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douglas firs
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douglas firs - Douglas Firs
douglas firs - (Genus: Pseudotsuga, Carrière. Famijy: Abietaceae.) G. Douglasie; F. Sapin de Douglas.
The douglas tree or Oregon Pine. Named after David Douglas (1799-1834), Scottish botanist, who first sent seeds to Britain from British Columbia in 1828. Douglas also introduced other valuable conifers from this region. The scientific or " Latin " name " Pseudotsuga is a compound of ancient Greek and modern Japanese, and indicates the tree's superficial resemblance to the Hemlock tree.
The Douglas Fir is one of the most remarkable trees in the world. It is one of the biggest and tallest, exceeding 300 feet in height and 6,000 (six thousand) cubic feet in volume in its native country, where it forms great forests covering hundreds of square miles. In these areas its conversion to timber is the principal industry, employing directly and indirectly many thousands of people. It is exported in great quantities to other parts of America, and to Europe. Introduced into Britain, its rapid growth has opened up new possibilities in the supply of home-grown timber, and it has become one of our principal plantation trees. It seeds well in Britain, and may even regenerate itself naturally here.
The leaf spray of Douglas closely resembles that of the Silver Fir. Only long-shoots are found, and the fiat needles lie in two ranks on either side of them; on being detached they leave a flat, round, scar. The most distinctive feature by which the genus can be immediately recognised, is the bud, which closely resembles that of the beech tree, being long, narrow, pointed, with many chestnut brown scales. The foliage is pleasantly odorous. The lower branches persist on the tree for a long time, and the foliage as a whole casts a dense gloomy shade. The bark is at first smooth, but rounded resin-warts soon develop; this resin is clear-coloured, sticky, and has a pleasant odour; in older trees the bark becomes very thick and rugged.
Flowers are borne at an early age, as young as 15 years, the male flowers being conspicuous in May by reason of their dense masses of yellow pollen; they are small oval structures borne near the tips of the shoots, on the undersides. The female flowers are inconspicuous green oval structures at the tips of side shoots. After pollination they grow quickly, ripening from green to brown in the following October. The mature cones are beautiful oval structures, rather resinous, with a series of rounded scales each bearing behind it a narrow, three-pointed bract. This trifid bract enables the tree to be identified with ease. These pendulous cones persist on the tree, opening in the dry weather of the next spring to release showers of small triangular seeds, each with an oval wing.
Few of the thousands of seeds which fall in a good seed year, alight on favourable ground. The parent trees cast a dense shade, and so deep is the humus of needles beneath them that the slender root of the seedling can seldom penetrate it to reach the mineral soil beneath. But those seeds which fell in open spaces, or under trees of other species, make rapid growth from the start. The seedling is rather large, with many cotyledons.
Cones for seed are usually obtained from fellings, sometimes from the standing tree. After extraction under mild heat, the seed retains its germinative power for two or three -years. Around 50,000 seeds go to 1 lb. Seeds are frequently attacked by a minute insect (Megastigmus), the larva of which eats out the kernel and escapes through a small hole, by which sign the damaged seed can be detected. After sowing, the seeds are rather slow to germinate, and hence they usually escape frost damage. They require no shelter and are highly resistant to drought; usually they put forth quite a strong shoot during their first year, but as growth commences late in the season, they are seldom big enough to transplant until their second year. Large surplus plants from the nurseries are sometimes used as Christmas trees.
Transplants that have grown on for one year after two years' seedbed growth, are usually over a foot high and fit for planting. A year later they may be twice that size, but are still plantable. If left a third year, i.e. until they are 5 years old, they grow too big for normal forest planting. Douglas is usually used in pure plantations, fox the simple reason that it outgrows and outshades nearly every other tree that could be used in mixture with it. Because of its rapid growth, it is frequently set at wide distances, but little or no advantage accrues from this practice. If set at 8 feet apart each way, no thinnings may be needed for 20 years, but when thinning has to be done it will be difficult - side branches will be enormous and expensive to " shed " off, and every tree which has failed will cause a gap at least 16 feet across in both directions; also, the timber will be wide-ringed weak, and coarse. At 4 feet apart, four times as many trees are needed, and thinnings may begin as early as 12 years after planting; but the trees will clean their side- branches much better and, as Douglas poles are usually in demand for rustic work, each thinning will bring in an early profit.
As may be expected from its quick rate of growth, Douglas Fir requires a reasonably good soil. On most old woodland soils it thrives excellently, but when used for the first afforestation of waste lands it is less reliable; thus, it is apt to fail on heather land, but where bracken has been the previous ground growth, it does much better. It is not successful on shallow chalk or limestone soils, but where a good depth of loam or clay overlies a chalk subsoil, growth is excellent.
