine leaves in a similar fashion (Owen, 1978). Ultimately however, the same purpose is served – the insect uses the living tissues of a leaf to complete its life cycle, and fuel further generations.
Fallen leaf litter, as briefly touched upon earlier when discussing spiders, may also be of marked benefit to many arthropods. Ants, beetles, and spiders are but three examples of groups that will utilise leaf litter as a means of habitat (Apigian et al., 2006). Beetles will, for instance, rely upon leaf litter to attract potential prey, though also to provide niche micro-climates that remain relatively stable in terms of humidity, light availability, and temperature (Haila & Niemelä, 1999). Their abundance may, according to Molnár et al., (2001) be greatest at forest edges, perhaps because prey is most abundant at these edge sites (Magura, 2002). Of course, this does not mean that edges created through artificial means will necessarily improve beetle populations, as research has shown that there are few ‘edge specialists’ and therefore populations usually will go into decline where there has been significant disturbance. Unless management mimics natural mortality events of forest trees, then constituent beetle populations may thus suffer adversely (Niemelä et al., 2007).
With regards to ants, Belshaw & Bolton (1993) suggest that management practices may not necessarily impact upon ant populations, though if there is a decline in leaf litter cover then ants associated with leaf litter presence may go into – perhaps only temporary (until leaf litter accumulations once again reach desirable levels) – decline (Woodcock et al., 2011). For example, logging within a stand may reduce leaf litter abundance for some years (Vasconcelos et al., 2000), as may (to a much lesser extent) controlled burning (Apigian et al., 2006; Vasconcelos et al., 2009), though in time (up to 10 years) leaf litter may once again reach a depth suitable to support a wide variety of ant species. However, the conversion of forest stands into plantations may be one driver behind more permanently falling ant populations (Fayle et al., 2010), as may habitat fragmentation (Carvalho & Vasconcelos, 1999) – particularly when forest patches are fragmented by vast monoculture plantations of tree or crop (Brühl et al., 2003). The conversion of Iberian wood pastures to eucalyptus plantations is one real world example of such a practice (Bergmeier & Roellig, 2014).
Also of benefit to many arthropods are nectar and pollen. Bees, beetles, butterflies, and hoverflies will, for instance, use nectar from flowers as a food source (Dick et al., 2003; Kay et al., 1984), and generally (but not always) a nectar source will lack significant specificity in terms of the insect species attracted (Karban, 2015). Despite this, different chemicals secreted by different flowers, and the toxicity of certain nectar sources to particular insects, means certain tree species may only be visited by certain insect species (Adler, 2000; Rasmont et al., 2005). Tree diversity may therefore be key to sustaining healthy insect populations (Holl, 1995), and where species may prefer to frequent herbaceous plant species the presence of a diverse woodland canopy above may still be very influential (Kitahara et al., 2008). This may be because a diverse array of woody plant species increases the diversity of herbaceous species. At times, pollen may also be a reward, as may (more rarely) a flower’s scent. Karban (2015) remarks that all are collectively dubbed as ‘floral rewards’.
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A quick heads up that the Ancient Tree Forum summer conference at Epping Forest is now open for bookings at this link. At £50 for the two days it’s certainly worth it, and I can attest to the quality having gone last year to the one in Dorchester.
Expect more additions to this blog next week, though having been very busy lately I haven’t had a chance to get much uploaded!
Grazing rights on commons must be safeguarded, for these rights are an historical relic of an otherwise aggressively-advancing culture. Indeed, there are a wide range of benefits from grazing, including the ecological, socio-economic and cultural, though the New Forest – and probably many (or all!) other sites where grazing occurs under tree canopies – is also subject to the damage associated with unrestricted grazing.
Certainly, the number of horses within the New Forest, the unrestricted nature of their movement and the lack of safeguarding measures (and probably food) around veteran trees has resulted in some quite substantial (yet currently rather isolated and sporadic) damage to the beech trees. I would expect much of the damage comes during winter, when the horses are searching for food that is not in such great abundance, and luckily (or not!?) I managed to watch a few horses de-barking a fallen limb and the butt of one particular beech tree (whilst another horse was grazing upon the lower branches of holly), in addition to some recent examples of damage on other beech.
I was forunate to be able to spend some time in the New Forest yesterday, having driven back from Somerset after picking up a microscope (more on that, in due time). When last down there, which was during mid-summer, I spent a few hours sojourning around the Bolderwood / Knightwood Oak ornamental drive, with specific focus upon the myriad of mature and veteran beech pollards that dressed the roadside. One beech, even then, alluded to fungal parasitism, given its dire vigour and evident crown retrenchment (perhaps associated with ground compaction, given its close proximity to a car park and the Knightwood Oak). Therefore, I paid a visit to this beech, with the hope of finding some fungi – and I wasn’t disappointed!
I’ll actually be honest and say this beech is testament to the ability for the species to provide for many wood-decay fungal species. I really don’t think I have ever seen a tree more covered in fruiting bodies of many species than this one, and we’ll run through the suspected species below. First, we’ll look at the tree as a whole, however, and from the first image I don’t think there’s any debate over its poor condition. Granted, with the impending demise of a tree, weak fungal parasites and saprotrophs can enter, and this alludes to the cyclical aspect of energy transfer. In time, this beech will be the food for other plants and trees, though for now it’s fungal food.