DECIDUOUS TREES – trees that lose their leaves

D eciduous trees shed their leaves at the approach of winter, or in some countries that have hot rainless seasons at the advent of the annual drought. They are also called broadleaved trees because nearly all of them bear leaves with broad thin blades that contrast with the narrow needles of the conifers. Another common name is hardwoods, because the wood of most, though not all, is distinctly harder than that of coniferous trees, which are therefore called softwoods. Yet another name demands explanation, and that is evergreen. If a tree, whether broadleaved or coniferous, never sheds all its leaves at once, it is called an evergreen, even though individual leaves eventually drop off after a life-span of two, three or more years.

Seasonal leaf pattern

The distinctive feature of typical deciduous trees is their annual rhythm of life and growth. They stand leafless through the winter. When the warmth of spring stirs them, their remarkable winter resting buds burst. Scales fall away and release delicate shoots that bear green leaves of a shape peculiar to each kind. These leaves function actively all through summer, carrying out photosynthesis in just the same way as the leaves of smaller plants. When autumn arrives, the green colouring matter called chlorophyll, that is essential to this process, breaks down. The leaves change colour to yellow, orange or scarlet, due to pigments called carotins, or else t%< to purple hues caused by other pigments known as xanthocyanins. At the same time the food reserves of the leaves are withdrawn into the tree’s woody stem. After a brief display ot brilliant colour, each leaf is cut oft’ from the twig that bears it by a thin layer of cork, which forms across the base of its stalk. It then falls, usually on a windy day, or after a sharp night frost has caused ice to form at the point ot attachment; the morning thaw. snaps the link. Temperature is the controlling factor that causes these leaves to be shed, but it works’ in a curious way. The leaves can only function whilst they have an ample supply of sap sent up by the roots, but the roots in turn can only draw water from the earth when the soil temperature is over 4°C (39°F). If an ordinary broad-leaved tree retained its leaves in the winter, it would quickly lose more water than it could replace, and,it w.ould.die pfdro.ught.;Evergrc€n broadleavcd trees have special devices, including waxy surfaces and small breathing pores, to limit transpiration and these enable them to survive. But they rarely grow so fast as the true deciduous or ‘summcrgreen’ trees, because the thin blades of the latter are very efficient mechanisms for using the sun’s energy for growth. So efficient indeed that these trees can afford to grow a fresh set of leaves each spring and discard them each autumn. In the forest, however, the spent leaves enrich the leaf mould at ground level. The nutrients they hold, chiefly mineral salts, are picked up by the tree’s roots and recycled into the tree’s crown, so they are not lost for ever.

The woody stem

The leaves of deciduous trees are supported on the woody twigs and branches of a sturdy trunk, which increases in thickness as the tree grows older. All these woody stems, large and small, have an unseen but vital pattern of seasonal growth that keeps in step with the leaf life rhythm. In spring, when the fresh leaves are opening and demanding large supplies of sap, a ring—more strictly, a cylinder—of cells, called the cambium, forms a fresh band of open-textured cells and larger conductive elements, known as vessels, to transport it. This is the springwood. In the summer, when more support is needed for the enlarged crown of foliage, it adds closer-textured material, with thicker cell walls and less pore space, called the summer-wood. The two bands of tissue, taken together, form one annual ring, and the last-formed ring is always farthest out. In this way, each trunk and branch is built up of successive layers of wood, each encircling those created earlier.

So long as the wood is actively carrying root-sap upwards, it is called sapwood. As stems become older and larger, a central core of wood is gradually transformed into heartwood, which ceases to carry sap and serves only for support. Heartwood is usually darker than sapwood, and owing to chemical changes it becomes, in some species only, more durable. Oak, for example, has very durable heartwood, but that of beech or ash is ‘perishable’ — it will not last if used in damp surroundings. Rays are another essential feature of woody stems. These are narrow bands of tissue that radiate out from the centre of each stem, at right angles to the annual rings. They transport nutrients sideways rather than up or down, and so store them through the winter, when photosynthesis is suspended by cold and leaf-fill.

Outside all this hard wood lies another circle, or cylinder, of softer tissue called bast. This thin, but vital layer carries sugar sap down from the leaves, where it is manufactured, to the lower stem and roots, which need it tor life and growth. The true bark, which lies outside both bast and wood, protects everything within it from drying out, from casual injury’and from sudden changes of temperature due to sharp frosts or blazing sunshine.

