Although trees appear to be unchanging components of the landscape, as living things they must ultimately die. Before this point they may be removed as a result of disease, change in land use or for safety reasons. In order to ensure that there are always mature trees to enjoy in the future it is important that the semi-mature and young trees now growing are cared for. Therefore it is necessary to produce a population where all ages of trees are represented, so that there are trees for the future and also because young trees have landscaping value of their own.
To avoid the problems associated with monoculture it is important to have a mixed population of tree types. This is so that if a disease or pest were to damage one type of tree, the losses would not be too significant as there would still be immune trees in the locality. In addition, like flowering plants and shrubs, each tree type has a season at which it is at its most glorious and a mix of trees creates year round interest.