In many ways, trees and shrubs are no different from other flowering plants – they just happen to be bigger. They all grow, produce leaves, flowers and fruits in order to reproduce, and compete with other forms of life in the struggle to survive. The following is a basic review of the biology of trees and shrubs, which will help any reader unfamiliar with the subject to gain a better understanding of these fascinating organisms.
The principal way in which a tree or shrub differs from other, herbaceous, members of its family is its ability to produce woody tissue; this serves to conduct materials around the plant, and leads to the production of permanent shoots. In the case of perennial herbaceous plants, the shoots die back at the end of each growing season, or in the case of annuals, the whole plant dies and a new generation arises from seeds formed by the previous generation.
Trees and shrubs have an important layer of cells enclosing shoots, buds and roots, called the cambium layer. This is an active layer that is constantly producing new cells on its inner and outer surfaces. Cells that grow on the inside of the cambium develop into woody tissue or xylem; this conducts water from the roots to the shoots, buds and leaves. Eventually it forms the bulk of the trunk and branches of the tree as a new layer is laid down each year. Cells that grow on the outside of the cambium form the conductive tissue, known as phloem, that carries sugars from the leaves down to the roots. This vital layer must not be damaged. If a complete ring of this tissue is cut away from the trunk of a tree the roots will be deprived of nourishment from the leaves and the tree will eventually die.
A cross section through a Pedunculate Oak trunk reveals its many layers.
When the tree begins to grow vigorously in the spring it forms large conductive cells that allow sap to flow freely through the trunk. As the season advances, cells produced by the cambium layer become smaller, with thicker walls for support, so they give a more dense appearance. In winter, cell production slows down and then ceases for a while; come the spring, however, there is a sudden burst of cell production and large cells are produced once more. The new growth of large cells immediately next to the thinner layer of dense cells gives the appearance of a ring. By examining a cut stump it is possible to count the rings and therefore discover the age of the tree, and also to find out which were the best growing seasons (see pp.).
The bark is an important part of a tree, protecting the vital growing layers of cells below from varying environmental conditions. It is produced by a layer of cambium cells and grows to accommodate the increasing girth of the tree. It may be thin and papery, smooth and shiny, or thick and deeply furrowed. Each type of bark is matched to the tree’s environment, so tree species that are subject to heat and strong sunlight in their native ranges have a thicker bark than those that come from cooler, humid climates (see also pp.).
Himalayan Birch bark is relatively thin and peels readily.
while Pyrenean Oak bark is thick and corky.
The first part of a tree to emerge from a seed is a tiny root whose first function is to draw up water and dissolved minerals from the soil. In the case of most of our tree species, successful germination is dependent upon this first root making contact with a species-specific symbiotic fungal partner, a relationship that continues for the rest of the tree’s life. This relationship is discussed in more detail on p. 34. From this simple start the tiny root will grow and divide, eventually forming a large network of powerful roots, side-branches and fine root hairs that spread out in all directions around the base of the trunk. The main roots will be woody and very strong, but their many branches terminate in fine root hairs that are only a few cells long; these have thin, permeable walls through which will pass all the water and minerals needed for the survival and growth of the tree. Although the sturdy roots strengthened with woody tissue help anchor the tree in the ground, it is the millions of fine root hairs that keep the tree alive by supplying it with water and nutrients. These fine root hairs are very short-lived, being constantly replaced as the main roots grow through the soil.
The root system of a large, mature tree does not penetrate far down into the soil. The most useful supply of dissolved nutrients for the tree lies in the shallow layer of topsoil and the adjacent sub-soil, so it is more beneficial if the roots spread outwards through this layer rather than penetrate to a great depth into a rather sterile and hostile layer. A 50mtall tree will probably have a spread of smaller branching roots all around the bole, the extent approximately equal to the spread of the branches or, sometimes, to the height of the tree. The proximity of other trees, the nature of the soil, and the presence of obstacles like rocks or river banks will all influence the final extent of the root system, however. This knowledge of the spread of the roots is useful when planning where to plant large trees that may damage drains or the foundations of buildings when they reach maturity, and it should also be borne in mind when digging ditches or ponds near large trees.
Spreading Beech roots.
In order to be able to function at all, roots require a supply of nutrients from the leaves, so within the root system there is a two-way traffic of water and minerals up from the soil to the leaves, and dissolved sugars and other nutrients down from the leaves.
The root hairs are living cells that require oxygen in order to be able to carry out respiration. They give off carbon dioxide as a waste gas, so they need access to air in the soil to allow these gases to circulate. Most trees, and most land plants, cannot grow in completely waterlogged soils and those that do have special adaptations for survival.
