| Added on 30/12/2011 | Article |
Rooting Excavations
By Andrew Cowan
Do you condemn a tree because a decay fungus has been identified growing on it’s roots?
Is the mere presence of a fungal fruiting body sufficient to sign the order to fell a tree?
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Or should we be giving such decisions greater consideration and asking more questions before we issue the death sentence to a tree which may have taken well over a hundred years to reach maturity?
Andrew Cowan puts forward an argument for a more cautious approach and considers how much we still have to learn about the coexistence of trees with the fungi that live in association with them.
Consider the scenario as you walk into a client’s back garden to inspect their pride and joy, a mature beech tree with a well structured branch framework and broad branch spread. Your client is keen to hear you describe how some judicious pruning work will enhance the aesthetic appearance of their tree, which was the reason they recently bought the property. However, when you explore behind the brambles, which are growing up around it’s base, you find some fresh Meripilus brackets. Now what do you do?
Those long sweeping branches and that climbing frame structure are calling you and the thoughts of chainsaws buzzing all day, branches crashing to the ground, perhaps even the planks you could cut from the trunk, and not to mention the money you could charge for such a big job. But wait are you jumping the gun? Should you be thinking about felling just because you have identified a fungal bracket? Should you not be thinking how long have the tree and fungus been living together and more to the point how many more years, if not decades, they could survive without a concern for the threats of storm damage or wind throw?
Now we were faced by just such a choice recently with a back garden beech tree, where a couple of Meripilus brackets appeared in summer this year. No one could say whether there had been any there last year, or the year before for that matter.
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So what should we do? Further investigation using decay detection equipment such as the picus or even a resistograph would be unlikely to tell us much about the roots, even if we had the budget to pay someone to do the survey. The only way to get more information about the spread of decay into the roots was to start digging.
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Not so long ago we would have had to get out the pick and shovel to try and scrape away around the buttress roots to see if we could work out how far the decay had spread through the root system. However, now we have the opportunity to use supersonic air-stream technology, in the form of an AirKnife or AirSpade, depending on who manufactured the tool in question. These tools channel high pressure air at upwards of 125 psi through a hand held trigger mechanism and down a tub to a nozzle.
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Finding the fruiting bodies at the base of the tree told us one thing that the Meripilus was growing on the tree, but how far had the decay progressed and was it affecting the root system. There was only one way to find out, roll out the high pressure air hose and plug in the AirKnife.
With the tree sitting close to a boundary fence, it was not going to be practical to excavate all the way around the base of the tree, but we did not think this was really necessary anyway. The tree had a slight lean and the Meripilus had appeared on the back of this lean, so we were most concerned about the strength of the roots on this side, and luckily this was the easiest part of the tree to get to. So we decided to explore around the roots nearest to the fungal fruiting bodies which were growing out adjacent to two main buttresses.
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The excavation work was quick, with the light sandy/gravel soil blown out to reveal the main roots within an hour or so. Although the process was slowed by the need to remove the larger stones, which were not blown out by the air-stream. The compressed air channelled through the pipe and nozzle effectively breaks up porous material such as soil, but leaves non-porous objects such as stones, pipes or tree roots, untouched.
An area of decayed wood could clearly be identified associated with one of the Meripilus fruiting bodies, and the adjacent buttress root was exposed. The seam of decay appeared to be associated with the lower trunk bole and the underside of the root buttresses where it extended out roughly half a metre from the trunk. This is perhaps not surprising because this is the oldest part of the trees root system, where a significant proportion of the wood will have become totally dysfunctional. So considering that the decay may only be associated with the more mature root system, how would this affect the stability of the tree?
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Having established that the decay was starting to spread from the trunk bole into the lower half of the buttress roots, it was important to find out weather the reactive growth was still maintaining a strong connection with the main anchoring roots further out. The stretch marks visible in the bark around the upper surface of the buttress indicated the strength and vigour of the trees reactive growth and this could be followed down into the root. It was quick and easy to expose the area of root, down to where it branched off below ground level, and confirm that the tree appeared to be well anchored. Whilst we were there we explored a little further to learn more about where the roots were.
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We were happy with the additional information provided by our investigations and could feel comfortable with recommendations for a light reduction to shape and balance the tree, rather than having to consider options for felling. However, it was considered important not to take too much foliage away from the tree, so the reduction work was limited to no more than 10% of the trees leaf canopy area. Pruning was restricted to just foreshortening the branches that extend out beyond the outer crown, while also maintaining the beech within the shelter of adjacent trees.
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Clearly it will be important to monitor the tree and record the annual growth of Meripilus fruiting bodies, not to mention the progression of the associated decay. However, this will increase our knowledge of how the tree’s reactive growth compensates for the loss of residual strength in the main trunk bole, as it continues to producing structural reinforcements reminiscent of those built to support the Eiffel Tower. This type of decay pattern is now commonly associated with Inonotus dryadeus on Oak, where we now recognise that the growth of this fungus between the buttresses does not warrant an order to fell.
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Why do we know so little about the co-evolutionary relationship between trees and fungi? Because, more often than not we don’t give them the chance to exist together, let alone stop to consider that their relationship may be mutually beneficial. Are we too quick to jump to conclusions about things that we don’t understand?
By Andrew Cowan