RPA's for fat trees?

[benedmonds]

Is there a reference some-place that allows for the reducing of RPA's on trees with unusually large diameters?

 

Things like grafted cherries for example? Or old trees that have retrenched or been reduced/topped? Or old coppice stools? I know the veteran tree people like increasing the RPA's but there are circumstances where a trunk is just to fat and the 12x diameter RPA just turns out massive.

[MATTMOSS]

I know the standard mentions something about measuring dbh away from an unusually large swelling but I don't think you can reduce the rap because you think it's too much. Your answer lies in bs5873 in any event.

[benedmonds]

I know the standard mentions something about measuring dbh away from an unusually large swelling but I don't think you can reduce the rap because you think it's too much. Your answer lies in bs5873 in any event.

 

The answer doesn't lie in BS5837 as far as I am aware. That's why I am asking if there is any other reference.. There are lots of things BS5837 doesn't mention.. The RPA's as calculated in BS 5837 should not be sacrosanct. They offer a start point. But in reality often offer very little relation to the actual rooting area of a tree. I just wondered if there was a publication that I could site that supports the view. There are people out there who follow the BS blindly when in fact it can be BS..

[MATTMOSS]

I am not aware of any other guidance.

[daltontrees]

BS 5837 Clause 4.6, max RPA capped at 707m2. Work it out what that means for DBH. 5837 is not the law, but if you can't justify deviation from it, then you're out on a limb, as it were.

 

Put it another way, there are reasons for the RPA, and they are imperfect reasons, but if you don't undertand the reasons well enough to be able to convince designers and planners that a different RPA will not compromise the ongoing 'viability' of the tree, then there's no simple get-out.

 

Personally I think that the RPA is not so much a function of diameter but of what is known as the CAI or current annual increment. If so, there is no justification for reducing the RPA for large trees, quite the reverse if anything. And when you get in to veterans and ancient trees, I can hardly imagine more important situations for root protection. I don't mean this in a tree-hugging way, I mean that BS5837 affords veteran and ancient trees the same retention desirability category as younger, better condition trees.

 

Do you have a copy of 5837?

[tree79]

is there a reason you need to reduce the rpa?

but as already stated if you can back up why you have reduced the rpa you shouldn't have to many problems. You could always try digging a few sample pits to try a determined the size of the rpa.

[benedmonds]

Put it another way, there are reasons for the RPA, and they are imperfect reasons, but if you don't undertand the reasons well enough to be able to convince designers and planners that a different RPA will not compromise the ongoing 'viability' of the tree, then there's no simple get-out.

 

to convince the planners it is nice to have someone else to quote hence the question! I am not talking about veterans here although I can see instances where that might occur. Root zones will die back in relation to the crown. If you had a 5 m tall old hollow stemmed tree with a 1250 diameter stem i can not see it requiring the 707m2 or 15 m radius rpa.

 

digging is a option but tcps are used to help design developments and so digging not normally completed at that stage in the process.

Edited April 1, 2015 by benedmonds

[daltontrees]

to convince the planners it is nice to have someone else to quote hence the question! I am not talking about veterans here although I can see instances where that might occur. Root zones will die back in relation to the crown. If you had a 5 m tall old hollow stemmed tree with a 1250 diameter stem i can not see it requiring the 707m2 or 15 m radius rpa.

 

digging is a option but tcps are used to help design developments and so digging not normally completed at that stage in the process.

 

I suspect you are heading for a cul-de-sac there. Any tree that big will, if it's in good condition, need the full RPA and is likely to be notable or a regional champion (see ancient tree forum guidance). if it's not in good condition it may already be ancient or a transition veteran/ancient tree, and will need the full RPA. But to weaken the argument further, the LA could quote back at you the following guidance from the ATF

 

"In the case of ancient and other important trees this [12 times the diameter] RPA may be insufficient to ensure their roots and the rooting environment are properly protected. A minimum root protection area of 15 times the trunk diameter or two metres beyond the canopy of the tree, whichever is the greater, was set out as a standard in the handbook ‘Veteran Trees: A guide to good management’ published by English Nature (now Natural England.) A greater RPA will reduce the possibility of damage or disturbance to these very special trees, so they have the best chance to provide a lasting contribution to the development."

 

It might seem counter-intuitive, but big trees are more likely to need RPA in excess of the standard than below it. The old hollw tree you present as an example might be less effective at gathering water and nutrients than one the same size in good condition. It might well need a bigger RPA.

[benedmonds]

I suspect you are heading for a cul-de-sac there. Any tree that big will, if it's in good condition, need the full RPA and is likely to be notable or a regional champion (see ancient tree forum guidance). i

 

I am not talking about notable champions.. I am talking about old willow pollards and grafted cherries. These can often have artificially large calculated RPA's due to a disproportionate girth.

 

The issue is that there are planning officers (and arbs) out there who can be pedantic and will quote BS5837 and other documents as if they are sacrosanct. The BS should give guidance not be followed blindly. Just as you would ignore the BS and quote the ‘Veteran Trees: A guide to good management’ for notable trees I was after a reference that indicates that root area is proportional to crown area.

[daltontrees]

I am not talking about notable champions.. I am talking about old willow pollards and grafted cherries. These can often have artificially large calculated RPA's due to a disproportionate girth.

 

The issue is that there are planning officers (and arbs) out there who can be pedantic and will quote BS5837 and other documents as if they are sacrosanct. The BS should give guidance not be followed blindly. Just as you would ignore the BS and quote the ‘Veteran Trees: A guide to good management’ for notable trees I was after a reference that indicates that root area is proportional to crown area.

 

Fair enough, I have seen some horrible big old willows recently that are in the process of self-destruction and/or decay. The RPA issue bacame insignificant because the trees did not deserve A or B categorisation, but occasionally there was one in good condition which was B or snuck into A. But their canopies were vast (almost as big as the RPA).

 

Maybe your pollards do not deserve retention? The RPA might be a moot point.

 

BS5837 is from start to finish full of generalities that can be challenged tree-by-tree. AS far as RPAs go, it is one size fits all species and that is plainly wrong. But I have never seen a good explanation of what is important about root protection of that extent. Is it for support? For water? For nutrients? For the avoidance of permanent roots that wouldn't heal if wounded? It's l;ikely to eb an amalgamation of all of these.

 

If you really want to knock yourself out finding evidence to challenge the RPAs for individual trees, then 'Tree Roots in the Built Environment' might help. Table 8.18 suggests that BS5837 is overly protective of roots. But it might do the opposite. For example table 10.1 shows that a Tulip Tree and a Monterey Pine both of 20 metres have water demands at opposite ends of the range. Do they need the same catchment area? And Table 10.4 gives 5 different examples of authoritative texts that rank trees for their damage potential due to roots, but the rankings are wildly conflicting e.g willow is ranked 6th, 7th, 8th, 9th and 2nd. How do we assess the relative rooting of these species if the root experts don't concur?

 

I would be reluctant to relate RPA to crown spread. Poplars have roots that go on forever because they need lots of water, but they can be fastigiate.

 

In the end if a LA is being difficult, it runs the risk of losing a refusal at appeal. But at the appeal somene will have to persuade an Inspector with perhaps very little tree knowledge that a deviation from a British Standard is sound. I think the onus would be on you to make the case for disapplying the BS rather than the COuncil proving that it does apply.