A notable characteristic of trees is how unrelated they are from each other.
You can learn to identify many plants easily because they can be classified into families which share many similar structural traits. For example, many native flower species in the rose family have five petals.
However, trees are an example of ‘convergent evolution’; this is when one or more unrelated livings things adopt the same biological traits to solve similar problems. In the case of trees many unrelated plant families have evolved a single woody stem to achieve height in order to get enough light. Consequently trees can be found in many plant families and there are a number of trees in the rose family alone – such as the wild service tree, white beam and rowan.
One obvious way to classify trees is simply to divide them into their biological class which gives us two main groups; Gymnosperms and Angiosperms.
- Gymnosperm means ‘naked seed’ and these were the earliest trees to evolve. They do not produce flowers and have seeds which are directly exposed to the air for wind pollination. Pine cones are one example. For ease you might think of the gymnosperms as conifers, such as yew and Scots pine.
- Angiosperm means ‘hidden seed’ and these are typically flowering trees which have seeds hidden inside a fruit. These trees evolved alongside insects, birds and mammals and usually make use of them for pollination. The angiosperms can be thought of as hardwoods, such as oak and beech.
When categorising trees, it is important to be aware that there are usually exceptions to the rule. Even within the strict biological classifications described above there are variations. For example, some angiosperms make use of wind pollination; examples include ash, birch and elm.
If you were to categorise trees into needle trees and broadleaf trees then, here too, there are problems. The gorse is a broadleaf tree (angiosperm) with needle-shaped leaves and the ginkgo is a conifer (gymnosperm) with broad flat leaves.
Finally, you may have heard of trees being categorised as evergreen or deciduous (deciduous trees shed their leaves at the end of the growing season); typically it is thought that needle trees are evergreen and broadleaf trees are deciduous. However, larch is a needle tree (gymnosperm) which loses its leaves in winter and holly is a broadleaved tree (angiosperm) which keeps its leaves all year round.