Introduction

Trees are an integral part of urban landscapes and provide important aesthetic and environmental contributions to making towns and cities more pleasant, safer and healthier to live in. Trees can give shelter from noise and wind, reduce chemical and particulate air pollution, provide shade and add value to nearby properties. Trees also benefit urban ecosystems, by sustaining biodiversity, and they reduce storm water run-off and prevent erosion. Removal of city trees will lead to a decline in the quality of urban landscapes and large-scale felling programmes would not be acceptable to the public.

There are about 100 million trees in the urban environment in the UK. The Highways Agency has an extensive roadside estate that extends to 30,000 hectares which supports more than 25 million trees, ranging from the common London plane to rare wild service trees. There is a significant population of trees aged 40 years or more; some of these are very old. There are even more trees on non-highway land, on both private and public property. Many trees are situated close to structures, buildings, road pavements and footways.

Unfortunately, structural damage is sometimes associated with the close proximity of trees to lowrise buildings (Figure 1). Trees can extract water from below the foundations causing some particular clay subsoils to shrink, ultimately leading to failure of the foundations and cracks in the superstructure (Biddle, 1998). The cost of repairing the damage caused by subsidence of domestic house foundations in the late 1990s has been estimated to be of the order of £300–£400 million annually. Not all of this can be attributed to the presence of tree roots. However, most of the subsidence incidents in the UK occur in areas with clay soils and, in these areas, tree roots are claimed to have an effect on subsidence incidents in 73% of cases (Loss Prevention Council, 1995). The potential for reducing or avoiding remedial costs by reducing the need for rectification work may be around £200 million per annum.

Currently, no methods exist that reliably predict which trees may cause damage, although not all trees near buildings are implicated. However, decreasing water uptake by trees may lessen subsidence risk by conserving soil moisture and reducing clay subsoil shrinkage.

Tree pruning is thought to be a potentially effective method for conserving soil moisture that could prevent unnecessary removal of trees, but this idea has never been tested on amenity trees in an urban environment. Cyclical pruning is recommended in a risk limitation strategy developed by the London Tree Officers Association (1995) for reducing subsidence claims.