A SURVEY OF DAMAGE TO ROADSIDE TREES IN LONDON CAUSED BY THE APPLICATION OF DE-ICING SALT DURING THE 1990/91 WINTER

Abstract

Summary The severe 1990/91 winter provided an opportunity to quantify the effects of de-icing salt on the roadside trees of London. Between late July and mid September 1991, 25 paired main road and side road plots were established in the London area. Each plot consisted of 30 roadside trees, giving a total of 1,500 trees. London plane (Platanusx hispanico) comprised 25 per cent of the main road and 19 per cent of the side road trees. Common lime (Tilia x europea), small-leafed lime (T. cordata) and flowering cherries (Prunus spp.) were also abundant. From records of the distances traversed in order to assess the trees, estimates of the total roadside tree populations were produced. One hundred and forty two main road and 70 side road trees showed current salt damage. By extrapolation this amounts to over 20,000 roadside trees in the London area. The most frequently damaged species was London plane; 39 per cent of the main road and 30 per cent of the side road trees being affected. With this species damage mainly took the form of ‘post flushing dieback’. Symptoms affecting more than half the crown occurred in a quarter of the affected trees. Norway maple (Acer platanoides) was also frequently affected and other common species with a moderate incidence of damage included Sycamore (A. pseudoplat anus) and the common and small-leafed limes. Cherries were almost entirely unaffected. The influence of site factors was examined by analysing separately the data from London plane and those from all the other species together. Damage incidence was higher on main roads than side roads but the most striking association was that between the incidence of damage and the close proximity of the tree to a shop or other public building. This indicates that de-icing conducted specifically for access to such buildings is a major cause of damage and that if this operation could be conducted in some other way, much damage could be avoided. A resurvey was conducted in late summer 1992 to determine the fate of the damaged trees. Most trees with symptoms affecting less than half the crown in 1991 had either recovered completely or only showed relatively small amounts of dieback. By contrast, of the 47 trees in which at least half the crown was affected in 1991,16 required felling or major pruning and 9 had already been felled. By extrapolation this amounts to 2,500 trees for the London area. The direct costs in felling and pruning would amount to some £70,000, and the costs in terms of lost amenity value were calculated to be more than £2 million.