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Summary
Of 2232 street trees surveyed during 1986 in Manchester (13 per cent of the total), 30 per cent were causing damage to pavements and 13 per cent damage to kerbs. Trees planted into pavements caused significantly more damage than trees planted in planting strips. Trees in planting strips > 3 m wide caused significantly less damage than those in strips < 3 m wide. The most severe pavement damage usually occurred within a 2 m radius from the base of the trunk. Significantly more damage occurred to pavements sealed with tarmacadam (asphalt) than to those paved with slabs. However, displaced slabs more commonly pose a safety hazard to pedestrians.
Of the 8 commonest species and species groups of trees in the survey (lime (Tilia sp.), ash (Fraxinus excelsior L.), whitebeam/rowan (Sorbus spp.), sycamore/maple (Acer spp.), cherry (Prunus sp.), oak (Querus spp.), birch (Betula spp.), horse chestnut (Aesculus hippocastanum L.)), lime and ash caused the most pavement and kerb damage, whitebeam/rowan and cherry the least pavement damage and cherry and oak the least kerb problems. Most trees started to cause damage when they were 11—20 cm dbh but oaks and horse chestnut not generally until >20 em dbh. The distance apart at which trees were planted had no significant effect on pavement or kerb damage.
To minimise pavement problems it is recommended that trees are planted in planting strips, which should be as wide as possible. If planted into pavements an unsealed area, ideally at least 2 × 2 m, around the tree base will reduce the severity of damage. Kerb problems can be reduced by planting in such a position that the trunk will be at least I m distant from the kerb when the tree is mature. When replacing large trees such as lime and ash along narrow and restricted streets it is advisable to plant inherently smaller species such as whitebeams or cherries. Where space is not so limited oak, sycamore or Norway maple are likely to cause less damage then lime, ash or horse chestnut.