Alnus glutinosa

Alder is the first catkin-bearing tree to be considered in this booklet so let us look first at these remarkable structures. Figure 14 shows fertile catkins in March: they open in early February, before the leaves appear, because in some years that is the best time for the wind to carry pollen from male to female flower. The long, dangling male catkins are seen on the left, with the short, club-shaped female catkins just above them: the other structures are the winter buds from which leaves and shoots will emerge in April.

Each male catkin has a long central stalk, from which little groups of flowers, called dkhasti. spring off at intervals. A single dichasium is shown, at six times its natural size, at the top right-hand comer of the drawing. At its foot is a stalk, while at the top you can see one large bract - a modified leaf, with just below it four smaller bracts, called bracteoles. Each of the three structures in the centre of the drawing is an individual male flower, consisting of four petals and four stamens, displayed like the arms of a cross. A male catkin consists of about 40 dichasia. holding in all some 120 flowers with 4S0 stamens. Its only function is to shed clouds of yellow pollen; then it withers and falls.

The female catkin, seen enlarged to three times its natural size at the lower right-hand corner, is a smaller and simpler structure. Each of the bracts represents a dichasium. and within it there are two female Bowers, each with two stigmas; therefore, you can see four stigmas protruding from each bract Each female Sower is very simple indeed, being nothing but a single seed chamber or carpel, ending with the two stigmas that catch the wind-borne pollen

After pollination the female catkin ripens into a woody 'false-cone' that looks rather like that of a coniferous tree. Each bract gets bigger and becomes hard and woody, while within it four small bracteoles, not seen in the sketch, likewise get larger and

FIGURE 12

Leafy twig of alder in October (x{).

Left; male catkins maturing for following spring.

Centre; current year's female catkins, ripening to shed seed.

harder. Below each woody, compound, cone-scale, each of the two flowers develops a single nut-like fruit which is shown enlarged in Figure 12. In the same drawing you can see the ripe cones, typical leaves, and the young male catkins (on left) which will expand next spring. The cone-scales open in autumn and the fruits, each holding a single seed, fall out. Empty cones hang on the tree for several years, and make alder easy to identify.

The little seed of the alder is spread, to some degree, by the wind, but it is also carried by water, since it is light and readily floats. It will only germinate easily on damp mud, and what usually happens is that the seed is carried down a stream and left stranded on its bank Alder is very much a riverside or lakeside tree, as Plate 6 shows. You can grow it as a planted tree on any reasonably moist soil but it rarely occurs naturally away from water. Another factor that limits its spread is its dependence on another organism for much of its nourishment. Its roots develop curious ball-shaped nodules, containing a living bacterium, Fiankia. Alder is only vigorous where this bacterium is found; it is common on marshy soils, but not elsewhere. Its function is to fix nitrogen from the air, and this helps alder to thrive on infertile soils.

Alder is easily identified at most times of year by its stalked side buds (Figure 13). Its leaves are simple, round in shape, with shallow teeth on their edges; the notch at the tip of their main vein is a key feature for identification. Alder bark is very dark greyish brown, almost black, in colour and becomes broken up into small squares.

Stalked winter bud of alder (x3).

Occasionally alder becomes a tall timber tree, up to 25m high. More often it is seen as a many-branched bush, because most alders have been cut over in the past to give a harvest of poles or small timber. The only regular use for alder wood today is in turneries; it is easily worked, but hard and strong enough for simple uses, which include broom heads and cheap tool handles. In the past much alder was used for carving into soles for clogs -these are working shoes with a wooden base and leather uppers, which carry iron runners below the sole to take the wear; they were once universally worn on farijis and in mills in Lancashire and neighbouring counties.

Another major use of alder poles was to provide charcoal for gunpowder. It is better than that of most other trees, and gunpowder mills were usually sited beside alder swamps. The bark jfex jfex figure 13

figure 14

Leafless catkin-bearing twig of alder in February (xj).

Left: hanging male catkins with dub-shaped female catkins just above.

Above right: dichasium of male catkins with three flowers (x6).

Below left: female catkin in bud (X2J-). Below right: open female catkin

was orjce harvested for tanning. When it is stripped from the logs, a brilliant orange colour develops, and alder is in fact the source of many traditional textile dyes.

Alder swamps are known in England, and locally in Scodand, as cans, a word derived from the old Norse word kjarr. In Wales they are called gwem and in the Highlands Seam, They always occupy fertile alluvial soils brought down by rivers, and therefore, they have - with but few exceptions - been cleared and drained for agriculture. Much of our fenland was once dominated by this singular tree. Wherever alder grows beside running water, its roots play a valuable part in holding up the river banks and checking the erosion of the soil. Scarcely anyone planted it until recently since there is little demand for its timber, although it was sometimes used to screen pit banks, but now it is more widely planted as a native tree whose seeds feed small finches, in many amenity schemes.

The Italian alder, Ahius cordata. is now often planted. It grows rapidly into a taller, more shapely tree than the Common alder, with much bigger catkins and fruit. The leaves are heart-shaped and have tufts of hair on the veins on the underside. No aider shows any autumn colours.

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