Survival and relative frequency of native woody species and their specialist Lepidoptera on Coll, Inner Hebrides, two millennia after deforestation

The Hebridean island of Coll has almost certainly been predominantly treeless for at least two millennia; it was certainly largely deforested when the first surviving written description of the island was made in the sixteenth century. A survey of the surviving native trees, shrubs and lianes carried out between 1997 and 2011 showed that only Salix aurita is present in any quantity, with all other species being reduced to small, relict stands or scattered individuals. Juniperus communis (at least 650 bushes), Populus tremula (450 trees) and Sorbus aucuparia (121 trees) are the commonest of these species and represent contrasting strategies, with P. tremula present as long-lived suckering clones and the others as freely seeding but probably short-lived individuals. Betula pubescens (42 trees), Hedera helix (50 plants), Corylus avellana (17 trees), Quercus spp. (11 trees) and Salix cinerea (4 stands) are the rarest surviving native species. Historical records indicate a decline since the 1930s of Betula pubescens, Corylus avellana and caninoid Rosa species. The commoner native species have the highest representation of the regional species of host-specific Lepidoptera, with Salix aurita supporting 42% of the regional species pool whereas Betula, Corylus, Hedera and Quercus support ,10%. Woody species may become increasingly frequent in the future now that subsidised fencing allows areas to be grazed more selectively and broad-leaved trees are currently being established in plantations from which they may spread.