Appendix F: Preserving trees when carrying out roadworks and building new developments

1.1 Trees are an important feature in our urban and rural landscapes and make a significant contribution to the character and quality of our roads. They play an important role in providing screening, filtering traffic noise and absorbing dust and other pollutants. Trees on or next to the highway can be severely damaged by construction and maintenance work to roads and footways. However, damage can be limited, and in many cases prevented, by following the guidelines contained in this appendix. The guidelines cover design principles and site practices, which, if followed, protect existing trees within new projects.

1.2 Wherever possible, our policy is to retain, preserve and protect existing healthy trees when carrying out road construction and maintenance work. A large number of trees are lost each year due to many types of land development and so it is important that trees with significant value and potential are retained.

1.3 It is better to retain healthy, semi-mature and mature specimens with significant life potential rather than retain over-mature specimens with limited life expectancy. Protection measures must always be thorough. You should put them in place before the works begin and maintain them until the works are finished. Inadequate or half-hearted protection measures are a waste of money and can lead to significant damage, which may only become apparent months or years after the works are completed. A damaged or deteriorating tree can present a significant danger to users of the highway.

1.4 Trees are complex living organisms which can be badly affected by direct or indirect damage caused during construction works. Even minor changes to their living environment can affect growth. As the tree forms a trunk and crown above the ground it also produces a root system below ground which supports the crown. Tree roots need to absorb oxygen to survive, so most of a tree’s root system is found in the aerobic (oxygen-rich) soil within the 600mm immediately below the surface. The tree’s roots absorb nutrients and moisture from the soil and can extend well beyond the area taken up by its crown.

1.5 Water and nutrients are transported from the roots, up the trunk and into the crown of the tree through tissues called xylem vessels which lie just below the tree’s bark. Similarly, carbohydrate produced by the tree’s leaves is transported through phloem vessels down the tree to its trunk and roots. Here it combines with absorbed oxygen to produce food energy which, in turn, supports future growth. And, water lost through the tree’s leaves is replaced by moisture absorbed through its root system and transported through the tree.

1.6 Consequently the tree’s roots and its trunk and crown above ground are interdependent and even minor damage to one can badly affect the other.

1.7 Highway construction and maintenance design should allow for all healthy existing trees and where appropriate, planting of new trees. You should involve our forestry officer or your consultant arboriculturalist who should advise at the planning and design stage on retaining existing trees and planting new specimens. You should consider the potential growth of retained trees, their future compatibility with new and existing highway features and how near new and existing service runs will be. You should identify an appropriate protection zone around the trees you are retaining and, wherever possible, you should exclude this area from the construction site. All necessary tree maintenance work, both before and after construction, should be carried out by trained operatives in consultation with our forestry officer.

1.8 When planting new trees, you should consider their future growth and habit and the development should include design features which allow for their growth.

1.9 You should carry out a survey of the construction site at the planning stage before you design the development or improvement works. The survey should include details of both the site and individual trees and should provide the following information.

1.10 Detailed information on the species, health, condition and status of each tree should be collected and assessed by an appropriately qualified and experienced arboriculturalist.

1.11 When designing a new development or road, you should, wherever it is safe and practical, incorporate existing trees worth retaining into the overall layout to provide a mature and established appearance.

1.12 Trees within the development site should be assessed in accordance with the recommendations included in section 5.2 of British Standard (BS) 5837 ‘Trees in relation to construction’. The assessment should cover their condition, significance and landscape and environmental value and potential. The tree survey should classify each tree into one of the retention categories below.

1.13 Where there is a conflict between retaining a category A or B tree and ensuring road safety, the first option should be to amend the proposed design to avoid the conflict and retain the tree. We will consider situations where it is not reasonable or practical to do this on a site-by-site basis.

Trees in this category:

Trees in this category are:

Trees in this category are:

Trees in this category are:

1.14 You should plan or design the roadworks or new development to ensure all category A and B trees are retained and, where possible, category C trees. However, you should not impair road safety or access to the development.