Woody stems, like the shoots of other plants, increase in length through the work of growth points, meristcm’s,-4if TheifTipsV Afteivtheir first season the tissues of the soft young, shoot’s veins link up to form a continuous ring, or more strictly a narrow cylinder, of cambium cells, so creating a growth layer. By creating new wood on its inner side, and more bast cells on its outer side, the cambium causes the stem to grow stouter but only the meristem.at.its tip .can. make it .growIorigerV The highest Emd at the tip of a tree usually grows more vigorously than the others, causing the main trunk to grow taller than side branches.

The wood of deciduous trees is a more complex substance than that of coniferous trees. It holds pores or vessels, composed of clusters of cells, that permit rapid upward transport of sap; they are never found in conifers. There are marked differences between timbers ot various kinds of deciduous trees, which man makes use of. Ash, for example, gives tough springy tool handles; but elm is preferred for chair scats, because it will not split when the legs are driven in.

Life pattern

The deciduous tree that bears these leaves and builds up this woody stem starts life as a little seedling, which opens seed-leaves, always two in number, when it sprouts. Juvenile leaves, often simpler in outline than the familiar adult leaves, next appear on a slender upright shoot. After its first summer, the seedling sheds all these little leaves and forms resting buds, one at its tip and others at the sides. Next spring, it expands more shoots, bears more leaves and thickens its stem. This chain of processes, repeated yearly, forms first a sapling, then a large adult tree. Few deciduous trees exceed 36m (120ft) in height or s-im (18ft) in circumference, which is measured 1.2m (4ft) above their base. Exceptional specimens, such as a lime tree at Duncombe Park in Yorkshire, approach 45m (150ft) in height. A few old oaks exceed 12m (40ft) in circumference. The average lifespan of a deciduous tree is around 150 years, but counts of annual rings show that some old oaks may stand for 500 years or more. ft a tree escapes being felled for timber or blown down by a gale, it eventually falls victim to decay caused by fungus. Fungal spores, spread by the wind to some surface no longer protected by bark—such as the stub of a broken branch—germinate and give rise to hyphae or fungal threads that spread through the tree’s inner timber, and feed on it destructively. Some fungi kill trees by blocking the phloem vessels, as in Dutch elm disease. After some years have elapsed, the trunk collapses. Before this happens the fungus has usually borne sporophores— shaped like mushrooms or brackets, that spread further spores on the wind, to perpetuate its race. The appearance of these sporophores is often the first sign of hidden decay.

Reproduction

Most deciduous trees do not flower until they are 20 years old or so. By then they have grown fairly tall and have expanded their crowns of branches and leaves. Some, like birch, then flower annually. Others, such as oak and beech, flower in some years but not in others. Flowering, when it occurs, is always abundant. A birch may bear 100,000 seeds each year for 50 years, or 5 million seeds in all, though only one seed is needed for its eventual replacement!

The flowers of deciduous trees fall into two distinct groups. Some, like cherry and horse chestnut, bear typical ‘perfect’ or hermaphrodite flowers. These have rings of green sepals to protect them in bud, and bright petals, either white or gaily coloured, to attract pollinating insects. Within these rings stand stamens with golden anthers that dust the visiting insect with pollen, and a pistil which receives pollen carried from another flower. Nectaries at the base of the blossom secrete nectar to reward the pollinating bee, or similar insect. Perfumes attract it.

In contrast, many other trees bear wind-pollinated flowers that lack all such attractions— they have no petals, scent or nectar. As a rule the male stamens are borne in a separate structure from the female pistils. Sometimes, as with poplars and willows, males and females are found on separate trees. Instead of sepals, we find small leafy blades called bracts that protect the sexual elements in bud. This kind of inflorescence, or flower-cluster, is called a catkin, though most are very different from the ‘pussy-willow’ catkins that originated this name, which means ‘kitten’. Male catkins often hang down in long ‘lamb’s tail’ bunches yellow with golden pollen. Female catkins, usually very different in appearance, may resemble buds, as in hazel, or cones, as with alder. Most catkins open before the leaves do. This makes it easier for the pollen to spread on the wind from male to female flowers.

