Classification and Identification
Different types of trees actually have very little in common.
Many other plants can be easily identified because they can be categorised into ‘families’ of plants which share certain traits. For example, many native flower species in the rose family have five petals. However, trees are an example of what we call ‘convergent evolution’. This is when one or more unrelated living things adopt the same biological traits in order to solve similar problems.
Trees evolved from various unrelated plant families which all developed a single woody stem in order to to grow taller and therefore get more light. This means that trees can be found across many different plant families. There are a number of trees in the rose family alone – such as the wild service tree, white beam and rowan.
One way to classify trees is to divide them into biological classes. This gives us two main groups; ‘Gymnosperms’ and ‘Angiosperms’.
- Gymnosperm means ‘naked seed’. These were the earliest trees to evolve. They do not produce flowers. Instead, their seeds are directly exposed to the air, so that they can be carried away by the wind, a process called ‘wind pollination’. Pine cones are one example of a ‘naked seed’. You can think of Gymnosperms as conifers. Some examples of Gymnosperms you might be familiar with include conifers such as the yew and the Scots pine.
- Angiosperm means ‘hidden seed’. These trees typically have flowers, in which seeds are hidden. The part of the flower where the seeds are produced is called the ‘fruit’ of the tree, but this is not necessarily ‘fruit’ like those we eat. These trees evolved alongside insects, birds and mammals and usually use them for pollination. You can think of Angiosperms as hardwood trees, such as oak and beech.
When categorising trees, there are often exceptions to the rule, even in the strict biological classifications described above.
For example, even though wind pollination is a feature of gymnosperms, some angiosperms also use wind pollination. These include, ash, birch and elm.
Another way to categorise trees would be by leaf type, giving us two groups: needle trees and broadleaf trees. However, these categories also cause problems. For example, the gorse is a broadleaf tree (angiosperm) with needle shaped leaves, and the gingko 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, evergreens do not. Usually, needle trees are evergreen and broadleaf trees are deciduous. However, the larch is a needle tree (gymnosperm) which loses its leaves in winter (deciduous) and holly is a broadleaved tree (angiosperm) which keeps its leaves all year round (evergreen).
Size and shape: trees come in all shapes and sizes, from small and squat to tall and thin.
Leaves: Size, shape and colour and important when it comes to identification.
Flowers, fruit or cones: these can also be useful but may only be found at certain times of the year.
Trunks: tree trunk or bark can become familiar to you with practice; they differ in colour, pattern and texture.
Winter twigs and buds: identifying deciduous trees in the winter can be tricky, but twigs and buds are usually quite distinctive. Their size, shape, colour and layout along the twig are all good indicators.
It is helpful to work out whether a tree is broad-leaved or coniferous, as this can allow you to identify the species:
- Most broad-leaved trees are deciduous and shed their leaves in winter.
- The leaves of conifers are either needle-like or small and scale-like. They are normally present all year round.
As always, there are exceptions to the rule; for example, holly is an evergreen broadleaf, and larch is a deciduous conifer.
You can use the same characteristics as those listed above to identify conifers. However, instead of examining leaves, you compare the number and structure of the needles, their colour, and scent.
Trees of the same species can look quite different. This can be due to the immediate environment that surrounds them. For example, an oak that has grown on its own in an open field may look different from one that has grown between other trees in a wood.
Leaves can vary even on the same tree, according to how much light is available to them. This means a tree typically produces ‘sun leaves’ and ‘shade leaves’. Sun leaves tend to be near the top of the tree and have the most exposure to light. They are smaller and because they have less chlorophyll, they are paler in colour and able to tolerate bright light without wilting. Shade leaves are situated in parts of the tree that receive less light, such as the lower branches. Shade leaves are larger and contain more chlorophyll, making them darker in colour. This helps them to absorb what little light is available to them.