Protection of retained trees
Trees are often overlooked during development and, as a result, many are either lost, or given inadequate protection, that results in their demise within a few years. The British Standard BS 5837 Trees in relation to design, demolition and construction - recommendations, was reviewed and updated in April 2012 and is the benchmark document for how to successfully take account of, and retain, suitable trees in proximity to development. Where there are trees that could affect, or be affected by a planning application, we may require a tree survey to be carried out and submitted in support of the application.
Trees have to adapt to their immediate surroundings and any changes will have some affect. Therefore, it is essential that a detailed tree survey which complies with the British Standard is undertaken before a scheme is designed. This will schedule the trees according to their suitability for retention and identify the extent of land required to ensure that they have the best chance of survival. Older trees are more vulnerable and they are often the most desirable to retain for both their amenity and conservation value. The tree survey should be carried out by an arboriculturist, as detailed within the British Standard, otherwise an application will not be considered valid. Of particular assistance to anyone submitting a planning application is Figure 1: The design and construction process and tree care, and Annex B, Trees and the planning system.
Common damage to trees during development
- abrasion of bark and wounds that leave wood tissue exposed;
- crushing of roots by vehicles / plant equipment and / or storage of materials;
- severing and removal of roots by excavation;
- broken branches, leaving wood tissues exposed;
- poor pruning;
- fire damage;
- poisoning of roots from spillage, or storage of fuel, oil, chemicals and any other potentially noxious materials;
- changes in soil levels around trees, resulting in root death;
- installation of impermeable surfaces.
The part of the tree most susceptible to damage is the root system because:
- roots cannot be seen and their extent is not realised;
- of a lack of understanding of root function and their importance for the health of the tree.
The affects of damage can be serious, but often it takes several years for this to become evident and is not always linked back to the actual cause during development work. Often, by the time the damage becomes evident, the developer may no longer own the site, leaving the new owner with the problem and the potential need for costly tree work. Lack of protection can also result in damage to bark and branches that can disfigure a tree and result in disease and decay that also reduces safe life expectancy.
Tree Root Systems
Roots have three main functions:
- absorption of water, oxygen and nutrients;
- tree 'food' storage in the form of starch;
- structural support.
Tree root development is entirely opportunistic and spreads horizontally to a distance and depth entirely dependent upon the ground conditions encountered. Very few trees have a 'tap root' after the first few years. Roots require oxygen and water to function and therefore most will remain close to the surface. Research has shown that 90% of tree roots are to be found in the top 600mm of soil. Roots may extend horizontally for considerable distances and, where conditions are suitable, this distance may be equivalent to two, or even three times, the tree height.
The majority of roots are the easily overlooked, fine, fibrous roots that absorb water, oxygen and nutrients from the soil; these are easily damaged by crushing and removal during soil stripping operations. The main structural support roots are usually found within a few metres of the tree stem and these are linked to the fibrous roots by a network of cable-like roots that also provide additional anchorage. All tree roots are important.
Root protection to prevent long-term damage
The guidance contained within BS 5837 Trees in relation to design, demolition and construction - recommendations, identifies a Root Protection Area (RPA) based on the stem diameter, but protective measures may need to be increased, for example, to the extent of the branch spread, to avoid damage to the above ground parts of the tree. Tree protective measures are detailed within the British Standard, the default specification for a protective barrier is shown in the diagram below.
More detailed information regarding appropriate tree protection is detailed within BS 5837 Trees in relation to design, demolition and construction - recommendations available from the British Standards Institution, 389 Chiswick High Road, London W4 4AL, or visit their website www.bsigroup.com/standards.
Anyone submitting a planning application where trees are affected should ensure that they seek advice from an appropriate professional adviser, to reduce the chance of delay with their application.