If a garden is left untended for a long time it becomes overgrown. The flowerbeds fill up with weeds and bushes may spread onto a lawn. If the garden is left for years brambles may sprawl through the bushes and new bushes and even trees may start to grow. This change in plant life is natural and does not just occur in neglected gardens. It occurs everywhere.

The change in plant life takes place in a certain way. It is not random. For example, you do not get trees growing first then small plants taking over for a while to be followed by bushes later. If an area of ground is cleared of all plants it is colonised in an orderly way. The first plants to grow are annual plants. These plants are usually small and they germinate, grow to full size, flower and set seed in less than a year. This life style allows the annual plants to colonise an area quickly. The plants arrive on the soil as seeds that are blown on the wind. The seeds of trees and shrubs may also arrive at the same time as the annual seeds but they do not germinate and grow. The reason for this is that the conditions are suitable for annual seeds to germinate but unsuitable for tree and shrub seeds to germinate. In time when the annuals have colonised the soil they make the soil damp and shady. These are the conditions that the tree and shrub seeds need so if they land on the soil at this time they will germinate.

Shrubs and trees are perennial plants with woody stems. Perennial plants live for many years and there woody stems grow larger and larger as the years go by. In the early years of growth the woody stems are small and produce few leaves. These do not cast much shade so other plants can grow next to them. Shrubs have a very short trunk and send out shoots upwards in all directions. As the shoots grow they sprout more leaves and eventually shade the ground beneath them. Eventually so little light reaches the ground that other plants cannot survive there and they die leaving bare soil around the shrub's small trunk.

Trees have much longer trunks that shrubs. They may grow to a height of two or more metres before they send out branches. As the branches grow they produce huge numbers of leafy twigs which shade the ground below. The branches of a tree may grow out towards the branches of other trees close by. In time the branches may grow together and form a kind of roof over the ground below. This roof made by trees in a wood is known as the woodland canopy. It creates great shade in summer when all the trees are in leaf. It is too shady for many plants to survive there in summer so many woodland plants produce leaves and flowers early in the year before the leaves on the trees come out.

Once a group of trees have formed a wood the wood can remain for many centuries. Although individual trees in the wood may die and leave a space for other plants many are soon shaded out leaving only young trees and bushes to fill the gap.