CONIFEROUS TREES – trees with cones and needles

Conifers are a peculiar, but geographically and economically important, group of trees that ripen their seeds on the scales of woody cones. Two other common terms, coniferous and cone-bearing, mean just the same thing.

Conifers are also called gymnosperms, from the Greek meaning ‘naked-seeded’. This applies to the position of the seed, and the ovule from which it develops, on the surface of the cone scale. It is not enclosed within an ovary, as in broadleaved trees and flowering plants, which are called angiosperms, or ‘hidden-seeded’. As the fertilized conifer seed develops, it becomes tightly enclosed between the cone scales, which protect it from birds, mammals and insects. Finally, when the cones ripen and the scales open, it is exposed again and eventually falls out.

Conifers are also called softwoods, because the timber of most species is distinctly softer than that of most deciduous trees, or hardwoods. It is therefore easier to work, and under modern industrial conditions this makes it more valuable for all the everyday uses to which timber can be put, such as house-building, packaging, fencing and the manufacture of chipboard and plywood. Softwoods have long fibres, which makes them highly suitable for making paper —basically an interlocked sheet of wood fibres. Naturally some kinds of conifers are better for each job than others. But softwoods are adaptable materials, and in practice the industrialist adjusts his process to match his source of supply.

The leaves of nearly all conifers are narrow and needle-shaped, and therefore the group as a whole are often called needle-leaved trees. This leaf structure is better than that of broadleaved trees for restricting the loss of water into the air through transpiration. Therefore conifers can thrive in drier places, like the subtropics of Spain, South Africa or California, than can broadleaved trees. Because cold soils reduce the water supplies available for plant life, conifers also thrive farther north, and higher up mountains, than do deciduous trees. They also need smaller supplies of mineral nutrients, so grow faster on poorer soils than deciduous trees can do. Foresters, the world over, naturally tend to make new plantations with profitable conifers, rather than with the deciduous trees that demand better soils for slower timber outputs.

Conifers are often known as evergreens. Nearly all conifers have evergreen leaves, which live for three or four years before falling. Therefore their branches are never leafless. There are two common exceptions—the larches, Larix species, which grow high on the mountains, and the swamp cypress, Taxodium distichum, which lives under difficult soil-water conditions in the swamps of the southern USA. The evergreen habit is efficient, because it means that photosynthesis can be carried on, whenever the temperature is high enough, all the winter through.

Another peculiarity of the conifers is the presence of resin—they are sometimes called resin-bearing trees. Resin is a transparent sticky fluid, composed of a solid wax, called rosin, dissolved in a lighter spirit, known as turpentine. It is formed in channels or small chambers in wood, bark, needle-shaped leaves, cones and even seeds, and is easily detected by its sharp distinctive smell. If the tree’s tissues are injured, resin oozes over the wound. The vola- r tile turpentine evaporates and a protective coating of hard rosin remains. This shields the tree—not always successfully—from attacks by fungi, insects, mammals and birds, as well as man-made injuries.

Resin is harvested commercially in southwest France and the southern USA, where it is called ‘naval stores’, by tapping living pine trees. The turpentine is distilled off for use as a solvent for paints and chemicals. Most of the rosin goes into printing inks; it gives them quick-drying properties essential for high-speed machine printing.

Simple woody stems

The wood of conifers is simpler than that of deciduous trees. It has no pores or vessels— those special structures for rapid transport of sap in spring that the deciduous tree needs when its broad-bladed leaves expand. The conifer’s need for more sap in spring is met more simply by developing broad bands of springwood, composed of long thin-walled fibrous cells with large cavities available for sap-flow. In summer the tree’s need for greater support for its enlarged crown is met by a change to surri-merwood, composed of similar cells with thicker walls and smaller cavities. Many conifers, especially pines, Pinus species, and Douglas firs, Pseudotsuga, develop very clear annual rings in this way. In others, such as spruces, Picea, the summerwood is less obvious.

The thickening of conifer stems is due, like that of the woody stems of deciduous trees, to the activity of a layer of cambium cells on the outer surface of the true wood. This cambium layer lies between the sapwood, which carries the main sap stream upwards from the roots, and the bast, which carries the downward flow of nutritious sugar-sap from the leaves to the lower stem and underground root tissues.

