Trees and shrubs are perennial plants whose stems become woody and last for many years. Trees usually have a single erect stem or trunk which at a certain height branches out to form a crown, whereas shrubs branch from near to, or at, ground level. Trees and shrubs are also differentiated according to height. Small trees attain a height of about eight metres, those of the second category fifteen to twenty-four metres, and those of the first category more than twenty-five metres. The thickness of a tree is usually given by the diameter of the trunk measured at breast height, I.e. 1.3 metres above ground.
The trunks of conifers generally extend to the very tip of the slender, conical crown with layers of fairly thin branches usually growing out at right angles. This type of trunk and branching is characteristic of the spruce, fir, Douglas fir and most larches; in the broad-leaved trees it is to be found in the alder and the pyramidal forms of such species as the Lombardy poplar (Populus nigra). The trunks of most broad-leaved trees divide at a certain height into a few, thick, upward-or outward-growing branches which divide further to form the crown which may be broadly oval to round. This shape provides the leaves with the greatest amount of light and air, thus assuring the best conditions for the nourishment of the tree. The tree is anchored in the ground by means of roots which also serve to supply it with water and mineral nutriments.
The complex of a tree’s brandies and twigs is called the crown, and the trunk or bole and crown combined form a characteristic shape or habit which enables the expert to identify them even from a distance. Frequently, however, the habit is influenced by the environment in which the tree grows and is subject to a certain degree of variation. The most important factor is whether the tree grows in the open or under forest competition. Conifers grown in the open, fully exposed to light and wind, have a conical trunk with a centre of gravity below the mid-point and a crown that reaches almost to the ground. Broad-leaved trees grown under the same conditions have a short, thick trunk with a broad, low-placed crown. Only certain light-demanding trees such as the pine, larch, birch and aspen, have the lower half of the bole clear of branches and the crown placed high even when grown in the open. Trees grown under forest competition are taller with a long trunk and a high-placed crown taking up only one-fourth to one-third of the tree’s height. This is mainly the result of shade and associated microclimate. Trees growing on the forest margins have irregular crowns, fully developed only to one side.
The twigs of all trees are covered with buds which, especially in the case of broad-leaved trees which are without leaves in winter, are an important means of identification. Buds are actually embryonic shoots, containing immature leaves or flowers protected by scales. They are differentiated according to their position on the twig. Those appt aring at the apex are called terminal and those borne in the axils of the leaves are termed lateral. Lateral buds are either alternate or arranged in a spiral (oak, alder, hornbeam] or in opposite pairs (maple and ash). The positioning of the buds is identical with that of the leaves, in the axils of which they are borne. In other words, a tree with buds arranged in spirals has the leaves also arranged in spirals. Buds are protected against drying out and frost damage by modified leaves known as scales; either by just a single scale (willow or plane tree), two scales (alder), or several scales (beech, hornbeam and oak). Distinguishing features of the scales are colour and pubescence. Some trees that bloom in early spring can be identified by the (lower buds, which are of different shapes, e.g. willow, elm. Poplar and cherry.
A further good means of identification in some trees are the short, peg-like projections known as spurs (cherry, apple) on which the flower buds are borne. Some trees’ twigs have distinctive large or small leaf scars which remain after tin-leaves fall. The horseshoe-shaped ones of horse chestnut are noteworthy.
Spines and thorns are another good means of identification, e.g. on the twigs of the wild pear, black locust arid honey locust. The twigs of other trees have conspicuous lenticels (bird-cherry, white ash), waxy warts (birches) or corky plates (hedge maple and smooth-leaved elm).
Under normal conditions trees are the strongest and fittest members of the plant realm. In comparison with non-woody plants they have the great advantage of height and longevity. Before man introduced intensive land cultivation most of the Earth’s surface was covered with forests. Even today the forest would soon reclaim the land, should man cease to tend and cultivate it. This is borne out not only by examples in tropical Africa and southeast Asia, but also by all the lands in Europe that have been left lying fallow. Only where insufficient rainfall, high temperatures or severe winters and frozen soil prevent the growth of trees do they leave such areas to grass, shrubs and succulents.