Edited sentence: Around 20 million elm trees, most of them hedgerow trees, were felled or died through Dutch elm disease in the late 1960s.Hedgerow trees are both an important part of the English landscape and are valuable habitats for wildlife.Hedgerow trees are the theme of the 2009 Devon Hedge Week.It is suggested that hedgerow trees cause gaps in hedges but it has been found that by cutting some lower branches off, sufficient light can get through to the hedge below to allow it to grow.Many hedgerow trees are veteran trees and therefore of great wildlife interest.New trees can be established by planting but it is generally more successful to leave standard trees behind when laying hedges.The age structure of British hedgerow trees is old because the number of new trees is not sufficient to replace the number of trees that are lost through age or disease.The distance allows the young trees to develop full crowns without competing or producing too much shade.The most common species are oak and ash, though in the past elm would also have been common.There are thought to be around 1.8 million hedgerow trees in Britain with perhaps 98% of these being in England and Wales.Trees should be left at no closer than  apart and the distances should vary so as to create a more natural landscape