Conifer bark, which is formed as a protective layer, outside the bast, by its own cork cambium, shows marked variation according to kind of tree. Some species, like the western red cedar, Thuja plicata, have thin stringy bark. Others, like the coast redwood of California, Sequoia sempervirens, bear extremely thick fibrous bark, which can even protect the living stem against the heat of a forest fire.

As conifer stems grow larger, their inner zone of wood ceases to carry sap, as sapwood. It is then converted to heartwood, which serves only to support the tree’s crown. This change is effected by the deposition of resin and related chemicals. With some kinds, there is an obvious change in colour and durability. Larches, Larix species, develop red-brown heartwood that resists decay much longer than does the pale-brown sapwood. But in spruces, Picea species, the hcartwood is scarcely darker, and certainly no more durable, than the sapwood.

Conifer stems also hold ray tissues, which radiate from the centre, or from points near it. They carry nutrients sideways, and store food reserves through the winter, but they are never so thick or obvious as those of some deciduous trees.

Life pattern

Conifers start life as seeds, small brown oval grains that fall from the cones of their parent trees, attached to thin triangular or oval wings. These wings aid their spread, enabling the wind to carry them a fair distance from the parent tree before they strike the ground. With the warmth of spring, the seed takes in moisture from the damp earth, swells and grows a little root. Once this root has become anchored and can draw in water through its root hairs, it raises the rest of the seed upwards. It bends its stalk until it stands erect. The seed coat or husk then splits and falls away, and the seed-leaves within it expand. Many conifers, unlike other plants, have numerous seed-leaves; common numbers are two for western red cedar, Thuja, three for hemlock, Tauga, but twenty or so for pines, Pinus species. These seed-leaves quickly turn green and begin the photosynthesis that nourishes the seedling tree, once the food reserves in the seed have run out.

A leading shoot soon emerges from the centre of the whorl of seed-leaves and bears green needles. These early juvenile needles are often different, in arrangement or character, from the adult needles that the tree will bear later. Pines, for example, bear grasslike solitary juvenile needles, but their adult needles are always grouped in twos, threes or fives. After its first summer the seedling develops a main resting bud at its tip, with smaller side resting buds around it, and growth ceases for the winter.

Next spring these buds burst. Their scales fall away, and the shoots within them make remarkably rapid growth for a spell of a few weeks. The terminal shoot goes upwards, to become a leader. Side shoots go outwards, to become the tree’s first branches. As they extend, the needles that were already hidden in the bud expand all around them. After this rapid expansion a pause ensues. By midsummer growth has usually ceased and the winter buds, that will expand in the following spring, are already being formed.

Sudden spurts of growth on a geometrical pattern, are characteristic of conifers. The age of most young, and indeed middleaged, trees can be ascertained by counting the numbers of annual growth whorls, or clusters of branches, up their stems. The length of the unbranched shoot, or internode, between successive nodes of branches, shows how far the tree grew taller each year; older trees, however, lose this regular pattern.

Conifers, overall, do not grow much larger or faster, than deciduous trees, nor do they live much longer. But the wood they produce is concentrated in a convenient form in straight lightly-branched main trunks, rather than the heavy branching crowns of the deciduous trees. Further, the conifers can be grown closer together. This makes them more profitable to the forester and more attractive to the timber merchant.

Record-breaking conifers exceed the largest and oldest deciduous trees. Coast redwoods, Sequoia sempervirens, the world’s tallest trees

Left: New needles of Scots pine, Pinus sylvestris, breaking through the brown bud scales which have been protecting them as they develop. today, soar to 117m (384ft) in California. Douglas firs felled at the end of the last century almost certainly exceeded 122m (400ft). Their allies, the giant sequoias, Sequoiadendron giganteum, also Californian, can reach circumferences up to 30m (98ft) measured 1.2m (4ft) above ground level. They show the greatest timber volumes of any known tree. Bristlecone pines, Pinus aristata, in the Arizona desert, may live for 4,000 years, remaining stunted trees in a harsh environment.

Reproduction

Conifers that grow in the open and receive full sunlight, often start to flower when only 15 years old. Those crowded in plantations may postpone flowering to an age of 30 years or more. Once flowering starts, it continues annually, with occasional years of poor seed production. During its lifetime a big spruce or pine may bear a million seeds, in a lavish provision to ensure the survival of its kind.

