Leaves on the line

What is happening to trees in Britain? Horse chestnuts now turn brown in July. A microscopic caterpillar eats out the green insides of the leaves; only the outer skins remain. Horse chestnuts also weep dried blood from their bark, and sometimes the huge trees spectacularly die. Alders have been weeping bloody tears and dying. Newspapers warn of sudden oak death and acute oak decline. The Forestry Commission has stopped planting Corsican pine because of red-band needle blight.

The problem is globalisation of pests and diseases. Diseases which for millions of years evolved to come to terms with their local hosts are introduced to other countries and find new host trees that have not adapted to living with them.

The horse chestnut itself symbolises globalisation. It began as a small tree clinging to Balkan limestone cliffs, whence gardeners took it 400 years ago. Now, I suspect, a native pest has caught up with it.