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Trees: Appreciating the Value of the Underground World

Environment

by Mark Chester - Cedarwood Tree Care

 

There tend to be two views regarding tree roots. The first is that they reflect the crown spread above ground. The second is that once trees have been planted, we tend to forget about them. The issue of the extent of root spread is one which can influence the design and layout of developments. It can be used to restrict the extent of development, and in some cases, to justify refusal of consent. With care and an informed approach, opportunities can be pursued and trees be successfully retained. The needs of both existing and new trees can also be realistically accommodated.

In an ideal situation, with good soil and unrestrictedaccess to nutrients and moisture, roots tend to growin an even range. However, such situations rarely exist, especially in our urban settings. When surveying trees for suitability to be retained within a potential development site, a calculation is made to determine how much space should be allocated to the roots. This space is typically shown as a circle, and is called the Root Protection Area. For those seeking to regulate planning and the extent of development, the Root Protection Area (RPA) is often seen to be sacrosanct, and any encroachment in to it can meet with resistance unless it is fully justified and supported by detailed technical evidence. Fortunately, the document which informs how trees are retained on development sites, BS5837:2012, is a recommendation, a series of guidelines. It facilitates a degree of flexibility. It is my role, in guiding my clients, to identify solutions where they may be possible, and to provide the technical evidence to support a proposal. Sometimes, this needs lateral thinking. Equipped with this and an array of technical tools, I have been able to secure consent for clients, and to ensure that mitigation is proportionate.

The starting point is to appreciate that the RPA is below ground. With care, it can be possible to install infrastructure above ground. Whatever happens above ground, we need to ensure that we avoid or minimise compaction, maintain drainage and retain and possibly enhance aeration of the ground below. The use of cellular grids which are laid on the surface and filled with gravel helps to meet these objectives. It spreads the weight of passing traffic and thereby limits compaction. It is a recognised method for providing access. I have used it to enable access to a site where there was a tree off-site (and within a Conservation Area). The site, within my adopted Hereford, where there is increasing pressure to develop, was a garden to a larger property. The principle of development was recognised, but there was local sensitivity, especially with this being a Conservation Area. The planners needed to be reassured that the proposed new dwelling could be constructed in practice.

There was a small area of hard standing by the entrance, which has been used for parking a car. However, this was clearly insufficient to accommodate the machinery and materials associated with construction. By using the grid method, I was able to demonstrate that the additional hard standing could be created without compromising the root area. It ticks the boxes of minimal compaction and of unrestricted water infiltration and air movement. I remember that case well, not only because the application was successful but also because the report was used as the template for an accreditation scheme I developed with the training organization Lantra.

Working with known, or likely, areas of roots is one thing. One of the challenges sometimes is to establish whether the roots are following the expected parameters. This can be very useful in informing the extent of mitigation works required, and can justify not implementing costly, but unnecessary measures. Several years ago, I worked on a proposal to install wind turbines on an exposed hill side in south Wales. Wind turbines are tall, with long blades. They are
substantial pieces of equipment, supplied as parts with considerable lengths.. This particularly site, accessed via a long-established forestry track, needed the construction of a new road, with substantial foundations, to facilitate delivery. This new road was to follow the existing track, which passed close to a mature Beech hedge. In order to construct the new road, the existing feature needed to be excavated, and there were concerns that the construction process could be
detrimental to the trees.

The hedge had open fields to one side, and the forest track to the other. My observation was that roots were more likely to be flourishing on the field side than growing under the track. The planning officials required this reassurance ahead of making an approval for the proposals. My solution was to dig a series of small pits between the trees and the track, ideally to 600mm depth, and record any roots that I found. If there were few roots in this compacted ground, it was very unlikely that they would extend under the track. In the absence of roots, the track could be excavated and the new road installed without compromising the trees. I actually found the ground to be so heavily compacted that I was unable to dig any of the pits beyond 100mm depth and, unsurprisingly, I didn’t find any roots. I have used this approach repeatedly to inform both the direction of construction works and the extent of mitigation works.