All conifer flowers are borne in separate male and female clusters. Each individual tree is capable of bearing flowers of both sexes, though it does not always do so in the same year. Pollination is always effected by wind, possibly because conifers evolved when there were no insects to carry pollen. Consequently their flowers have no green sepals, no brightly coloured petals, no nectar and no attractive scent. A few—notably the female blossoms of larches and the Korean silver fir, Abies koreana—are very pretty, but this is just an accident! Nearly all conifers flower in spring. The true cedars of the Cedrus genus are an exception; they flower in autumn.

Male conifer flowers open in clusters, oval or cylindrical in shape like the male catkins of deciduous trees. Each flower consists ot small bracts and yellow anthers, which shed abundant golden pollen. For a week or two the flower cluster looks like a golden cone or caterpillar; then it fades. Most female conifer flowers are bud-shaped, green and inconspicuous; they open at or near the tips of branches. Pines bear round red flowers, each smaller than a pea, at the tips of new shoots. Larches have larger ones, red or white, shaped like rosettes. Spruces and silver firs bear small upright green conelets. Each female flower consists of scales, set spirally on a central stalk. On the inner face of each scale lie the ovules which, after fertilization by male pollen, become the seeds. In most conifers, each scale bears two ovules, but cypresses, Chamaecyparis, have four or five, and monkey puzzle, Araucaria, only one.

After fertilization, the female flowers enlarge and develop through a soft green stage into hard woody cones. With most kinds, this process takes about six months. Pines need 18 months; after 6 months one female flower has only grown into a brown pea-sized structure, which needs another year to mature. Cedars, Cedrus, take two or three years. Cones vary markedly in size and shape, and provide a handy means of identifying each kind of tree. In a typical cone, each scale is attached to the central stalk by a hinged joint. When the weather gets drier, one side of this joint contracts, the angle of the scale alters and the cone opens. When the air gets moister, the scales close again, and this explains why ripe cones can be used, in a rough and ready way, to forecast the weather.

Ripe seeds fall gradually, day by day, and usually during dry spring weather, from the open cones. Each has a papery wing, which delays its fall as it travels on the wind to some point away from its parent tree. There, as soon as rain falls, it takes in moisture and sprouts, and the whole life cycle begins anew.

Conifer seeds have many enemies. They provide tasty nutritious food for woodmice and squirrels. In America, chipmunks regularly collect cones and store them in hoards called caches, as a food reserve for the winter. With his needle-sharp teeth a squirrel can easily bite through a cone scale to reach the tasty seed below. Birds gobble up fallen seeds. One kind of finch called the crossbill, Loxia curvirostra, has its two mandibles crossed; this enables it to prize back cone scales to reach the seed below. Only a small percentage of seeds survive to germinate, and the seedlings only thrive if they start growth on bare, or nearly bare, soil. They grow very slowly for their first few years and are easily choked by other plants. Some young conifers can thrive under partial shade, but others, including pines and larches, need full sunlight for successful growth.

Coniferous forests

The world distribution of coniferous forests comprises a broad band that reaches right round the Northern Hemisphere, across Canada and the northern United States, then across Europe from Scotland to Russia, and finally across Siberia to the Pacific Ocean. Farther south there are great conifer forests spreading along mountain chains, including the Rocky Mountains and Appalachians in North America; the Alps, Pyrenees and Apennines in Europe; the Caucasus; the Himalayas in Asia; and the higher hills of Japan. A few conifers grow on tropical mountains, and a very few, like the monkey puzzle of the Andes, in the Southern Hemisphere. The general pattern is that the conifers grow in colder places than deciduous trees, though some, like the maritime pine, Pinus pinaster, of the Mediterranean region, thrive under climates that are too hot and dry for deciduous trees to tolerate. In general, the conifers are able to flourish on the poorer soils, too infertile to attract clearance for agriculture, and also on steep hill slopes unsuited to cultivation.

