RPA for standing dead trees?

[kevinjohnsonmbe]

Any one had any experience or offer any insight into calculating a suitable RPA for a standing dead tree?

 

It's not a question that relates to a specific scenario so I don't offer the detail of 'it's this tree, with this height & spread, in this situation'... Just a train of thought I was having.

 

So, assuming there was a standing dead tree which it was desirable to retain, and assuming the indicative RPA calc's in 5837 take account of the full extent (and some room for expansion) of a living root system, and assuming that a standing dead tree would only require that portion which would provide sufficient anchorage to retain stability, and assuming the necessary anchorage requirement would reduce proportionately in relation to a reducing crown or height of a standing stem....... And that's a lot of 'assumings!'

 

What would you think?

[daltontrees]

Firstly I wouldn't call it 'RPA' I would invent another term, say 'support zone'.

 

Cutting back the roots of a dead tree will probably, almost certainly, result in the structure failing more quickly by admitting subterranean decay that could accelerate the loss of strength and woody material in structural roots. But I imagine you'll get nowhere by telling a developer that he'll get another 2 years out of his standing deadwood if he allows a full RPA-type support zone.

 

I think more importantly is the issue of risk. Particularly if loss of support in the invisible subterranean area is a real possibility the risks associated with retaining a dead tree are predictably ever increasing but unmeasurable b any VTA or even most of the more advanced techniques. The most logical outcome I see is that an exclusion zone around the tree where people and property are excluded or cna be predicted to be rarely present is ahat is needed. It could hbe the area within which branches could drop or it could be the falling distance zone in any directions where root zone failure would allow the weight of the dead tree to go that way.

 

I am sure there would be exceptions, but I instinctively expect that the risk exclusion zone will be bigger than the support zone for all dead trees. But it needn't necessarily be a circle. If the dead tree is leaning and could only go one direction the risk exclusion zone could be a segment of only 30 degrees.

 

I'm making this up as I go along (as if you couldn't guess) and I a now thinking that the but differences between compressive reaction wood strategies of support in conifers and tensile on broadleaves might mean that for leaning trees the support zone will be directionally the same as the risk exclusion zone for conifers and directionally opposite for broadleaves.

 

For an upright dead tree, I would be inclinded (after a quick flick through Tree Roots in the Built Environment) to go for a support zone of no more than 5 x DBH. Beyond that roots are most unlikely to have structural significance. I suspect a more rigorous approach would be to relate the SZ to the estimated extent of live canopy when the tree had last been in normal vigour.

 

As ever, just chatting, don't act on this as formal advice. I'd be interested in any other opinions.

[Paul Melarange]

Interesting question. I assume you want to know how much of the roots need to be retained in order to maintain the stability of the tree? If that is the case Mattheck's graph showing 'root-plate radius/stem radius plotted against the stem radius from field studies of wind thrown trees' in 'The body language of trees' is probably going to be useful. However, this applies to living trees, so the wind load would be much higher than a dead tree.

 

Using this approach might work if you are thinking along the lines of trenching for the installation of utilities etc. However, if it is for a structure or similar to be constructed near to the tree, I would allow for an area equal to the height of the tree as an exclusion zone.

[armybloke]

Just a quick though as you have not mentioned the height of your retained dead tree. Consider the hazard and whether you are able to reduce the tree so should failure occur then the risk is significantly reduced ie, failure would not cause structural damage (the reason you ask for an 'RPA').

You could add a natural prop (sturdy limb or similar) to ensure the tree falls the opposite way to the prop if that is acceptable or desirable. I have seen that approach on the continent.

In terms of RPA, I concur with Jules, maybe a new name?

[kevinjohnsonmbe]

Great responses, more food for thought and, as often happens, taking the thought process in another new direction - many thanks!

 

I was originally thinking predominantly of standing stems but the responses have broadened it out to full (or as full as remains) dead crowns.

 

I started thinking about it after an ancient tree forum at Lanhydroc recently. There were some comments made about the need to educate arborists in the preservation of existing, and facilitation of future, veteran / ancient trees – that’s a bit of an aside, but that’s where the thought process started.

 

It rather felt like the presenter considered the arborist as ‘the weak link’ for want of a better phrase in destroying potential future veteran trees. I felt that view failed to acknowledge the constraints under which the arborist might be operating for example:

 

The instruction received from the tree owner (assuming no TPO / CA constraints apply) – we’re trying to make a living after all!

 

The advice / guidance of the arborist (or consultant if engaged) in provided recommendations to the tree owner.

 

That which is actually achievable.

 

The cost implication for the tree owner.

