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. The Yugoslav discoverer of the micro-caterpillar named it Cameraria ohridella, after the city of Ohrid near the horse chestnut’s homeland.

How serious are these threats? Are they a re-run of Dutch elm disease, which took out big elms in the 1920s and again in the 1970s? The micro-caterpillar has not been here long enough to tell. But horse chestnut, a robust tree, so far seems not to mind having its effective growing season cut in half. In this country, despite its name, sudden oak death has not mainly been an oak disease: it has hit rhodendrons, a plant that conservationists dislike. Acute oak decline is an unknown quantity: there have been mysterious oak declines in the past, as in the 1920s.

Matters could be much worse. Kyoto, Japan, is surrounded by richly wooded mountains. This year alone a sixth of the trees have died, but not because anyone has neglected the Kyoto Protocol on climate change.