These coniferous forests form an enormous, and fortunately renewable, reserve of raw material for industry. Wherever the woods can be reached by modern transport, the loggers go in and harvest the timber. The larger logs are sawn into planks and other structural timber for house-building, packaging, fencing and everyday furniture, or are peeled to provide flat sheets of veneer for making plywood. Smaller logs are used as telegraph poles, mine props and fence posts. Some are cut into chips, which are then stuck together with plastic glues to become chipboard, a ‘man-made’ wood. Very large-quantities are ground or cooked with chemicals to make paper pulp, the source of all our newsprint, writing, book-printing and wrapping paper. Nowadays the owners of these forests, who may be governments, timber or paper companies, or individual landowners, plan this harvest for many years ahead. It is only worthwhile to build a big modern sawmill or paper mill if it is known that timber can be cut to feed it, far into the future. Ideally, the amount of timber harvested each year should equal the amount that grows again in the forests within economical transport distance.

Pinewoods

Conifers of many kinds are loosely called ‘pines’ or ‘firs’, but all the true pines belong to the genus Pinus and are identified at once by their slender needles grouped along the twigs in twos, threes or fives. Each needle is tough and leathery, and has bent surfaces, concave within and convex without. In hot dry weather it contracts and becomes almost cylindrical in cross-section. This lessens water loss through transpiration and enables pines to grow in hot dry places. There are many species, each adapted to peculiar climates, from the tropics to the far north. The long-leaf pine, P. palustris, of the southern USA, has needles up to 30cm (i2in) long. The Scots pine, P. sylvestris, of northern Europe, and the Jack pine, p. bimksiana, of northern Canada, bear very short ones, only 5cm (2in) in length. Some pines have small cones; others, like the big-cone pine, P. coulteri, of California, ripen enormous ones, the size of pineapples. Pines cast only light shade and the ground beneath them is usually covered with short grasses or low shrubby plants like heather and bilberries. They are the haunt of red and roe deer, agile squirrels, black grouse and a giant grouse called the capercaillie. Sprucewoods

Spruce trees can be identified by the little woody pegs, set at the base of each needle, which is always solitary, never one of a-.gro.up,.the needles down opposite sides of the twig form a two-ranked herringbone pattern, lying in one flat plane. The cones are long, cylindrical, and always hang down when ripe. Spruces grow typically on wet swampy land all round the world’s northlands, and on mountainsides with ample rainfall farther south.

Spruce trees tolerate deep shade, and can crowd close together, forming dense woodland. They cast an even deeper shade. Hardly any light reaches the floor of most sprucewoods, and no green plants can live in such places. The ground is covered with dead needles and twigs that break down slowly to form an acid mould or humus. But the nutrients this holds gradually return, via the roots, to the trees, and spruce forests yield high volumes of useful timber in perpetuity. Birds and mammals are scarce; pigeons nest and deer hide among the spruces, but feed elsewhere. Larchwoods

Larches are easily recognized by the grouping of most of their needles in clusters on little knobs along their twigs; at the twig tips, however, the needles are borne singly. These needles fall each autumn, but all through winter the woody knobs on leafless twigs make identification easy. The cones are barrel-shaped. Larch needles are thinner and softer than those of other conifers, bright green in spring, they turn bright orange when they fade in autumn. They cast only a light shade, and soft grasses, ferns and some flowering plants flourish beneath them. Natural larchwoods are characteristic of high mountain ranges, where they provide shelter and pasture for sheep and cattle, as well as homes for wild deer. Larch timber is exceptionally tough, so is used for exacting work like ship’s planking, bridge-building and beams and rafters in wooden buildings. Quaint survivors from the prehistoric past Linked with the true conifers are several groups of primitive trees which survive in relatively small numbers of species and rarely form substantial forests. Best known are the yews, Taxus species, ‘which have needle-shaped evergreen leaves, but bear their seeds in fleshy pink berries, not woody cones. Every yew tree is either male or female; male flowers open as yellow catkins in spring; female ones are green and bud-shaped. The rare maidenhair tree, Ginkgo biloba, from China and Japan, bears fan-shaped leaves like the fronds of a maidenhair fern; they spring from woody knobs and fall each autumn. Maidenhair’s male flowers are catkinlike; the small female flowers, on separate trees, ripen plumlike fruits with hard stone-shaped seeds. The cycads, such as Cycas revoiuta, are palmlike trees that grow in the tropics and expand huge feathery leaves arranged spirally on an unbranched stem. They bear male and female flowers in separate cone-shaped structures, and ripen large seeds resembling plums, with a fleshy outer layer and a hard stone within. Their leaves are used for the thatched roofs and walls of huts, but the wood has no value.

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