A number of trees, especially members of the Fabaceae, such as the Honey Locust, have many rounded nodules on the roots that contain colonies of nitrogen-fixing bacteria. These are able to use gaseous nitrogen and turn it into compounds vital to the growth of the plants.
Leaves are among the most conspicuous and distinctive features of any tree. They grow in a huge variety of shapes, sizes, colours and combinations and are usually the best feature for identifying the tree because of their unique structure. Leaves may vary from one species to another but they all perform the same vital function as the principal producers of food for the tree.
The first pair of leaves to emerge from the seed are simple, and bear no resemblance to the true leaves of the tree; they are derived from the seed’s food store. They are green, however, and supply the tiny seedling with its first food made from sunlight energy. Once the seedling has begun to produce leaves that are miniature versions of its true leaves, growth can begin very rapidly. Tiny seedlings are vulnerable to grazing, trampling, drought and competition, so very few survive.
Recently germinated oak seedling.
A plant’s leaves are its powerhouse, trapping energy from sunlight and converting it into basic food.
Leaves are basically thin layers of living tissue with the ability to trap light energy and use this to convert the raw materials of water and carbon dioxide into a simple sugar. This reaction, known as photosynthesis, is arguably the most important chemical reaction in the world, for it is the basis of all other food production. Animals do not have the ability to convert these simple materials into food; they have to rely on plants to do it for them. The simple sugar produced in the leaves is glucose, and this can be formed into a variety of other important materials, particularly starch, which many plants store, or pack into their seeds. A vital by-product of this reaction is oxygen, the gas essential for the respiration of all members of the animal kingdom. This explains the vital role of trees in the ecosystem: they are major consumers of carbon dioxide, one of the so-called ‘greenhouse gases’; and they are major producers of oxygen, the gas we need for our respiration. They are also major producers of food for much of the animal kingdom.
Contained within a leaf are numerous specialised cells. Some are concerned with the transport of materials in and out of the leaf, some are the vital energy-trapping cells that utilise sunlight, and others are concerned with the regulation of water movements. The cells that trap light-energy contain a light-absorbing pigment called chlorophyll, which gives leaves their green colour. Other pigments of different colours may be present in varying amounts, and it is this variety that gives leaves of different trees their own subtle shade of green. Without the green chlorophyll or other light-absorbing pigments, leaves would be unable to perform their important function, and also, if deprived of light, they would be unable to manufacture the tree’s food.
Leaves arrange themselves in such a way to absorb the maximum amount of sunlight, so spreading canopies or trees growing taller than their neighbours, are both ways in which trees maximise the light-gathering power of their leaves. Some leaves have paler patches that lack green chlorophyll; these are known as variegated leaves and certain trees, such as some cultivars of the Highclere hollies, regularly produce green-and-yellow leaves. If the leaves were completely lacking in chlorophyll they would be unable to manufacture food for the tree; the small areas of green tissue in the leaf produce all the food needed by the whole leaf.
All leaves have tiny pores in their surface (normally just the lower surface) called stomata. These allow water to evaporate into the atmosphere. To some extent the tree can regulate the opening and closing of these stomata, but during daylight hours, when the tree is trapping sunlight, they will be open, allowing water out and also allowing the circulation of the gases involved in photosynthesis. This can lead to problems for trees growing in hot, dry areas, or in well-drained soils where little ground water is available. In order to allow the essential gases to circulate, and at the same time minimising water loss, many leaves have become reduced in size, such as the needles of firs and pines, or have thick waxy upper surfaces such as the glossy green leaves of hollies and magnolias. This reduces water loss to a minimum without impeding photosynthesis.
The great variety of leaf shapes and sizes is an indication of the variety of ways in which trees can cope with environmental conditions. Some trees grow in areas where water is at a premium, so they have small leaves, to cut down on water loss through their thin skins. Some grow in shady conditions, so they may have larger leaves that can trap the maximum amount of light energy. Some trees are subject to grazing by animals, so their leaves are spiny or prickly, or protected on tough, thorny stems.
Beech
OVAL AND ENTIRE
Hornbeam
ELLIPTICAL AND TOOTHED
Small-leaved Lime
CORDATE
Red Oak
LOBED
Horse-chestnut
PALMATE
White Ash
COMPOUND
Yew
NEEDLE-LIKE
Scots Pine
WITH PAIRED NEEDLES
Leaf types.
