A tree is a plant form that occurs in many different
orders and
families of plants. Trees show a variety of
growth forms, leaf type and shape, bark characteristics and
reproductive organs.
The tree form has evolved separately in unrelated classes of plants, in response to similar environmental challenges, making it a classic example of
parallel evolution. With an estimate of 100,000 tree
species, the number of tree species worldwide might total 25 percent of all living
plant species.
[7] The majority of tree species grow in
tropical regions of the world and many of these areas have not been surveyed yet by
botanists, making species diversity and ranges poorly understood.
[8]
The earliest trees were
tree ferns,
horsetails and
lycophytes, which grew in
forests in the
Carboniferous period; tree ferns still survive, but the only surviving horsetails and lycophytes are not of tree form. Later, in the
Triassic period,
conifers,
ginkgos,
cycads and other
gymnosperms appeared, and subsequently
flowering plants in the
Cretaceous period. Most species of trees today are flowering plants (
Angiosperms) and conifers. For the listing of examples of well-known trees and how they are classified, see
List of tree genera.
A small group of trees growing together is called a
grove or
copse, and a landscape covered by a dense growth of trees is called a
forest. Several
biotopes are defined largely by the trees that inhabit them; examples are
rainforest and
taiga (see
ecozones). A landscape of trees scattered or spaced across grassland (usually grazed or burned over periodically) is called a
savanna. A forest of great age is called
old growth forest or
ancient woodland (in the UK). A young tree is called a sapling