Speaking at Water: Challenges for the 21st century conference in Winchester, Professor Baines said: “Trees across the UK are being damaged by the digging of trenches ­ but to my amazement, no one has yet performed an environmental impact assessment (EIA) on the effect of mains repairs. The roots will be damaged and although the effect may not be immediately apparent, the tree may die later as a result. And has anyone considered the fact that some urban trees might need the water from leaking pipes?”

Mike Hedges, planning and capital works manager at Portsmouth Water, rejected Professor Baines’ claim: “I’m afraid I have to take exception to the idea that mains repair work damages trees. We have recently been commended for the way we have carried out mains repair work in the vicinity of trees. For instance, before carrying out mains repairs in Chichester near to a line of mature trees we asked Chichester Council’s tree protection officer to come out and advise us on how to complete the project without damaging the trees. We have since been commended by local residents for the way we completed the work. We always carry out work to the National Joint Utilities Group’s (NJUG) guidelines on protecting trees.”

Professor Baines replied: “Despite 10 years of campaigning to get some effective legislation, there is little doubt there is damage to street trees due to trenching. I am not trying to single out the water companies, but I am concerned that there have been no studies into how their work will affect trees. Trees are a very valuable resource to water companies. Not only are they an important part of our heritage but they can also help trap and break down pollutants in the soil and slow the rate of run-off which reduces the risk of flooding.”

This is not the first time that Professor Baines has warned of the effect of trenching. In an article “Cables of Destruction” published in Tree News (Summer 1994), the magazine published by the Tree Council, he claimed: “Service trenching obviously poses a potential disaster for tree roots. There is trenchless technology available, which can tunnel beneath tree roots. Mostly, though, services are installed and maintained by simple trenching and more often than not, the trench is excavated by machine.” Most of the article levelled criticism at cable-laying companies, which at the time were being issued licences by the Department of the Environment, Transport and the Regions (DETR) without planning permission in order to deliver the government’s objective of an ‘information superhighway’.

However, water companies have also been trenching on a large scale in their drive to reduce leakage. The NJUG and Arboricultural Association provide the Government’s guidelines for utility trenching. NJUG booklet no.10 (1995), Guidelines for the planning installation and maintenance of utility services in proximity to trees, says: “Contrary to popular belief, the root system of a tree is not a mirror of the branches, nor is there usually a ‘tap root’. The majority of the root system of the tree is in the surface 600mm of soil extending radially in any direction for distances frequently in excess of the tree’s height. Excavation or other works within this area are liable to damage the roots. The effects of damage from different causes will be cumulative and while roots may regenerate, they will not necessarily provide their original anchorage.”

Professor Baines spoke of a recent case in Birmingham where an urban tree fell over onto a line of traffic, killing three people: “I wrote to the coroner and asked for an investigation of the tree’s roots, but as yet I have not received an answer.” He went on to describe the situation in his own street: “The road behind my house has been dug up 45 times in the past five years, and in that time three mature trees have died.”

The NJUG booklet asks utilities to dig carefully, and if possible with hand tools, leaving as many roots as possible intact in a ‘precautionary’ area four times the trunk’s circumference. The Arboricultural Association’s leaflet Trenching and Trees suggests a ‘protected’ area equal to the branch spread or half the tree’s height (whichever is greater).

Rotary-wheeled excavation machines are often used to cut trenches rapidly through pavements, but this is not advisable in the vicinity of trees. The leaflet says: “Do not cut or damage roots greater than 25mm diameter in the protected area. If you think you need to cut larger roots, consult a tree specialist first.” Avoiding tree roots is difficult because the majority are found growing towards the front garden of a house in a typical suburban street, and they are attracted to the cool damp surface of supply pipes.

Although Mr Hedges of Portsmouth Water made a convincing case for the work done in his area, it is difficult to imagine how trenching work can be carried out without some damage to tree roots, especially if the same area is repeatedly disturbed. Professor Baines said there should be a limit on repeated trenching and said the UK could learn by example: “In Ireland, some local authorities have already started enforcing a ‘no return’ rule for trenching of two years.” In Dublin, developers are also required to make a £10,000 deposit before beginning work in order to protect trees but there is no such requirement in the UK.

In the UK, some trees have Tree Preservation Orders (TPOs) and if they are damaged local councils can prosecute those responsible. The majority of urban trees do not have TPO status and prosecutions by the local council or a government body such as English Nature could be raised under the water companies’ more general duties of care towards the environment.

According to Environment Agency (EA) spokeswoman Gill Aylward: “The EA does not bring prosecutions with direct regard to urban trees, unless damage can be shown to result in flooding or increased pollution.” A more effective legal framework is needed as Professor Baines predicts trenching will result in the loss of 20-30% of the UK’s urban trees over the next decade. Unless they can prove otherwise, water companies may be found partly responsible for the decline.

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