Part 1 of this article described the Root Protection Area (RPA) and how its size and shape is calculated. Part 2 will explain how the wrong sort of development can harm trees and what the likely consequences are for the Developer.
As mentioned in Part 1, the RPA is identified within British Standard 5837:2012 Trees in relation to design, demolition and construction – Recommendations 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 and where the protection of the roots and soil structure is treated as a priority”. It is therefore assumed that when it comes to development, a Local Planning Authority (LPA) Tree Officer would only take a view of the tree roots growing within the RPA and not tree roots growing beyond the RPA. There may be exceptions to this rule where for example, the tree is a veteran or ancient tree, although I have yet to see any evidence that the RPA of veteran and ancient trees need to be extended and that there is nowhere within the British Standard requiring the RPA of these trees to be extended.
There may also be a need to extend (modify) the RPA in a particular direction where there is a reduced rootable soil volume caused by the close proximity of structures or other barriers to root growth. I mentioned in Part 1, that the close proximity of existing buildings may result with the RPA being modified by reducing the RPA to align with the existing building. The LPA Tree Officer would also be justified in requiring the RPA to be extended so that the overall area of the RPA is not altered. I am not sure how the science would accurately inform this and how subjective the calculation for this modification would be. I guess that there would be a conversation between the Arboricultural Consultant and the Tree Officer based on each’s opinion. I would suggest however, that a suitably experienced and qualified arboricultural consultant is involved, who would provide fair and unbiased advice which hopefully the LPA Tree Officer will agree with.
Where development is allowed within the RPA of a tree, suitable Method Statements are normally required by the LPA to ensure that the tree is not harmed. Some examples of development may include: the laying of services, erecting fences, construction of decking or walls, construction of paths, demolition of structures. Some extreme cases will include the construction of out-buildings, garages, house extensions and even whole houses. It is these extreme examples which I want to focus on in this article but first, I would like to explain why we even need Method Statements and what possible damage could development do to trees and their roots.
We will need to go back to basics and some school biology.
As mentioned in part 1, tree roots grow where tree roots grow and tree crowns grow where tree crowns grow. In other words, the shape of each when viewed in plan form are rarely circular but grow amorphously. This is because the tree roots exploit soils to achieve optimum growth and if this soil is predominantly to one particular side of the tree, the roots are more likely to grow in greater concentrations on that side of the tree. Similarly, tree crowns tend to grow towards the light and will grow at a greater rate where it can exploit more light. Sometimes, and this is often observed on exposed hills, the shape of a tree’s crown is dictated by wind conditions and may grow more to the leeward side than the windward side of the tree.
Within the soil, there is another world going on. I have already explained why tree roots grow into amorphous shapes but it is also important to realise that tree roots also tend to grow at particular depths. The science suggests that approximately 85% or tree roots grow within the upper 600mm of soil (Image 1). This is often observed when we see a fallen mature tree laying on its side within a field or woodland. Often the root plate partially sticking out of the ground is around the 600mm depth. It must be noted that the observed root plate of fallen trees do not comprise of all the roots but mainly the buttress and major anchoring roots and it is highly likely that the tree roots will extend for many more metres.
Image 1 – Root Protection Area – Twelve times trunk diameter measured at 1.5 metres above ground level.
Although tree roots mainly grow within the upper 600mm of soil, there are still 15% or so which grow deeper than this and I would expect that tree root depth is similar to tree root spread in that the roots exploit soils at whatever depth to gain the most benefit. Generally, though, trees require certain physical properties from soil to grow optimally. The optimum soil is likely to have a mix of sands, silts, and clays to provide a suitable texture with the addition of organic material, water, air and minerals, although some trees are happy to exploit waterlogged, clay or dry sandy soils. In addition, optimum soils will have a good structure which means the clumpyness of the soil is generally even thereby allowing the free passage of water and air through the soil as well as allowing tree roots to grow through the soil.
The reasoning why tree roots mainly grow within the upper 600mm of soil is that this is the soil horizon which contains the best growing conditions. Normally this soil is permeable for air and rain water as well as allowing gases produced by the roots and other soil organisms to escape. This horizon also contains many of the nutrients required by the tree.
Now, back to the proposed development.
There are certain activities carried out by the developer which often involve harm to trees. These include the obvious such as lighting fires in close proximity to trees but also include, collision damage, cutting of roots, and compaction.