Deciduous tree seeds, whether they develop from perfect or catkintype flowers, show a surprising variety of size and form. Some are fleshy nuts, tiny in hornbeam but large in beech, or as the acorns of oaks. These depend for dispersal on the birds and squirrels that cat a large proportion, but drop and forget a tew. Others are hard stones within fleshy fruits that attract birds. The cherry is a good example. Others again have wings to aid dispersal by wind, as in the maples and the birches, or tufts of hairs that act as parachutes, as with the willows.

The larger seeds produce big seedlings with strong primary roots that can penetrate turf or gain a roothold amid weeds. Oak, for example, can colonize grassland. The little seedlings that arise from smaller tree seeds are only able to thrive on ground that is bare or nearly so. Elsewhere they are soon smothered by faster-growing herbs or grasses. This explains why forests, once cleared, return slowly unless replanted by man. But a natural established forest, left to itself, can renew its kind indefinitely. Whenever an old tree dies and falls, leaving a gap, natural seedlings soon spring up, under the varying conditions of light and shade. They compete for growing space until one takes the lead and replaces the fallen adult tree.

Forests and deciduous trees

To sustain their vigorous seasonal growth, deciduous trees require both fertile soil and ample spring and summer rainfall, combined with adequate warmth and sunshine. These conditions are met in the northern belt of broad-leaved forests that extends across North America, Europe and much of Asia. It lies between the cold arctic tundra and the hot deserts. Because of its fertile soils, this deciduous forest zone has long attracted farmers and settlers who have made clearings in it, to grow their crops or to pasture their sheep or cattle. It has, in consequence, been broken up into fragments by farm land. In the north, and on the upper slopes of high mountains, deciduous forest gives way to coniferous forests of pines or spruces. To the south, it is replaced by evergreen broadleavcd forest, and eventually by arid deserts. Oakwoods

Typical timber trees of the temperate zone deciduous forests are the oaks, which make up the genus Quercus. The leading species in Europe is pedunculate oak, Q. robur, which bears its acorns on long stalks called peduncles. North America has over 40 species of oak trees. The best-known deciduous one in the east is the white oak, Q. alba. All oaks can be recognized in winter by the way their buds are grouped in clusters at the tips of their twigs. This arrangement leads to a characteristic pattern of twigs and branches that spread in all directions and so build up a huge spreading crown. This is supported on a stout trunk clad in rugged thick grey bark. Oak leaves have a distinctive lobed outline. Male flowers, which open when the leaves do, in late spring, are catkins composed of groups of greenish-yellow flowers on long stalks. The much smaller budlike female flowers are wind-pollinated. They ripen, by autumn, as the well-known acorn crop. Each acorn is a single seed or nut, sitting in a neat round woody cup.

Nearly all the acorns that fall, in a good seed year, from a large oak tree are eaten by birds, including pigeons, pheasants, jays and woodpeckers, or by mammals such as squirrels, mice, pigs or deer. Those few that escape destruction sprout next spring as strong deep-rooted seedlings, and oak is well-fitted to colonize forest clearings or abandoned farmland. Many others are actually distributed by the animals that store them for future use, like squirrels and jays, and then forget them.

Every oak trunk or stem of reasonable size-holds a naturally durable core of heartwood that is exceptionally hard and strong. Yet it can be readily worked to shape when the right tools are used. From the earliest times man has felled oaks to get timber for building houses, ships and fences, knowing that it would last for scores, or even hundreds, of years, without breakage or decay. Oak forms the uprights and beams of all substantial timber-framed houses, and the roof timbers of cathedrals and barns. It provides the framework of keel, posts and ribs for all large wooden ships built in Europe or America, from the warships of Nelson’s day to fishing vessels constructed today. Oak is also a first-rate timber for making attractive strong, if somewhat heavy, furniture; planks are specially sawn to display its beautiful grain.

