In his illustrated talk at the Conservation Society’s AGM on November 1, Guy Litchfield of the Sussex agricultural college at Plumpton outlined how the trees that once covered all of Britain were reduced to today’s level.
And now the huge modern expansion of global trade is bringing new threats to our trees in the form of pests arriving from abroad.
Britain’s tree population developed by a process of succession after the last ice age. About 5,000 years ago, rising sea levels cut Britain off from Europe, resulting in fewer native species of trees than on the continent: Britain has only 33 native species, including just three conifers.
Neolithic peoples began the process of deforestation at about the same period as Britain became an island. They cleared land for farming and used timber for other purposes, and developed coppicing as a management technique. The Romans accelerated the process, but also introduced various new tree species.
Thus from 1000 BC to AD 1000, tree cover fell from 80% of the land surface to 15%. The following centuries brought additional demands, and by 1900 woodland cover fell to 5%. During the First World War, a third of the remaining timber was used. After the war an afforestation programme began, and the Forestry Commission was set up in 1919.
Globalizaion of trade in recent years has brought a significant rise in tree disease and pest threats from abroad.
Illustrations for some fungal diseases appeared in Newsletter 140.
Chalara, a fungal disease, threatens to wipe out the ash. Yet there is hope that the ash trees here in the Southwest, being derived from southern European stock, may prove more resistant than ash strains in other parts of Britain that derive from northern Europe.
Horse chestnuts are being attacked by the leaf-miner moth, which can be serious if several cycles of attacks occur in one year, and by a bleeding canker or slime flux caused by bacteria.
Sweet chestnut is vulnerable to chestnut blight, a devastating fungus.
The caterpillar of the oak processionary moth can strip the tree of leaves, but the pest has so far been contained. Acute oak decline, caused by bacteria, is a more serious threat.
Pines and firs are vulnerable to red-band needle blight. Phytophthora ramonum is a fungus devastating larch trees. It is able to transmute into new strains and thus also threatens other conifers, beech, oak, and both chestnuts.
Other pests include the oak pinhole borer and pinetree lappet moth, which have arrived in Britain. Serious pests approaching these shores are the Asian longhorn beetle, capable of attacking many species, the eight-toothed European spruce bark beetle, the emerald ash borer and the citrus longhorn beetle.