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PROTECTION AGAINST WEEDS AND PARASITIC PLANTS.
Plants of various kinds may interfere with the growth of timber crops, either by overrunning the soil as weeds overtopping seedlings and plants in young plantations, or else as epiphytes, or as parasites and fungi growing on or in trees, and frequently causing disease. Fungous diseases, often epidemic, always damage the buds, leaves, roots, or timber of trees attacked, and may cause the death of single trees or of large blocks of plantations.
A. Weeds comprise all vegetation interfering with the vigorous growth or regeneration of timber crops. Natural regeneration is often rendered difficult, and sometimes even impossible, when the soil is overgrown with rank weeds, the roots of which mat the ground, thus preventing the rootlets of young plants from ramifying throughout the soil, besides intercepting rain, and interfering with the aeration of the soil. Special measures have then often to be taken to destroy the weeds before sowing or planting has any fair chance of succeeding. On moorland covered with heather the thick close network of roots often mats the soil, so that the cost of planting is often much heavier than it otherwise would be, owing to stout transplants being required.
A dense soil-covering of weeds extracts from the soil large quantities of soluble salts, part of which, at any rate, might have gone to nourish the timber-crops. Unless the weeds are removed and used as manure, litter, &c.,
, the mineral nutrients are, it is true, returned to the soil on decomposition; but they are again withdrawn by a fresh growth of weeds, so that they are practically lost to the timber-crop. Weeds also hinder light rainfall from penetrating into the soil, because much of it is caught on the leaves and quickly evaporated.
Grass and other quick-growing weeds soon outgrow and top young plants in plantations, and then at once interfere with their growth by withdrawing light, air, and dew from them. And when the weeds die in autumn they often overlay the young plants and press them down to the ground during heavy snow - storms, sometimes smothering them completely. Spreading weeds like brambles, and creepers like honeysuckle, convolvulus, and wild hops, often completely overgrow the young plants and gradually suffocate
them. Swamp-mosses increase the soil-moisture to an excessive degree and cause the formation of bogs; while in other places a strong growth of rank grass may soon cause the soil to become dried up, owing to rapid transpiration of moisture from the upper layers. For this reason plantations are often backward when there is a rank growth of grass. Through rapid transpiration and radiation of heat in spring plants standing among grass often suffer from late frost, while similar plants growing on similar soil, but not surrounded with grass, remain undamaged. A dense mass of tufts of grass also harbours mice, voles, and insects, which may sometimes do a great deal of damage in young plantations.
The trees most likely to be interfered with and hindered in growth by weeds are, of course, light-demanding kinds which are of slow growth while young. Quick-growing and shade-enduring kinds are less exposed to danger. Thus Oak, Ash, Elm, and Sweet-Chestnut among broad-leaved trees, and Silver Fir and Spruce among conifers, are, owing to their slow rate of growth at first, more liable to injury than most other trees, while Birch, Willows, Poplars, Larch, Pines, and Douglas Fir are quickest in outgrowing the reach of weeds, and thus getting clear of danger.
Weeds may, however, under certain circumstances, be of considerable use. They are undoubtedly useful in holding together the soil on very steep slopes, in binding drifting-sand, and in protecting very young plantations against frost and insolation if the weeds (such as furze, broom, and juniper) are high enough and not too thick. Heather often affords protection to Scots Pine seedlings during natural regeneration; and many a self-sown Oak owes its existence to the acorns having dropped by chance among holly or other prickly shrubs, that have protected it against the bite of cattle. Sometimes weeds can also be used as fodder, litter, or manure (heath, heather, bracken, broom, furze, dry grass), or it may sometimes even pay to have their fruits collected for sale (whortleberry, cranberry, raspberry, brambles).
The more important Weeds.—When timber-crops are grown in close canopy, the amount of light reaching the soil is seldom sufficient to admit of a dense growth of weeds overrunning the soil. But when the leaf-canopy becomes interrupted (e.g., as in old Larch- and Pine-woods, or in Beech-woods undergoing natural regeneration), or when mature timber is cleared, a thick growth of weeds soon makes its appearance. When any densely-shaded woodland area (like a Spruce or Silver Fir wood in close canopy) is thus suddenly cleared, it is extraordinary how rapidly the soil gets covered with a strong growth of weeds of all sorts, whose extremely light seeds, carried in the excreta of birds or provided with wings and feathery crowns making them easily wind-borne for a great distance, have perhaps long been lying undeveloped on the soil, and unable to germinate through want of sufficient light.