Because oaks come into leaf late in spring, and cast only a moderate shade, many kinds of smaller plants can flourish in their midst. Oak-woods are the home of bluebells, yellow primroses, pink campions and white windflowers or wood anemones. All these expand their leaves and open their flowers early in the year, when ample sunlight reaches the forest floor, making their photosynthesis effective. Oaks support a remarkably large number of insects that feed on leaves, buds, flowers and acorns. These seldom check growth severely, and rarely kill the tree. These insects in turn provide food for many attractive insect-eating birds, such as warblers, robins, blackbirds, thrushes, nuthatches, tree-creepers and jays. Fungi of many kinds grow on decaying oak timber, and insects that attack this substance form the prey of woodpeckers. In short, oakwoods provide habitats tor a remarkable range of animals and plants. They make excellent nature reserves, whilst at the same time yielding valuable timber. Beechwoods

Where the soil becomes poorer, or the climate cooler, oakwoods give way to forests of beech trees. This happens both in Europe where the native beech is Fagus sylvatica, and in America where there is a similar beech with somewhat larger leaves, F. grandifolia. In Britain, beech-woods are found mainly on thin soils over limestone rocks, on the Cotswold and Chiltern Hills, and the North and South Downs. On the European continent they form the highest broadleaved forests along mountain ranges like the Alps and the Pyrenees.

Beech trees are easily recognized by their smooth grey bark, their slender pointed pale brown winter buds, set singly along the twigs, and their simple oval leaves. Male flowers are grouped in yellow tassels at the tips of short stalks, which arise in little bunches. The green bud-shaped female flowers ripen to fruits that consist of hard green husks, clad in soft blunt spines, and holding shiny brown triangular seeds. Beech seedlings, which begin life with two broad, fleshy seed-leaves, thrive best on bare soil free from weeds. They can grow under considerable shade and are able to colonize oakwoods. As beech trees grow taller, they cast deeper and deeper shade until at length, photosynthesis becomes impossible for any green plant that strives to grow beneath them. The floor of a beechwood therefore becomes a lifeless place, holding no plants and attracting few animals. The ground is strewn with brown faded beech leaves, slowly rotting down to become fertile leaf mould.

Beech timber is pale brown in colour, shot through with little red-brown flecks or plates that represent the rays of the wood. Though it lacks the great strength and durability of oak, it is valued for its hardness, smoothness and the fact that it can be easily worked in any direction. It makes good everyday furniture, particularly chairs and school desks, and is used for mallet heads, plane blocks, short tool handles and other objects needing a handy reliable smooth hard piece of wood. It is a first-rate firewood. Birchwoods

Everyone recognizes birch by its white bark and slender drooping purplish-brown twigs. Its little leaves are triangular to diamond-shaped, with toothed edges, and hang from thin stalks. Pale green in spring, darker later, they turn to bright gold before they fall in autumn. Male birch catkins, shaped like hanging lamb’s tails, become golden in spring, when they scatter yellow pollen. At this time the female catkins resemble little green caterpillars, but by September they have ripened to drooping cylinders, which suddenly break up and release hundreds of tiny winged wind-borne seeds. Seedlings are often found springing up on bare soil; they thrive best on poor sands, where the weeds that might otherwise choke them grow very slowly. Silver birch, Betula pendula, is the commonest European kind; the American paper birch is B. papyri/era.

Birch trees always demand full sunlight. They cannot tolerate the shade cast by other trees, or even that of other birches. Every birchwood is therefore a light and airy place, with green plants carpeting the ground below. Typical birchwood plants are shrubby heather, heaths, bilberries, mat grass and wavy hair grass. There is usually a sprinkling of delicate flowering plants like yellow cinquefoil and golden cow-wheat. Birch forests spread right round the Northern Hemisphere, across Canada, Scandinavia, Russia and Siberia, on the cold poor soils just south of the frozen tundra. They also grow high on mountain ranges like the Alps and the Scottish Highlands. In these regions birch has always been an important firewood and a handy source of timber for small articles like tool handles, broom heads, bobbins and simple furniture. It is also used for veneers in plywood and as pulpwood for paper-making. Other deciduous trees

Several other trees are found growing, as a rule, in association with oak, beech or birch. Where the ground is particularly favourable, they occasionally form small forests of their own kind. Maples, like the American sugar maple, Acer saccharum, are recognized by their lobed leaves, always set in pairs, and their large seeds, also paired, with curved papery wings; their hard wood is used for flooring, furniture and paper pulp. Ash trees, Fraxinus species, are easily identified by their large compound leaves, each composed of many leaflets ranked on either side of a long stalk. They have winged seeds that hang down in bunches. Tough ash timber is used for the handles of big tools like spades and axes. Walnuts, Juglans species, have similar compound leaves, but are easily known by their fragrance when crushed. They ripen the familiar nut as their seed. Their strong wood, with its beautiful patterns of brown, grey and black, is used for gun stocks, decorative carving and high-grade furniture.

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