| Trees Of Britain And Ireland |
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ISSUES OF DEBATE 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 Hybrid s. In particular, the number and definition of species in the genera ''Sorbus'' ( Rowan s, Whitebeam s etc.), ''Ulmus'' ( Elm s) and ''Salix'' ( Willow s) 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 the British Isles , 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. 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 (see also Tree ). LIST OF SPECIES Native trees
Native large shrubs These larger Shrub s occasionally reach tree size:
Naturalised trees
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