The fresher and the more fertile any soil is, the more likely is it to be overrun with weeds, the stronger is their growth, and the greater their variety and number. On poor soil the reverse is the case, so that sometimes one weed, like heather, dominates extensive areas. Where many different
varieties of weeds are to be seen, this is always the sign of a fairly good, and particularly of a fresh soil. Climatic conditions, of course, also regulate the growth of weeds, as the mountain flora is different from that of the lower hills and plains. And the amount of light obtainable also determines both the kinds of weeds that grow, and the extent to which they cover the ground. Thus whortleberry, for example, grows best and thickest in half-shade, while grass, heather, furze, and broom grow most luxuriantly in full exposure to light.
The forest weeds may be herbaceous, like grasses, epilobium, foxglove, &c., which die down annually, or they may have woody-fibrous stems that are perennial; and in this latter case they may either be dwarfish plants, like heather, heath, and whortleberry, or else shrubs like bawthorn, dogwood, and dwarf elder. And a number of quick-growing softwoods, prone to spring up in large numbers and strong in growth where not wanted, must also at times be reckoned as weeds. Aspen and Saugh, or even Birch and Alder, often thus prove exceedingly troublesome, and have to be treated more or less as weeds, even though at other times, and in their own proper place when wanted, they are cultivated as useful and profitable trees.
The following are the more important weeds usually found in woodlands, and they are here classified according to the class of soil they naturally prefer :
1. On Wet, Boggy, or Peaty Soil : Peat-moss (Sphagnum), Hair-moss (Polytrichum), Bell-heather (Erica tetralix and E. cinerea), Cranberry (Vaccinium oxycoccos), Bogwhortleberry or Great-bilberry (Vaccinium uliginosum), Lousewort (Pedicularis), Dock (Rumex), Cotton-grass (Eriophorum), Horse-tail (Equisetum), and different species of Sedges (Carex), Bulrushes (Scirpus), and Rushes (Juncus).
2. On Fresh, Fertile, or Humose Soil: Raspberry (Rubus idaus), Blackberry or Bramble (Rubus fruticosus), Briar (Rosa), Foxglove (Digitalis purpurea), Willow Herb (Epilobium angustifolium), deadly Nightshade (Atropa belladonna), black Nightshade (Solanum nigrum), Balsam (Impatiens noli-me-tangere), stinging Nettle (Urtica dioica), Hemp-nettle (Galeopsis tetrahit), Vetch (Vicia), and Clover (Trifolium), Bracken and other ferns, as well as broad-leaved grasses.
3. On Dry Loam and on Sandy Soil : Ling or Heather (Calluna vulgaris), Whortle. berry, Bilberry, or Blaeberry (Vaccinium myrtillus), Myrtle-Bilberry, Cowberry, or red Whortleberry (Vaccinium vitis idæa), Furze, Gorse, or Whin (Ulex europaeus), Broom (Sarothamnus scoparius), Greenwood (Genista), Groundsel or Ragwort (Senecio), Mullein (Verbascum), Hawkweed (Hieracium), Spurge (Euphorbia), and various narrow-leaved meadow-grasses ; while on very sandy soil characteristic plants include Sand-sedge (Carex), Lyme-grass (Elymus), Bent (Agrostis), and Marram-grass (Ammophila, Psamma).
4. On Salt Soil : Marsh-samphire (Salicornia), Salt-wort (Salsola), Sea-milk-worts (Plantago, Glaux), and similar plants.
Shrubs, most frequently to be found among hills and valleys with fresh soil, include Dogwood (Cornus sanguinea), Alder Buckthorn or Black Alder (Rhamnus frangula), Blackthorn or Sloe (Prunus spinosa), Hawthorn (Cratagus), Spindlewood (Euonymus), Guelderrose (Viburnum), Barberry (Berberis), Holly (Ilex), Honeysuckle (Lonicera), Privet (Ligustrum), Elderberry (Sambucus); and on drier soil, Juniper (Juniperus). Sea-Buckthorn (Hippophaë rhamnoides) is characteristic of sandy soil.
Weeds not only give a general indication as to the nature of the soil, but also to a certain extent indicate its physical properties. The number of plants, however, that are more or less constant with regard to the mineral
soil (so as only to be found on clay, lime, or sand, for example), is very limited, although general preferences of this sort are clearly noticeable in most weeds. Like our trees themselves, the woodland weeds are divisible into the two classes of light-demanding and shade-enduring. To the former belong heather, furze, broom, and most of the plants characteristic of poor and sandy soil; to the latter belong holly, dogwood, and many other shrubs ; while whortleberry, bramble, raspberry, and grasses thrive in the subdued light of old woods.
