RPA's of reduced trees

[benedmonds]

If a tree (a cedar) has been reduced can the RPA also be reduced?

[Paul Barton]

No, the RPA is calculated from the stem diameter not crown size.

[Amelanchier]

Nope - as above.

[benedmonds]

Is that also the case with an old pollard?

[Amelanchier]

I would draw your attention to section 5 within BS5837

 

"5.2.4 The RPA, for each tree as determined in Table 2, should be plotted on the TCP taking full account of the following factors, as assessed by an arboriculturist, which may change its shape but not reduce its area whilst still providing adequate protection for the root system.

a) The likely tolerance of the tree to root disturbance or damage, based on factors such as species, age

and condition and presence of other trees. (For individual open grown trees only, it may be acceptable to offset the distance by up to 20 % in one direction.) (See Note 1 of 11.3.5.)

b) The morphology and disposition of the roots, when known to be influenced by past or existing site

conditions (e.g. the presence of roads, structures and underground services).

c) The soil type and structure.

d) Topography and drainage..."

 

Whilst this is the guidance, I would talk to your tree officer. It will come down to his decision essentially so if you can establish a comprimise regarding the extent of a the pollards rpa then all the better.

 

Do you have a copy of the BS?

[benedmonds]

I've got the BS, just wondering..

[Andy Collins]

As above, the RPA is simply calculated as 12xcircumference of trunk at chest height. This is the quickest and easiest way to calculate and is the way my LPA works.

[Pete Mctree]

As above, the RPA is simply calculated as 12xcircumference of trunk at chest height. This is the quickest and easiest way to calculate and is the way my LPA works.

 

I've had that quoted to me by several people. I does seem a little arbritary imo, it does not take into account canopy spread, species or site topography.

 

Anyone know how and by who the formula was developed?

[Amelanchier]

You're damn skippy its arbitrary. The key point in the BS is the bit I highlighted in bold above

 

"...which may change its shape but not reduce its area..."

 

So; take for example the TPO'd x Cupressocyparis leylandii (oh yes!) on a development site I looked at today.

Mature specimen tree in the corner of a raised lawn. Corner is formed by two roads. Lets say a 1m dbh for arguements sake (it was bigger and multistemmed) Developers RPA?

 

12m circle. Shown on the map as including the roads...durrrrr why would the roots opt for road when there is a perfectly good lawn?

 

Well they wouldn't so a more accurate rpa would use the same area of the circle (452m squared) and apply it to the lawn to give a 21 x 21m square. (bye bye to your proposed footed hard surfaced access driveway mate!) But that assumes the roots grow in a square I hear you say. Yep. so lets calculate another way...

 

Its in the corner so maybe its fair to say the roots would have exploited the space in a quarter circle fashion? Seems ok? The guidance says we need the same area so we need to calculate a circle that has four times that area that we want (a quarter remember).

 

4 x 452 = 1808m squared

 

1808 / 3.142 = 575

 

square root of 575 = 24m radius

 

So a quarter circle 24m radius from the tree base out into the lawn. Bloody big but slightly more representative of what the RPA might be. Now apply you knowledge of root morphology. Soil type, species, age obstacles...This particular tree, we might comprimise on a 18-20m radius quarter circle given the vigour and age of the tree and its ability to deal with the loss of area. But don't tell the developer that - he hasn't bothered to do the calculations and I don't work for him!

 

 

Pete -

The 12x modifier is referenced in table 2 of the BS as follows:-

 

"NOTE The 12× multiplier is based on NJUG 10 [9] and published work by Matheny and Clark [10]."

 

Where the refernces are:-

 

[9] NATIONAL JOINT UTILITIES GROUP (NJUG). Guidelines for the planning, installation and

maintenance of utility services in proximity to trees. Publication number 10. NJUG, April 1995.

 

[10] MATHENY, N. and J. CLARK. Trees and development. A technical guide to preserving trees during

land development. 1998. International Society of Arboriculture, Champaign, IL 61826-3129. 184 p.

[Pete Mctree]

I get your point about the system currently in use. I prefered the old one with no determinate formula.

 

The hard/soft lasndscape around root has always presented issues. A bowling green of a lawn (constantly rolled and cut therefore well compacted) surley would be as restrictive to root growth as a shallow metaled surface?

 

I'm just trying to illustrate the inflexibility of the current system in my eyes. Anyone agree?