British woodlands tend to have one major characteristic which sets them aside from woodlands in many other parts of the world. They are frequently neat and tidy, cleared of their fallen trees and woody brash. Leaving fallen trees and dead logs provides a food source for fungi, as well as invertebrates, and is essential if the diversity of an area is to be maintained. Some wood-rotting fungi persist unseen in the canopy or bark of a host tree and quickly fruit when a branch falls to the ground. Other fungi require much longer to colonise before fruiting and several species are found only in very old forests. Indeed the range of wood-rotting fungi is used in Scandinavia as a measure of the suspected age of the woodland.