 

Future liability issues that may follow from either the consultant or the arborist providing recommendations.

 

So that’s the background, I wasn’t thinking of questioning RPA’s for veteran trees (didn’t want to open that can!) but since that forum I’ve been noticing more and more standing deadwood stems – particularly at the road side through the Glyn valley between Trago / Bodmin (for Cornish natives), in country parks (Lanhydroc / Cothele) and in private estates. So that’s when I started thinking, in the absence of the majority of VTA indicators, how is it (reasonably) practicable to make a judgment of how long a standing stem might remain stable – and provide the valuable diversity and beneficial habitat (and a certain visual amenity.)

 

So accepting that the usual variables apply – wind, soil, height of stem / dead canopy, target area, population etc….. How are the people responsible for these situations making that judgement (assuming they are not doing a pull / stress test??)

 

 

 

Firstly I wouldn't call it 'RPA' I would invent another term, say 'support zone'.

 

Yes, agreed! That is the nub of the question, what is required to support a dead tree in an upright position. (and therein lies the problem; over what time frame, what size / shape of remaining crown, how do you test, how long (or even – can it ) be assumed as ‘safe’ or acceptable (depending upon the traffic / hazard zone)

Cutting back the roots of a dead tree will probably, almost certainly, result in the structure failing more quickly by admitting subterranean decay that could accelerate the loss of strength and woody material in structural roots. But I imagine you'll get nowhere by telling a developer that he'll get another 2 years out of his standing deadwood if he allows a full RPA-type support zone.

 

The original question didn’t relate to a development scenario but totally agree your point both relating to the developer (if in that scenario) but more broadly, any intrusion in the existing root area would likely accelerate the potential for decline in stability for the reasons you highlight.

 

I think more importantly is the issue of risk. Particularly if loss of support in the invisible subterranean area is a real possibility the risks associated with retaining a dead tree are predictably ever increasing but unmeasurable b any VTA or even most of the more advanced techniques. The most logical outcome I see is that an exclusion zone around the tree where people and property are excluded or cna be predicted to be rarely present is ahat is needed. It could hbe the area within which branches could drop or it could be the falling distance zone in any directions where root zone failure would allow the weight of the dead tree to go that way.

 

Yes, totally understood and agreed.

 

I am sure there would be exceptions, but I instinctively expect that the risk exclusion zone will be bigger than the support zone for all dead trees. But it needn't necessarily be a circle. If the dead tree is leaning and could only go one direction the risk exclusion zone could be a segment of only 30 degrees.

 

Good logic.

 

I'm making this up as I go along (as if you couldn't guess) and I a now thinking that the but differences between compressive reaction wood strategies of support in conifers and tensile on broadleaves might mean that for leaning trees the support zone will be directionally the same as the risk exclusion zone for conifers and directionally opposite for broadleaves.

 

Mmmmm, now there’s a line of thought I hadn’t considered. I was only thinking of standing stems but, as is so often the case, the question has resulted in answers (maybe more questions?!?) far beyond that which I was initially thinking.

For an upright dead tree, I would be inclinded (after a quick flick through Tree Roots in the Built Environment) to go for a support zone of no more than 5 x DBH. Beyond that roots are most unlikely to have structural significance. I suspect a more rigorous approach would be to relate the SZ to the estimated extent of live canopy when the tree had last been in normal vigour.

 

As ever, just chatting, don't act on this as formal advice. I'd be interested in any other opinions.

 

Jules, I was hoping the topic might catch your eye and I’m much obliged and fascinated by your thoughts. I think we’ve said this before but just to reinforce – it is just an informal exchange of ideas and discussion. It’s a poor reflection on the state of our society where ‘having a chat’ (albeit online) gives rise to worry about possible litigation. I know it probably doesn’t actually amount to much (in a legal sense), but I play by the ‘old rules’, if I do it, it’s my responsibility.

 

Interesting question. I assume you want to know how much of the roots need to be retained in order to maintain the stability of the tree? If that is the case Mattheck's graph showing 'root-plate radius/stem radius plotted against the stem radius from field studies of wind thrown trees' in 'The body language of trees' is probably going to be useful. However, this applies to living trees, so the wind load would be much higher than a dead tree.

 

Yes saw that, agree, wind load would be (probably amongst the most) significant factor(s) if a crown (albeit without leaf) was retained. Apol’s, I should have said (in this question) I was thinking only of standing stems.

 

Using this approach might work if you are thinking along the lines of trenching for the installation of utilities etc. However, if it is for a structure or similar to be constructed near to the tree, I would allow for an area equal to the height of the tree as an exclusion zone.

 

Makes sense.

