Many lists of trees of Great Britain and Ireland have been written. There are a number of issues surrounding the inclusion of a species in such a list. As can be seen from the outline of debate below, there is no 'correct' list of trees of Britain and Ireland.
There are a number of different opinions regarding the validity of some species, notably apomictic microspecies and whether some 'species' may actually be hybrids. In particular, the number and definition of species in the genera Sorbus (rowans, whitebeams etc.), Ulmus (elms) and Salix (willows) are open to debate.
Native species are considered to be species which are today present in the region in question, and have been continuously present in that region since a certain period of time. When applied to Britain and Ireland, three possible definitions of this time constraint are:
The only endemic tree species in Britain and Ireland (that is, that are native only to this region) are some apomictic whitebeams.
Species that were native in the region in prehistory before the last ice age, but not subsequently, are generally regarded as extinct and no longer native.
Many additional species have been imported by humans; the total list of all introduced trees numbers several thousand. A far smaller number of these have become widely naturalised, spreading by their own accord without recourse to further human assistance.
A tree can be defined as a large, perennial, woody plant with secondary branches supported by a primary stem (compare with shrub). There is no set definition regarding minimum size, though most authors cite a tree species as being one which regularly reaches 6 m (20 ft) tall (see also tree).
These larger shrubs occasionally reach tree size:
An endemic species is a plant only native to a certain area. Outside this area, unless spread naturally it is considered non-native, usually as a result of cultivation. Britain and Ireland have few endemic trees, most being micro-species of Whitebeam. But there are some interesting endemic trees nevertheless.
The whitebeams are members of the family Rosaceae, comprising subgenus Aria of genus Sorbus, and hybrids involving species of this subgenus and members of subgenera Sorbus, Torminaria and Chamaemespilus. They are deciduous trees with simple or lobed leaves, arranged alternately. They are related to the rowans, and many of the endemic restricted-range apomictic microspecies of whitebeam in Europe are thought to derive from hybrids between S. aria and the European rowan S. aucuparia; some are also thought to be hybrids with the wild service tree S. torminalis, notably the service tree of Fontainebleau Sorbus latifolia in French woodlands.
The scalloped hazel is a moth of the family Geometridae. The species was first described by Carl Alexander Clerck in 1759.
The scalloped oak is a moth of the family Geometridae. The species was first described by Carl Linnaeus in his 1758 10th edition of Systema Naturae.
The mottled umber is a moth of the family Geometridae. It is common throughout much of the Palearctic region. The species was first described by Carl Alexander Clerck in 1759.
The engrailed and small engrailed are moths of the family Geometridae found from the British Isles through central and eastern Europe to the Russian Far East and Kazakhstan. The western Mediterranean and Asia Minor and the Caucasus represent the southern limit of the distribution. In the north, the distribution area ends at the Arctic Circle. It also occurs in North America. Debate exists as to whether they make up one species, or whether E. crepuscularia actually refers only to the small engrailed, with the engrailed proper being separable as E. bistortata.
The grey dagger is a moth of the family Noctuidae.
The dun-bar is a moth of the family Noctuidae. It is a common Palearctic species.
Peavy Arboretum is an arboretum operated by Oregon State University and located on Arboretum Road, Corvallis, Oregon. It is open to the public daily without charge.
The Arran whitebeams are species of whitebeam endemic to the island of Arran, Ayrshire, Scotland.
Weeping trees are trees characterized by soft, limp twigs. This characterization may lead to a bent crown and pendulous branches that can cascade to the ground. While weepyness occurs in nature, most weeping trees are cultivars. Because of their shape, weeping trees are popular in landscaping; generally they need a lot of space and are solitary so that their effect is more pronounced. There are over a hundred different types of weeping trees. Some trees, such as the cherry, have a variety of weeping cultivars. There are currently around 550 weeping cultivars in 75 different genera, although many have now disappeared from cultivation.
The Arboretum La Alfaguara was formerly a tree nursery in the Sierra de la Alfaguara, in Alfacar near Granada in Spain. It contains samples of many of the trees that are scattered throughout the Sierra and the surrounding mountain ranges.