1.15 The following activities are common causes of damage to trees when construction is carried out within the trees’ natural rooting area.

1.16 A tree’s root system can extend radially (outwards) to a distance much greater than the tree’s height. Ideally the whole of this area should be protected and remain undisturbed during construction work. If works are necessary within the tree’s potential rooting area, you should identify a protection zone from the base of the tree’s trunk. This should equal the full extent of the branch spread or be equivalent to half the tree’s height, whichever is greater. This area should be protected with substantial fencing (see below) and be excluded from the construction site. If the works are necessary within the protection zone, you must consult our Forestry and Arboricultural Group at the design and implementation stages to identify specific measures to minimise damage. At this stage you should also consider the potential effects of the works on privately-owned trees next to the site.

1.17 On larger schemes where you are retaining trees within the site area, you should erect protective fencing, as specified in Figure RT4, before the works begin. This fencing should stay up until the work is finished. It is essential that the protection zone is excluded from the construction site and all associated activities including excavations and changes of soil level. Areas where machinery, materials, chemicals and waste material are stored should also be excluded from the protection zone.

1.18 However, due to the linear nature of many highway maintenance operations (for example, kerbing) and the fact that most highway corridors are narrow, it is not always possible to exclude a tree’s protection zone from the work area. In such cases you should adopt the following specific measures to minimise the affects of the works.

1.19 To survive, tree roots needs water and oxygen within the soil. Surface material should also be flexible to allow for soil movement and future root activity. Granular surfacing, such as chippings or other permeable materials, laid on to a geo-textile membrane would be appropriate in this case.

1.20 You should install any service runs or street furniture within a tree’s protection area using hand-digging methods to ensure that all roots with a diameter of more than 25mm are retained. You should follow the recommendations contained in NJUG 10. You should contact the forestry officer if you need more advice.

1.21 Do not include residual chemical herbicides in construction materials within a tree’s protection zone as these may be absorbed by the tree’s roots and have a serious toxic effect.

1.22 In some instances, it may be necessary to carry out tree surgery before work begins to accommodate access to the site and prevent physical damage occurring to nearby trees.

1.23 Tree surgery may also be necessary after the work has finished to repair any minor damage which has occurred. Tree surgery should always be carried out in accordance with British Standard (BS) 3998 : 1989 ‘Recommendations for tree work’. You should use a qualified tree-work contractor who is skilled in modern arboricultural techniques. If damage has occurred to the site during the works, techniques such as soil aeration and surface mulching with an organic material (for example, pulverised bark) may be beneficial to improve the soil and allow the tree to recover from any disturbance. Cultivating the soil, fertilizing, importing topsoil and raising levels are common site re-instatement techniques which are all potentially harmful to retained trees.

1.24 When designing and planning highway equipment, you should consider the location of nearby trees, whether they are on the highway or on nearby property. You should take care to avoid installing this equipment within a tree’s potential rooting area and within its crown or potential growing space. You should site equipment such as road signs, street lights and so on to avoid damage to trees during installation and to avoid the need for regular pruning to maintain visibility.

1.25 When carrying out development work or highway improvement works near to trees, it is essential that everyone working on the site adopts all the measures identified to retain trees and minimise damage. It is also essential that all site operatives (that is site managers, clerks of works, all contractors’ operatives including subcontractors and so on) understand what is needed. You should arrange a meeting before work begins between everyone involved to establish good communication and identify all the initial requirements for retaining trees. Regular meetings should then take place while the works are ongoing to identify additional requirements and potential problem areas. It is important that all appropriate practices and protection methods are written into contracts, along with penalty clauses which provide an incentive to adopt good practice.

1.26 We will adopt trees that have been successfully retained on verges and other highway-related land providing you pay a commuted sum to cover their long-term maintenance. (Please see Part 4, Section MC18 for further details on commuted sums). We will not adopt any tree retained within a development if we know that it has been damaged by poor practices during construction and the appropriate protection measures have not been adopted.