For another site, I faced a different challenge. The tree was situated with the trunk in a neighbouring garden but branches growing over the boundary. The normal expectation would be for roots to be growing beyond the boundary. If this was the case, then foundations using piles and beams would be needed, rather than the normal trench method. Again, I sought to excavate pits to explore the situation below ground. The ground on the application side was covered in concrete and compacted. Conditions were not particularly inviting for tree roots! It was not possible to excavate beyond 100mm and it was no surprise to me that no roots were identified. The application has been approved without the need for costly construction methods. If a pile and beam construction approach is needed, then it is a valuable method for achieving development. However, if there are no roots to protect, it becomes excessive.

It is important to provide the conditions of the rooting area by ensuring there is no compaction of the growing environment. It is possible to install infrastructure
above the ground. It is important, in doing this, to ensure that the ground level is not altered, by excavation or by the addition of materials. Where footways need to be installed within the RPA, I have used decking to avoid compaction. For one particularly challenging site, with a tree being retained and on a steep slope, the use of decking has allowed access to a previously inaccessible garden area. The slope was so steep that I needed to tred carefully when inspecting the tree, and I appreciated the dry weather and ground conditions.

The construction process is stressful to trees being retained. One of the factors that I consider when assessing trees for suitability to be retained is whether they have sufficient vigour to withstand the stresses ahead. It is possible to assess vigour in advance using chlorophyll fluorescence measurements, and whilst these can only be used when the tree is in leaf, they can inform management.

I mentioned at the start that so often, the environment below ground can easily be overlooked. It is, however, vitally important to the health of our trees, whether they be ones being retained or newly planted. There has been much research over the past decade or so exploring how soil conditions can be improved,
equipping trees to resist attacks from various pathogens. My associate Kevin Martin, who manages the trees at the Royal Botanic Gardens, Kew, where there is a collection of some 14,000 trees, has been trialing techniques to enable some of the specimens to respond to the effects of compaction.

He has found that the use of an air spade to break up the surface, the application of compost and a good covering of mulch is proving effective. One of the challenges that Kevin faces at Kew is that the collec-tion includes trees which are botanically or historically important, and whose life needs to be extended for as
long as possible. When one is considering trees planted by King George lll in the late 1700s, and irreplaceable, the choice of fell and replace, normally available, is not an option. The results so far are impressive, and the methodology is one which I am using. One can almost sense the relief and appreciation of the tree as the ground around is improved, a bit like giving a human patient a new lease of life!

On the subject of rejuvenating trees, Dr. Glynn Percival, who I find a valuable source of information on plant health, has been monitoring the use of biochar, a product of high temperature charcoal production, in field trials. He has found a good response from early research, and has even found ash trees where biochar has been incorporated in to the soil are resisting Ash dieback. I am concluding from this evidence that we have the tools to equip trees to withstand the stress of development and be equipped for the future. Valuing the growing media and ensuring the trees have access to a good resource is key to them becoming established and sustained in the long term.

One of the key elements of my work, beside identifying trees for retention, is selecting trees for landscaping schemes, usually to mitigate for the trees removed as part of the development process. I recognise the importance of planting the next generation of trees. Within the planning process, the provision of a landscape
scheme can often be seen as just one of a list of documents required to achieve consent, and for many in the process, its existence with tree planting as an element is sufficient to reassure.

For me, the landscape scheme is actually one of the most important documents. It is the one which places a development in to its setting. At the Trees, People and the Built Environment ll conference held at Birmingham University in 2013, the architect Sir Terry Farrell, key note speaker, spoke about how with developments, the focus is to often on the built environment. However, it is the space between buildings, so often overlooked, that matters most to those who will work and live in the development once completed.

I can provide a landscape scheme simply to meet the requirements of planning. However, with every scheme I produce, whether to fulfill planning conditions or as part of a more holistic site development, I carefully select the trees to suit the site. It is important that the trees planted are suitable, and also contribute to and enhance the setting. This is more than just conditions above ground. How often do we dig a hole and put a tree in it, and walk away, without considering what the tree might need. Trees are generally low maintenance, especially once established. However, the early years are critical, and they need to be equipped for the future.