Fires on development sites
Heat from a fire will easily damage both the trunk of a tree or branches growing overhead (Image 2). My general rule of thumb is not to light any fire anywhere near a tree, suggest at least 10 metres beyond the tree’s canopy. I would also say that if it is too hot for you to stand between a fire and a tree, it is definitely too hot for the tree.
Image 2 – Areas of harm caused by fires
Another activity which is a major source of damage to trees are collisions from plant and lorries, especially earth moving excavators and skip lorries. Any collision, no matter how minor is likely to seriously harm a tree and must be avoided. This is one of the reasons why tree protection barriers are set up around trees. Perhaps, tree protection will be a subject for another article!
Addition or removal of soils
As mentioned already, tree roots generally grow within the upper 600mm of soil, they grow within this zone because that’s where they want to grow and where they have optimised to grow. The addition of soils over the RPA (increasing soil depth) or the removal of soils is normally a big no-no as far as the tree is concerned (Image 3). This point is also identified within the British Standard and it is highly likely that any competent Tree Officer would take a dim view of any such activity.
Adding soils will mean that tree roots optimised for growing within the upper 600mm of soil, now need to either optimise to grow within the upper 1.0m of soil or whatever the new depth is after adding the soil or will need to grow new roots at the optimum depth and where soils have been removed, tree roots will need to adapt accordingly. This is likely to add severe stress to the tree by affecting its physiology and possibly making the tree more susceptible to harmful pathogens, whilst requiring the tree to use up precious energy growing new tree roots. All of this at a time when the tree’s ability to take up water and dissolved nutrients is reduced. All in all, a bit of a disaster for the tree. Of course, all of this is unseen and may take several years for the tree to show signs of decline. An analogy would be to liken the harm to trees to global warming. It takes many years for the signs to occur, no-one takes the blame and the damage is often irreparable.
Image 3 – Addition or removal of soils within the RPA
Another major detrimental activity which often occurs on development sites is soil compaction and, in my opinion, this is probably the most harmful activity a tree will suffer from. Anyone visiting a development site will see, compacted ground often with churned up mud and numerous puddles. Surface water doesn’t drain away which causes more surface compaction. The end result is that the permeability of soils is lost, water reaching the roots may stop altogether whilst noxious gasses within the soil cannot escape (Image 4). Often soils change from aerobic permeable to anaerobic impermeable. This has the added disadvantage of killing off soil micro-organisms which are likely to be essential for healthy root growth. Compaction can be avoided with the correct siting of tree protection barriers and temporary ground protection. In my opinion, any half decent Arboricultural Consultant and Tree Officer would be in full accord with the negative impact to trees caused by soil compaction.
Image 4 – Soil compaction creating impermeable soils
Cutting of roots
I have come across many proposed developments whereby the developer wants to use strip foundations and are often surprised that this is not allowed within the RPA. Often strip foundations are easy, quick and the most cost-effective foundation type but cutting a 2.0m deep trench through the middle of the RPA will also remove all roots to the outer side of the trench (Image 5). In effect, cutting a whole chunk of roots out of the RPA which in itself is a reduction to the whole of the tree’s rooting area. This is not allowed within the British Standard and I would hope, it would not be allowed by any Tree Officer. The same goes for piled foundations with a ground beam (Image 6). Usually ground beams are set into the soil with additional compressive material laid to its underside. As with the strip foundation, the use of ground beams will still require a trench to be excavated which will sever tree roots. The use of such ground beams is likely to have the same negative impact on tree roots as the use of strip foundations.
Image 5 – Excavation for strip foundation
Image 6 – Piled foundation with ground beam
There is a system which we have been developing at Open Spaces over the past 10 years which allows buildings and other structures to be constructed within a tree’s RPA whilst retaining all of the tree’s roots within the RPA, maintaining soil permeability and allowing rain water to penetrate into the ground. I had hoped to include this ‘Pile and Void’ system within this Part 2 of the Article but as you can see, I got carried away with explaining how tree roots grow and the conditions they require to grow successfully, which I think is really important to understand and hopefully will give reason to the Pile and Void system which will be highlighted in Part 3 of this Article.
BSc(Hons), DipLA, CMLI, FArborA, MBALI
Graeme is a Director of Open Spaces Landscape and Arboricultural Consultants Limited, he is also a Chartered Landscape Architect and a Fellow of The Arboricultural Association
Open Spaces is a Registered Practice with the Landscape Institute with expertise in Landscape Architecture, tree and ecology consultancy. Also experienced at completing major BIM compliant projects.