 

Just a quick though as you have not mentioned the height of your retained dead tree. Consider the hazard and whether you are able to reduce the tree so should failure occur then the risk is significantly reduced ie, failure would not cause structural damage (the reason you ask for an 'RPA').

You could add a natural prop (sturdy limb or similar) to ensure the tree falls the opposite way to the prop if that is acceptable or desirable. I have seen that approach on the continent.

In terms of RPA, I concur with Jules, maybe a new name?

 

Makes sense.

[David Humphries]

............since that forum I’ve been noticing more and more standing deadwood stems – particularly at the road side through the Glyn valley between Trago / Bodmin (for Cornish natives), in country parks (Lanhydroc / Cothele) and in private estates. So that’s when I started thinking, in the absence of the majority of VTA indicators, how is it (reasonably) practicable to make a judgment of how long a standing stem might remain stable – and provide the valuable diversity and beneficial habitat (and a certain visual amenity.)

So accepting that the usual variables apply – wind, soil, height of stem / dead canopy, target area, population etc….. How are the people responsible for these situations making that judgement (assuming they are not doing a pull / stress test...........

 

I suspect you've already pondered including type of decay present (brown, white, simultaneous etc) (and/or potentially incoming decay) in the lower section of the tree including the stem, buttresses and upper root crown, as additional elements to consider.

 

Also tree species comes into the equation, (oak & beech being more durable than willow & poplar etc.....) and then regularity of inspection of the deteriorating biomass and any permanent/seasonal change of occupancy.

 

 

 

 

.

[daltontrees]

I have been staring at a blank response screen for 10 minutes, realising how complex this issue is and contemplating what I could add to simplify that complexity. Unfortunately I fear it can't readily be simplified, since it's such a broad hypothetical question. We can all see ways of defining the crieria but I am now convinced that a simple DBH x number or even a stem height x number solution isn't possible.

 

Probably looking at an algorithmic answer.

[kevinjohnsonmbe]

Here's a pic of my favourite example of a stem in a local woodland. Thankfully, it's very isolated from casual foot traffic and no structures nearby so the 'risk' element is very low. I've been watching it for about the past 8 years. It's a beauty, the woodpeckers love it as a feasting station!

 

Conversely, those close to the roadside or in parks etc, if allowed to degrade to this level - and the value of doing so is self evident, must present considerable risk of collapse. I'm thinking of noting examples where I find them and asking those responsible what system they are employing to measure / mitigate the risk.

[David Humphries]

Here's a pic of my favourite example of a stem in a local woodland. Thankfully, it's very isolated from casual foot traffic and no structures nearby so the 'risk' element is very low. I've been watching it for about the past 8 years. It's a beauty, the woodpeckers love it as a feasting station!.........

 

are these Ganoderma brackets Kevin ?

 

They look to be fairly high up the trunk.

 

 

............ I'm thinking of noting examples where I find them and asking those responsible what system they are employing to measure / mitigate the risk.

 

Our standing dead wood populations are visited annualy and are given a tap with a sounding hammer, drill with the PD400 and/or pulled with a tag line to help us monitor the ongoing degradation.

 

For us (I guess I'm meaning me) its more a 'feel' for the residual risk and the need to reduce where/when neccessary, rather than trying to quantify the unknown.

 

 

I like to believe the statement given at the end of Justice Mackays summing up of the Felbrigg case, (para 43) is in part, the ethos that should be guiding us in how we manage trees.

 

 

".....risk assessment in any context is by its very nature liable to be

proved wrong by events, especially when as here the process of judging the integrity of a tree is an

art not a science, as all agree".

 

.

[kevinjohnsonmbe]

are these Ganoderma brackets Kevin ?

 

They look to be fairly high up the trunk.

 

 

 

 

Our standing dead wood populations are visited annualy and are given a tap with a sounding hammer, drill with the PD400 and/or pulled with a tag line to help us monitor the ongoing degradation.

 

For us (I guess I'm meaning me) its more a 'feel' for the residual risk and the need to reduce where/when neccessary, rather than trying to quantify the unknown.

 

 

I like to believe the statement given at the end of Justice Mackays summing up of the Felbrigg case, (para 43) is in part, the ethos that should be guiding us in how we manage trees.

 

 

".....risk assessment in any context is by its very nature liable to be

proved wrong by events, especially when as here the process of judging the integrity of a tree is an

art not a science, as all agree".

 

.

 

Good pics David! I went back today and tried to get a better look. Fought my way into the stem and found this (not so) little surprise... Final pic is right at the top, maybe 15m. I'll try and get closer pics tomorrow.