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The need for trees

Marshalls plc ammonite logo
Friday 2nd March, 2018

In recent years, there has been growing recognition of the need for more and larger trees in our cities and perhaps it’s only now that the benefits of mature urban trees are being fully recognised. Unfortunately, our existing urban tree cover is now under greater threat than ever before, whether from pests and disease, utilities companies, erratic water supply or insurance companies dealing with supposedly related subsidence claims.  With the recent re-launch of BS5837, and several related initiatives, there is a real resurgence of interest in the subject of tree planting and a flurry of activity from several quarters. Many of the reasons put forward in favour of trees are entirely practical and tangible - carbon reduction; improving urban biodiversity; pollution filtering and absorption (particulate, chemical and noise); providing solar shade and thermal insulation (countering the urban heat island effect and reducing urban temperatures by 4oC); reducing wind speed; absorption and slowing of stormwater run-off.  Others are more esoteric but no less important - a general contribution to wellbeing; improved mental health; crime reduction; making the city a ‘nicer’ place to be and giving it a sense of place; increasing house prices...

Trees reflected in a mirrored building

Whichever of the above might be deemed most desirable, many parties would gain from a deeper understanding of the issues around the planting of large urban trees and improved provision of the systems that support them:

  • Clients (particularly Local Authorities) – resulting in an improved aesthetic to their built environment schemes; addressing current policy requirements; a reduction in the maintenance costs of highways/footpaths (and claims from trips and falls); avoiding the disruption caused by tree replacement after project completion (pedestrian control, ripping up of newly laid paving…).
  • Designers - improved aesthetics and a softening effect to balance the built environment; providing a key element of integrated infrastructure systems; healthier tree growth (ending up with desired results, not stunted or dead trees); minimised future impact on any hard surface around the tree.
  • Contractors - simpler/faster installation; less risk of tree death therefore minimal post project work replacing failed trees; easier and more effective maintenance of a tree during its first critical years.
  • Utilities providers – the development of systems that allow trees to co-exist in harmony with the necessary sub-surface provisions in any town or city; less disturbance to underground utilities by potentially damaging tree roots; simpler and faster access allowing maintenance of their equipment.

In addition, the demand for well-grown urban trees can only increase in coming years due to:

    • extremely strong (and growing) support for the adoption of Green Infrastructure, with both national and local focus, particularly to support climate change mitigation
    • an increase in shared surface projects, where trees as well as pedestrians will need to co-exist with nearby traffic
    • mounting pressure from accessibility groups for better pedestrian provision in the public realm and safer passage generally.

Regardless of all the positives, many in the construction industry are blissfully unaware of the basic needs of a tree, and how to provide for these in a typical urban environment.  Trees need both irrigation and aeration within the rooting zone and providing both successfully is a fine balancing act.  The size of rootzone varies enormously according to species and the quantity of water required by an individual tree is also governed by the species (and size) of the tree.  The other fundamental requirement is that of uncompacted soil through which the roots may grow unhindered – a need entirely at odds with the limited and highly compacted medium that is generally to be found beneath roads and pavements. Freshly cut tree stump

The list of items required to support the healthy growth of trees in paved areas is extensive and available from dozens of suppliers:

    • Tree grilles for installation into hard surfaces
    • Pre-cast foundations to support grilles and other (cantilevered) surface materials
    • Physical tree protection (barriers/bollard type) to prevent vehicular impact or vandalism
    • Permeable paving/attenuation systems that might also support irrigation
    • Irrigation/aeration systems (possibly linked to the above)
    • Porous bound gravel
    • Kerbs/blocks/flags etc to surround the tree pit
    • Loadbearing substructures – stacking crate systems that allow unhindered root growth through uncompacted soil
    • Root barriers/geomembranes… to prevent roots affecting foundations, utilities etc
    • Underground guying systems
    • In-ground uplighters
    • Specialist topsoils
    • The tree itself

While there are a few effective tree pit systems being developed by specialist suppliers, seldom does everything fit together neatly in a confined space where many conflicts exist at every level beneath the ground.  There would still seem to be a real need for open discussion of the many issues and a quest for a ‘best practice’ solution through collaborative design through the industry. When things go wrong in a tree pit, as they very often do, the inevitable outcome is that the tree dies, or it has to be felled due to poor health or lack of growth.  Even if a tree survives, it may struggle to find water and nutrients below ground and the roots will often behave in unexpected ways, growing towards the surface and breaking through surrounding infrastructure.  Evidence of paving that has been negatively affected by root growth can be seen everywhere and the impact on surrounding hardscape can be devastating.  Damage to the fabric is not just costly in terms of ongoing maintenance and replacement, but the resulting trip hazards might also lead to extremely costly litigation. Tree canopy as viewed from the ground

While the cost of an initial installation of a large tree is extremely high – in the region of £5,000+ for a single semi-mature tree with associated accessories – the cost of replacement is generally much higher and hence often harder to bear and fund. When a tree is first planted in an urban area, it may be that the pit is within a construction site so that the public is excluded, the necessary plant is both available and accessible, and the planting can be carried out in the correct order ie paving immediately surrounding the tree being the last item to be installed.  Contrast this with a failed tree that has to be replaced and there are many additional issues and therefore costs:

      • Access – it’s likely that the area will be in full use and so pedestrian control will be an issue
      • Plant – some areas may not have been designed to take the plant required to bring in a big tree after completion and the necessary traffic may damage other site fabric
      • Disruption to local businesses – such replacements need a lot of room and a fair amount of time for the pit to be opened up fully
      • Disturbance to surrounding paving, particularly if rigidly installed with modified mortars – removal and re-laying is an expensive job that requires specialist contractors
      • Aesthetics – the tree may take at least a couple of years to show evidence of failure, by which time others might have grown well; this means there may never be a well matched avenue of trees if that was originally intended
      • PR – failing trees look bad for everyone and they can often take a long time to die completely, so it’s often a long, lingering process!

So we do need to get it right first time and this should be possible by taking account of the huge body of knowledge available to assist in the specification process.  Helpfully, to start this process, the Trees & Design Action Group (TDAG) has recently published a guidance document on urban tree planting, titled ‘Trees in the Townscape‘.

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