Foundations within RPA

[Paul Barton]

Mmmm. My post wasn't really about one set of roots versus another. I believe that nature doesn't do anything for fun and as such all roots are needed. The point is, if you remove all of the roots outside of the RPA which could be a considerably proportion of the whole amount of roots, how come that is OK in planning terms and OK in BS terms and OK in Tree Officer and Consultants terms. So whats the point of these roots outside of the RPA and where is the evidence that 12 times the trunk diameter is the magical number between tree being happy (Planning, BS etc) and unhappy (Tree Officer etc). Why not 15 times diameter or 11 times diameter.

 

First off, the RPA is not necessarily meant to indicate the exact location of roots - it is termed in the BS as a "layout design tool indicating the minimum area around a tree deemed to contain sufficient roots and rooting volume to maintain the tree’s viability"

 

So, as Chris points out above, this area within the tree's potential root system is treated as a priority to protect as this is where there will be the largest roots but probably also a great number of fine roots.

 

Having done a bit of air-spade work now and again, I was always interested to see the vast difference in the amount of fine roots close to the stem between species. Oak had hardly any fibrous roots close to the root collar, but Beech had lots. Perhaps this should indicate that the outer edges of the notional RPA are more important for Oak than for Beech?!

 

It is shocking that there is no evidence based reason that we use a formula of 12 x the stem diameter - this really needs addressing.

 

However, as a design tool it does its job relatively well by at least offering a starting point to retain roots and protect soil around a decent proportion of the tree.

[daltontrees]

First off, the RPA is not necessarily meant to indicate the exact location of roots - it is termed in the BS as a "layout design tool indicating the minimum area around a tree deemed to contain sufficient roots and rooting volume to maintain the tree’s viability"

 

So, as Chris points out above, this area within the tree's potential root system is treated as a priority to protect as this is where there will be the largest roots but probably also a great number of fine roots.

 

Having done a bit of air-spade work now and again, I was always interested to see the vast difference in the amount of fine roots close to the stem between species. Oak had hardly any fibrous roots close to the root collar, but Beech had lots. Perhaps this should indicate that the outer edges of the notional RPA are more important for Oak than for Beech?!

 

It is shocking that there is no evidence based reason that we use a formula of 12 x the stem diameter - this really needs addressing.

 

However, as a design tool it does its job relatively well by at least offering a starting point to retain roots and protect soil around a decent proportion of the tree.

 

Well put, Paul. It's not a rule, it's a starting point. Justification for accepting or rejecting it is always required, even if it's the acquiescence of teh LPA. And detractors of the arbitrary 12x may wish to consider how any alternative formula (10x, 15x, 20x...) would be justified.

 

Sorry Gary, it being easy to calculate at 10x is not a good justification. Nice try, though.

[Gary Prentice]

I'm a metric kid, not bought up on the imperial system of multiples of 12, 14 and 16, so ten woul be easier.

 

 

Personally, frequently I think that 12x doesn't seem a lot.

[oslac]

Well put, Paul. It's not a rule, it's a starting point. Justification for accepting or rejecting it is always required, even if it's the acquiescence of teh LPA. And detractors of the arbitrary 12x may wish to consider how any alternative formula (10x, 15x, 20x...) would be justified.

 

Sorry Gary, it being easy to calculate at 10x is not a good justification. Nice try, though.

 

The 12x is arbitrary as would be any other multiplication. Its not about the multiplication but the arbitrary nature of the multiplication. You seem to be suggesting that because a formula for the RPA is required it's OK to pluck a figure out of the air, in which case, why not 10x or 15x.

 

It is not for me to justify the formula of the RPA, that is why clever people are employed to work these this out for us but it would be helpful if there was some science in the system.

 

My earlier point was that roots can grow well beyond the RPA and if, as far as the BS is concerned and many TO's, consultants etc. are concerned, the tree roots can be reduced to the extent of the RPA and all these people are content with that, what was the point of the tree growing such extensive root systems in the first place. The comment from Paul that this is the the "minimum area around a tree deemed to contain sufficient roots and rooting volume to maintain the tree’s viability" doesn't stack up if the RPA is an arbitrary size. I have seen many a council owned tree have their rooting area sliced in half to accommodate a footpath yet continue to thrive.

 

It seems to me that a Standard was required, protection of tree roots are required in some way and so an arbitrary way of determining an RPA was invented. We all work to it because it is the system, but actually it's is a load of cobblers and not supportable.

 

I don't have any suggestions about a better system but then again I am not clever but I can still see the flaws in the present system

[Paul Barton]

To be clear, that wasn't my comment I was citing the definition of the RPA from the BS.

 

I agree with you Oslac - trees regularly defy the arbitrary rules that we try and place on them. I guess no one has come up with a better idea yet though!

 

 

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