Deciduous trees, such as Horse-chestnut, produce fresh leaves each spring, which burst forth from buds.
Autumn leaf colour is spectacular in many maple species: as chlorophyll and other pigments are withdrawn, remaining red pigments prevail.
Evergreen trees do not lose all their leaves at the end of every growing season; most leaves remain on the tree through the winter, although there is always some loss and some replacement. In many of the pines, for example, the needles will remain on the tree for about 3 years. As the shoot grows longer each year, a new set of needles grows on the tip of the lengthening shoot. The older needles, finding themselves further and further away from the tip, gradually fall off. Small leaf scars remain, and these are quite distinctive in some species and may be a useful aid to identification. Broadleaved trees such as Holly also replace their leaves gradually so there is always some leaf-fall, but plenty of green foliage remains on the tree.
Deciduous trees generally shed all their leaves at the end of the growing season, before the onset of winter. Many of them produce spectacular displays of colour before the leaves finally fall. These colour changes are the result of the gradual withdrawal back into the tree of all the useful materials in the leaf; as the various pigments are removed the leaf itself changes colour until finally a corky layer, called the abscission layer, grows at the base of the petiole or leaf stalk. This seals off the shoot and when the leaf finally falls, a scar is left through which mould spores and other harmful materials are unable to pass. The twigs of Horse-chestnut have very distinctive leaf scars that look like tiny horseshoes. If these are examined carefully through a hand-lens, the sealed-off ends of the vessels that conducted materials in and out of the leaves can clearly be seen.
There may be as many as 250,000 leaves on a mature oak tree, whilst a large spruce probably has 10 times as many, in the form of needles. The oak’s leaves will be shed at the end of the growing season, adding to the rich accumulation on the ground beneath it, whilst the spruce’s needles will be shed and replaced gradually, each individual needle remaining on the tree for about 4 years.
Trees normally produce flowers when they are several years old. Beech, for example, produces its first flowers at around 30 years old, repeating the process each spring for the next 200 years if it remains healthy. Some trees, such as apples or oaks, have years in which they produce a large crop of fruits or seeds, followed by other years with hardly any, whilst other species, such as some maples, produce a good seed crop year after year.
Even a slight breeze will liberate pollen from the male catkins of Hazel
Carry it to female flowers.
Some trees and shrubs produce conspicuous flowers to attract pollinating insects, something that, in ornamental trees, we also find attractive. Honey Bees are particularly important pollinators, but numerous other insects visit the flowers for nectar and pollen. Many flowering trees have also long been prized by gardeners for their scent.
Many trees are pollinated by the wind. Their flowers are less conspicuous, often taking the form of catkins, which are pendulous and usually open before the leaves so that nothing impedes the free movement of the pollen grains. Wind-pollinated flowers normally have flowers of separate sexes, the males usually being larger and more abundant. Many wind-pollinated trees are such prolific producers of pollen that on warm breezy days in spring clouds of pollen can sometimes be seen blowing from the trees.
Conifer flowers are either male or female, and borne on the same or different trees. There are no petals, but some of the flowers are still quite colourful and decorative. Male flowers are short-lived, falling off after they have released clouds of pollen, but the female flowers, often covered with brightly coloured scales, remain on the tree after pollination and develop into cones containing the seeds. They rely on the wind for pollination and also for seed dispersal. A few close relatives of the conifers, such as the yews, produce fleshy fruits instead of cones.
Mature cones open and close in response to temperature and humidity, releasing seeds in hot, dry conditions.
The flowers of broadleaved trees and shrubs are usually hermaphrodite, containing both male and female parts, but there are a number of exceptions. Both sexes usually have petals in some form or other and they may also be scented. Small flowers are often grouped together in larger clusters to help attract pollinating insects. Some are wind-pollinated and open early in the year before the leaves, but insect-pollinated flowers usually open in spring and summer when more insects are active.
The fruits of trees and shrubs are much more varied than the cones of the conifers. They range from tiny papery seeds with wings, through nuts and berries, to large succulent fruits in a variety of shapes and colours. Edible fruits are designed to assist dispersal of the seeds by animals and many are delicious to the human palate.
Hawthorn flowers are an attractive sight in spring and are irresistible to pollinating insects.
HAZEL – NUTS
CRAB APPLE – FLESHY FRUIT
SYCAMORE – WINGED SEEDS
GUELDER-ROSE – BERRIES
The fruits of broadleaved trees and shrubs come in a range of shapes and size, including winged seeds, hard-cased nuts, luscious berries and juicy fruits.
