RPA for a pollard

[jacquemontii]

I recently surveyed a small pollarded lime with a stem diameter of 500mm (RPA radius 6m).  As the tree is being well managed with a smaller crown size relative to the trunk (crown is 8m high, 3m spread in all directions) I'm now considering if this should be reflected in a reduced size of the RPA?   I'm thinking along the lines that root growth is limited by a reduced supply of carbohydrate as a result of crown pruning (root:shoot ratio as described in 'TREE ROOTS IN THE BUILT ENVIRONMENT' Roberts et al, 2006).

Has anyone else adjusted the size of an RPA in a BS5837 survey based on the management of the tree?

[Paul Barton]

I commonly adjust the shape of RPAs due to ground conditions, but rarely reduce the overall size of an RPA due to crown size.  I think your logic is pretty sound though - it follows that a small crown will require less moisture and soil nutrition to sustain it, and certainly large anchorage roots will be less than for a full-crowned tree.

 

That said, the opposite thinking prevails when it comes to veteran and ancient trees.  These often have small, retrenched crowns but the standing advice from Natural England and the FC is that the RPAs should be enlarged!  They're not really comparable situations though 

 

 

[Gary Prentice]

What about pollarded veteran trees Paul? Bigger or smaller RPAs

[benedmonds]

3 hours ago, jacquemontii said:

I recently surveyed a small pollarded lime with a stem diameter of 500mm (RPA radius 6m).  As the tree is being well managed with a smaller crown size relative to the trunk (crown is 8m high, 3m spread in all directions)

Has anyone else adjusted the size of an RPA in a BS5837 survey based on the management of the tree?

I do on trees with disproportionate trunk to crown size, but normally specify some exploratory digging to ascertain if the roots actually are present.  Lets face it the calculation has very little relevance to the real world.  It gives a starting point to work from. Unfortunately many seem to think it is fact.

Edited April 4, 2018 by benedmonds

[daltontrees]

I'm going to give an alternative perspective (otherwise known as disagreeing).

 

Naturally trees have a balance between the below ground and above ground parts, commonly called the root-stoot ratio. The absolute value of the ratio doesn't matter, and I am sure it changes gradually over the lifespan of the tree. The top of the tree puts gases and energy in, the bottom puts water and nutrients in, and the two parts mature together.

 

Cut some roots away, and the tree will immediately try and replace them. Likewise pollarding, remove crown and the tree will bounce back vigorously. The only constant measure of the balance that would be achieved naturally is the stem diameter. And by simplistic methods, this is used to determine the root protection area. The stem is the main plumbing for the tree, up and down. BS5837 assumes its cross sectonal area is proportional to the root protection area. Pollarding doesn't reduce the rooting area, it just forces a new crown to start growing. Reducing the rooting area at the same time as pollarding may reduce the overall growth rate of the tree, but it won't cause the stem to shrink. It will just make the tree unwell and unable to regenerate a crown. So you won't have a pollard, you'll have an ailing stick. A full BS5837 12x RPA is in my view needed to make pollarded trees viable, to constantly try and regain the root-shoot ratio. Push it too far by removing RPA and you are well into the area of not knowing of controlling what willl happen. The stem needs a certain amount of energy in to maintain its older wood as well as create new wood.  BS5837 defines RPA in terms of maintaining viability. It doesn't say keeping alive.

 

That's my view anyway. I have never reduced the RPA of a pollard.

 

All of the above is not book learning, it comes (for me) from never ever missing a chance to examine the ring growth pattern of a tree that I have seen standing before it was felled. Newly pollarded trees do not reduce their annual increments, if anything they increase them, with vessel-rich wood rather than fibrous-rich. I think it is more marked in ring-porous species, but I am still observing.

 

So it depends what the objective is. If it's maintaining the tree's vitality, I'd keep the RPA (and then some). If it's sneaking something past planning, by all means try it, but I'd be telling the client that's what I was doing and not promising ongoing vitality. Read pedantically (because that's what TOs and planners seem to do) there is no clear mechanism in BS5837 to reduce the RPA, only modify it's shape. That said, one could infer that parts c and d of clause 4.6.3 anticipate changing the size rather than the shape of the RPA. I have certainly changed it on many occasions based on soil depth. It is after all an imitation of a soil volume. Deeper soil, smaller RPA.

 

As I say, much of this is based at looking at stumps of topped or pollarded trees I've been involved in removing. Te DBH is reflective most of all of potential for water uptake, and that is a reflection of rooting rather than crown. Re-pollard so of thn that that isn't true any more, and you've got a dying tree. I've seen that many times. They succumb to infection too in their weakened state.

[Paul Barton]

1 hour ago, Gary Prentice said:

What about pollarded veteran trees Paul? Bigger or smaller RPAs

Look, I'm easily confused at the best of times so don't taunt me with your mind-boggling scenarios!!

[daltontrees]

My post went in a thte same time as Benedmonds. Not trying to stat an argument here, just questioning Paul's 'sound logic' suggestion.

