Trees eating roads

SIR – Roy Dunnett (Letters, June 3) asks why road surfaces deteriorate beneath overhanging trees.

In the surface-dressing industry, it is a recognised phenomenon, caused by acidic droppings from the trees. This causes the binder (bitumen emulsion) to oxidise and slowly lose adhesion to the stone aggregate.

To combat this we would increase the rate of spread of bitumen emulsion per square yard, by slowing the spray tanker through the affected areas.

No doubt new research may have other scientific answers. S J Bridgwater

Shrewsbury

SIR – As a parish tree warden I am aware that shadows cast by trees slow down the evaporation of rain and dew. Moisture has, therefore, more time to seep into the tarmacadam, eroding both the surface and substructure of the road.

In winter, if this permeated moisture expands upon freezing, the road is rent apart. With the smallest breach in the surface, pounding by traffic furthers the destruction,

Lynchmere, West Sussex

SIR – It is not the overhanging branches that create the problem but the equally widespread underground root system.

During normal summers there is less rain, yet trees are in full leaf and transpire more, thereby sucking more water out of the ground.

This causes land to sink, especially near a tree. Any brittle patch of ground, such as a road surface, cracks, leaving it vulnerable to penetration by autumn rains and winter frosts. Dr Bruce Denness

Whitwell, Isle of Wight