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.