Unless we are planting trees in a grass verge or amenity area, it is likely that the setting will have access to limited organic matter. This is usually the case with development sites. Indeed, developments on previously undeveloped sites (including gardens, which tend to be considered brown field sites) often start with the top soil being stripped and piled for possible use later. Whereas turf and shrubs may grow and thrive on a covering of soil that is deep not thick, trees need more. This is not surprising!

The Trees in Towns ll report, published a decade ago and based on a survey of tree management across the various local authorities within England and Wales,
found that as many as one quarter of newly planted trees were dead within the first two years following planting. The reasons are numerous, a factor highlighted
in BS8545:2014, a new industry standard for the nursery sector. One of the major factors which hinders the ability of newly planted trees to become establish and flourish is the growing conditions in to which the saplings are being place.

In 2014, I worked with Keith Sacre, who chaired the Drafting Committee for BS8545:2014 to develop training in the various elements of establishing young trees within the landscape. Keith has spent some 40 years working with trees, and has specialized in managing the establishment of young trees. Spending time with him, I have appreciated the science guiding our understanding of how trees function and the ingredients for successful establishment. One recurring comment I heard, as the course developed and I worked on marketing it, was that people did not need training in how to plant a tree. This was surely not that complicated. I appreciate the sentiment when considering the basic principles, but I am also very well aware that things that seem ‘obvious’ to me tend not to be recognised by everyone. There is so much more to establishing young trees than simply digging the planting pit. The science guiding the subterranean environment and informing best practice is considerable.

I have written in the past about a site in Hereford, a high profile regeneration project where I have surveyed the trees for several years. Landscaping including planting a group of five Birch trees, using raised containers to create an architectural feature. There is nothing wrong with such an approach, but it seems that no one thought about how the trees would be irrigated, especially during the first couple years. When I visited the site at the end of a notably dry and hot September (in 2014), three of the trees were already dead, and all five had been planted in the wire cages they were delivered in! Isn’t watering just common sense?

In addition to watering, the environment in which the roots are to become established needs to be welldrained and aerated. Roots do not contain chlorophyll and so need access to oxygen in order to respire. If the soil become water logged, anaerobic conditions will soon develop and the roots will be affected and unable to properly function. We may need to bring soil in, although it is generally better if we can use material from the site. So often, we create a planting station broadly similar in size to the root ball of the sapling and expect the tree to flourish. We then complain when roots near the surface begin to spread out and threaten to damage the surrounding infrastructure as they seek space and nutrients to sustain further growth.

How much space do trees need below ground, and what do they need to sustain growth in to the decades ahead? We can actually calculate the volume of soil
needed, and ensure that good quality growing media (which does not necessarily need to be top soil) is provided. I refer to growing media for a reason. Soil is not always appropriate and research in Sweden has identified that, for urban areas in that nation, trees have flourished on brick waste from the dereliction of urban renewal. The local conditions are such that the building material includes deposits of nutrients which are able to sustain tree growth.

Trees do need space. Typically, an area equivalent to around twenty cubic metres per tree may be needed. One aspect of this which I am appreciating more is that we can recreate the conditions of the forest or nursery field in the most challenging of urban landscapes. We can do this in the subterranean environment
using crates which can accommodate all that the trees need, and support usage above ground. This enables trees to be planted in the middle of car parks, and be equipped for the future.

We tend to create individual planting positions for trees, and expect them to grow in isolation. However, this need not be the case. We can link up the planting areas for trees, and other areas of soft landscaping such as shrubs. This enables roots to move between planting areas, and the fauna which inhabits this environment can circulate across a larger area, which is healthier for it and the trees.

Finally, when we plant trees, it is important to appreciate their future potential, and how this could impact the site where they are being planted. There is little
purpose in planting trees which will take several decades to mature if the site may be refurbished before they have matured. Similarly, if the trees will outgrow the site, or are noted for problems, such as invasive roots, we are incorporating problems in to the design.

I don’t expect my clients to recognise all of the opportunities and constraints of an individual site, and which trees would be most appropriate, and where. It is more than simply getting the right tree in the right place. This is where I can guide and inform. When the selection and planting of trees is an after-thought, this is often reflected in the results. When I am engaged in the development process at an early stage, I can ensure that best practice is successfully incorporated in to the design. Then we can work together to achieve a quality scheme.