Continental Note.—The moister and the better the soil, and the more light that falls on the ground, so much the more varied and luxuriant is the soil-covering ; while on inferior soil, and where there is merely subdued light, only few weeds cover the ground, mostly heather in the former case and whortleberry in the latter. The demands of the different kinds of weeds for light and for certain conditions of soil are often so very characteristic that the forester can draw important conclusions from their appearance. Thus, when a sprinkling of weeds is noticeable in Beech seed-fellings, this tells him that there is sufficient light for the future crop to germinate in, whereas in Oak pole-woods it is a sign that it is just about time when underplanting is necessary. Where heather grows rank, there is no use of trying to grow crops making high demands on the soil ; while raspberries and nightshade indicate that the soil is still moist and fertile, and rushes point to stagnant water.-(Fürst, Forstschutz in Lorey's Handbuch der Forstwissenschaft, 2nd ed., 1903, vol. ii. p. 66.)
Prevention and Extermination of Weeds.—If undesirable Birch, Aspen, or Willow have to be got rid of, it is often far more effective (especially as regards Aspen) to cut a broad (12 to 15 inches) girdle deep into the wood, as this dries and exhausts the tree. This is often far more efficacious, as well as much cheaper and less troublesome, than waging a long warfare by way of cutting shoots and suckers. The growth of ground-weeds in such quantity as to injure young plantations, and other woods, can best be prevented by taking early steps to drain wet land previous to planting, and in keeping polecrops and older woods in close canopy. The planting of woodlands as soon as possible after clearance, and the use of stout transplants in place of small seedlings, are both measures which of course tend to minimise risk from weeds.
There may already, however, be a strong growth of grasses or other weeds on the land to be planted; or they may at once overrun the ground as soon as the fall of the mature crop takes place; or they may spring up in blanks caused by fires, windfall, snow-break, or insects; and in all such cases the weeds have to be kept in check before young plantations can succeed in establishing themselves, and can begin to grow vigorously.
The removal of weeds can sometimes be arranged for without incurring outlay—e.g., if heather, broom, or bracken can be used as litter or manure. But before planting operations can be carried out, clearance of the weeds is necessary in one way or another, even if only a partial clearance be made in strips, as answers well in the case of heather and berries, though this may have to be repeated. The burning of heather tracts and the cutting over of furze are often also advisable (see vol. i. Part III., Sylviculture, p. 422), although in some cases heather can be checked sufficiently in growth by
herding sheep on it before planting. Where heather is burned as soil-preparation before planting, the block to be fired should have a 6- to 10 - foot fire-trace cleared round it, and the burning should take place against wind on dry still days in early spring.
A strong growth of grass, apt to overshadow and choke young plants in summer, and to overlay and crush them in winter, can be removed by being cut with the sickle, or by being pulled out along with the roots in wisps. It is better to tread down or to smash and injure bramble-shoots with a bludgeon than to cut them back, as this only makes them throw out a strong flush of fresh shoots and suckers. Bracken on hillsides can best be checked in growth by lopping close to the ground with a switch-bill in spring, just when their brittle rolled-up young fronds are beginning to open.
Shrubs and other woody-fibrous plants should be cut through or hacked out of the soil with a hoe or mattock. This should be done about the middle of summer, because the shoots which may spring from the stool will then not only be fewer, but will also be more likely to get nipped by autumn and winter frost before the shoots ripen into hard wood. Blackthorn and hawthorn are often very troublesome; they are difficult to cut because of their thorns, and they have a strong reproductive capacity in shooting from the stool. If such shrubs, and also softwoods, are cut back in spring, piling heaps of earth (not too small in size) over the stools immediately after cutting often hinders their throwing out stool-shoots.
To obviate a strong growth of weeds in nurseries, the forester must avoid sites with damp soil, or near open areas from which the seeds of weeds can be wind-borne. Whenever available, old arable land free from weeds is preferable. Caution, too, should be used in manuring with compost formed from heaps of garden rubbish, as this often contains many weeds removed about their seeding-time; and as these seeds retain germinative power for a very long time, the use of such compost often leads to a very strong growth of weeds on the nursery seed-beds. Garden rubbish should therefore be burned and only the ashes thrown on the compost heap. So too, when leaf-mould is brought from the woods to pile on compost heaps, fungi (Botrytis especially) are often brought, which ultimately infect the seed-beds and cause much loss, among coniferous seedlings especially. When, in spite of such precautions, weeds make their appearance, they should be hoed after heavy rainfall or during damp weather, and all their roots removed. Spreading sawdust between the nursery-rows also prevents the growth of weeds and keeps the surface soil moist, while it also prevents the soil being lifted by frost.
B. Epiphytic Plants. The chief plants of this kind usually found in British woodlands are Ivy, and Beard-mosses (Usnea) and other Lichens.
1. Ivy (Hedera helix) is a non-parasitic plant. It obtains its nourishment mostly from the soil, and the rootlets attached to stems and branches are mainly tentacles for support. Unless where desired for ornament, it should be cut through near the ground, the operation being repeated if necessary; because if allowed to grow unchecked, it soon interferes with the well-being
of the crop.