Definition of species 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.
Definition of native 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: • a species that colonised these islands during the retreat of ice at the end of the
last ice age • a species that was present in these islands when the
English Channel was created and the
land bridge between Britain and continental
Europe was flooded • a species that has colonised without human assistance; in some cases this is uncertain. 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.
Definition of tree 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 with a single stem. Species like
Blackthorn (
Prunus spinosa) and
Purple willow (
Salix purpurea), which may reach 6 m but not on a single stem, are not treated as trees. ==List of species==