Whilst it is reasonable to expect Douglas to grow best in the wetter western part of Britain, no part of the country is too dry for its growth. It is highly resistant to drought, and thrives on dry hilltops with a pervious subsoil. Frost damage is seldom severe, but snow-break may occur; as the Douglas has great recuperative power and readily forms a new leader, minor snow damage is soon overcome, more severe damage indicates lack of attention to thinnings.
Douglas Fir is reputed to have a low resistance to wind-throw, but this may be attributed to neglected, too-long-postponed, or injudicious thinning. Under regular and systematic thinning there is no reason why its roots should not become thoroughly well anchored and its stem well developed to resist the wind. Specimen trees, grown in the open throughout, reach a height of 150 feet and more without going over, often in most exposed situations. Douglas grows twice as fast as most conifers, and therefore needs thinning four times as often.
As regards shade tolerance, the behaviour of Douglas Fir is unique. Its leading shoots do not tolerate overhead shade, whereas its side branches will bear any amount. Thus it must always " have its head free and its side branches, unless pruned off, remain alive and green for many years. Next to no light penetrates a close Douglas canopy, the tree uses every ray it can - hence its rapid growth; and the ground beneath is bare of vegetation. Douglas Fir is unsuitable for under-planting other trees - unless, the latter are very sparsely placed.
The foliage of Douglas is often attacked by a sucking insect (Chermes) (syn. Adelges) which is covered in specks of white fluff, but damage is seldom severe.
Plantations of Douglas in Britain are still too young for the tree's full potentialities to be assessed. It will certainly grow for eighty years, exceed 150 feet in height, yield two or three hundred cubic feet of timber per tree, and seven thousand cubic feet per acre. At present it may be regarded as our fastest-growing timber tree, giving the biggest volume of timber per acre for each year of growth.
Douglas poles can be utilised in the smallest sizes for fence rails and rustic work, and, when a little larger, they may be cleft for fencing stakes. They are moderately durable, especially when containing heartwood; usually straight, smooth-barked and of even taper - all useful qualities for this work. In larger sizes, they make satisfactory pit props, and are used as telegraph poles in America.
The timber of Douglas Fir is known as Oregon Pine; it is strong, resinous, burns well, has a yellow or red heart, and a narrow zone of paler sapwood. In America it is a staple timber for constructional work, building of all kinds, furniture, plywood, box- making, and paper pulp. It is durable, even in contact with water, and its suitability for a wide range of purposes makes it one of the, world's most valued timbers. Timber exported to Britain is usually of the highest quality, and very free from knots. Such timber, known as "clears" represents only a small fraction of the total crop, the bulk of the timber nearer the centre of the log being knotty. As the American exporters have been exploiting vast virgin forests (often by wasteful methods) they have been able to pick and choose the finest logs for conversion and leave the rest.
Such selection cannot be practised under European conditions, where every tree must be utilised if the plantations are to show a profit; and by the time that plantations now being established in Britain are yielding mature timber, the virgin forests will probably be worked out. If attention is paid to quality of timber, by growing trees closely at first to suppress side branches early, judicious thinning, and high pruning, British-grown Douglas Fir should compare favourably with the average American as regards quality and freedom from knots. The timber so far used in Britain has come mainly from specimen trees grown in the open, and is consequently coarse and knotty, since the side branches persist indefinitely under those conditions.
The foregoing description applies mainly to the "Green" or Coastal Douglas Fir (P. taxifolia Britton), found at the lower elevations. The " Blue " or Colorado (P. glauca Mayr) form is an allied mountain race, but despite their names, the two can seldom be distinguished by the colour of their foliage. The common " Green " Douglas normally has deep blue-green, almost violet, leaves, but where it is not thriving the leaves are bright green or yellowish. The " Blue " form is only cultivated here as an ornamental tree. One other American species is known, and allied species occur in Japan, China, and Formosa.
Already well established as a plantation tree, Douglas Fir is likely to increase in importance as a timber producer in Britain. It requires careful handling, and cannot be expected to thrive equally well everywhere, but where conditions suit it, may be expected to increase in height, by at least 2 feet per year for 50 years. The forester should remember that twice the rate of height growth implies four times the rate of spread of crown; Douglas must therefore be thinned very often to keep it growing uniformly, and to prevent long-drawn-up stems which will never become wind-firm.
A Douglas Fir at Powis Castle, Montgomery, has reached a total height of 175 feet, and is probably the tallest tree in Britain. (Fig. 4). Another, at Walcot, Shropshire, reached a height of 122 feet, girth of feet, and volume of 722 cubic feet, after only 80 years' growth. Girths of 38 feet are attained in Oregon.
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