[daltontrees]

1 hour ago, Gary Prentice said:

What about pollarded veteran trees Paul? Bigger or smaller RPAs

Or, don't pollard a veteran that is about to have its rooting area compromised by development?

[Paul Barton]

4 minutes ago, daltontrees said:

I'm going to give an alternative perspective (otherwise known as disagreeing).

 

Naturally trees have a balance between the below ground and above ground parts, commonly called the root-stoot ratio. The absolute value of the ratio doesn't matter, and I am sure it changes gradually over the lifespan of the tree. The top of the tree puts gases and energy in, the bottom puts water and nutrients in, and the two parts mature together.

 

Cut some roots away, and the tree will immediately try and replace them. Likewise pollarding, remove crown and the tree will bounce back vigorously. The only constant measure of the balance that would be achieved naturally is the stem diameter. And by simplistic methods, this is used to determine the root protection area. The stem is the main plumbing for the tree, up and down. BS5837 assumes its cross sectonal area is proportional to the root protection area. Pollarding doesn't reduce the rooting area, it just forces a new crown to start growing. Reducing the rooting area at the same time as pollarding may reduce the overall growth rate of the tree, but it won't cause the stem to shrink. It will just make the tree unwell and unable to regenerate a crown. So you won't have a pollard, you'll have an ailing stick. A full BS5837 12x RPA is in my view needed to make pollarded trees viable, to constantly try and regain the root-shoot ratio. Push it too far by removing RPA and you are well into the area of not knowing of controlling what willl happen. The stem needs a certain amount of energy in to maintain its older wood as well as create new wood.  BS5837 defines RPA in terms of maintaining viability. It doesn't say keeping alive.

 

That's my view anyway. I have never reduced the RPA of a pollard.

 

All of the above is not book learning, it comes (for me) from never ever missing a chance to examine the ring growth pattern of a tree that I have seen standing before it was felled. Newly pollarded trees do not reduce their annual increments, if anything they increase them, with vessel-rich wood rather than fibrous-rich. I think it is more marked in ring-porous species, but I am still observing.

 

So it depends what the objective is. If it's maintaining the tree's vitality, I'd keep the RPA (and then some). If it's sneaking something past planning, by all means try it, but I'd be telling the client that's what I was doing and not promising ongoing vitality. Read pedantically (because that's what TOs and planners seem to do) there is no clear mechanism in BS5837 to reduce the RPA, only modify it's shape. That said, one could infer that parts c and d of clause 4.6.3 anticipate changing the size rather than the shape of the RPA. I have certainly changed it on many occasions based on soil depth. It is after all an imitation of a soil volume. Deeper soil, smaller RPA.

 

As I say, much of this is based at looking at stumps of topped or pollarded trees I've been involved in removing. Te DBH is reflective most of all of potential for water uptake, and that is a reflection of rooting rather than crown. Re-pollard so of thn that that isn't true any more, and you've got a dying tree. I've seen that many times. They succumb to infection too in their weakened state.

Top class reply Julian!

 

One thing about the 'RPA' is that is not intended to necessarily represent the extent of roots.  Although I see some logic in the root:shoot ratio theory, it doesn't allow for those roots that perhaps by necessity due to soil conditions, track an awfully long way from the stem in order to exploit available soil moisture.  I.e. if the soil in close proximity is moist and nutritious then the tree may have a fairly compact and fibrous root morphology, but if conditions are not so good a tree will throw out exploratory roots for some distance.  We've all seen roots tracking under roads and footpaths etc.

 

So if the RPA can't even come close to describing the radial spread of roots, it must be more about a sufficient volume of soil that is required to sustain the tree.  As Julian says, this means that soil depth is pretty critical.  If the soil is only 50cm deep before rock, then we should all be doubling the radius of RPAs as I recall the BS5837 radial RPA is based on a 1m depth!

 

Going back to the original question, is it reasonable to estimate that a pollarded tree will need less soil volume than a full-crowned tree.  Well, yes I think it is but perhaps only temporarily as pollards tend to produce prolific foliage to re-instate the root-shoot ratio. Research in to heavy pruning/pollarding to reduce water demand in subsidence prone areas shows that water uptake is resumed to previous levels in just two-three years after cutting so unless a tree is pollarded very frequently to control it, presumably root activity continues.

 

Julian - your observations about incremental thickening of pollards is very interesting.  I don't have any stumps or cross-sections to dispute your point, but I do recall hearing a talk a few years ago from Mr Barrell where he showed some slides of some small pollards in a church yard - the assumption was they were quite young as their stems were slender but when they were felled they were found to be really quite old. Maybe a long-term regime of pollarding does reduce incremental thickening?  Or maybe those trees were just growing in poor soils and had somehow struggled on for more than a century!

[Khriss]

I definately argue against reducing RPA - is it development pressure ? Is the pollard to re-establish a better tree crown -from the previous decaying / damaged one ? If so - would always retain or expand RPA or put in enhancements for future tree sustainability , K