by sowing and planting; and everything is done to protect the forest itself, and through it the pasture and other land below the woodlands. The replantation of woods destroyed is extremely difficult, as the avalanches usually form in the same places year after year.

(4) Hail.-Hail-storms beat down, injure, and often kill outright young plants in nurseries and plantations, and sometimes do such serious damage to older crops (on the Continent) by wounding the bark and stripping the foliage, twigs, and fruits, as to require their clearance. But in Britain the chief damage is done to the Osier-crops in the fen districts. Scots Pine and Larch are more easily damaged than Spruce and Silver Fir, which are protected by their thicker foliage and closer branches. When Oak-coppice is damaged the bark will not strip at the cicatrisation, while Osier-withes do not peel freely, and break off at the injured parts when being used for basketmaking, &c.

Prevention is impossible; but the better wooded the country is, the more do the woodlands tend to prevent the formation of hail, by helping to modify extremes and to equalise the distribution of atmospheric electricity during storms.

Remedy consists in coppicing the saplings or poles, &c., injured in young crops, and in filling blanks in older woods with whatever kinds of stout plants seem suitable.

(5) Ice. In mountain tracts, and especially on N. and E. slopes, ice often weighs down branches and twigs during cold N. and E. winds in winter and early spring. Evergreen conifers are naturally most liable to such damage, and especially the long-foliaged Scots Pine, on which ice easily accumulates and snaps off its brittle branches, unable to resist the heavy weight. The Alder, the most brittle of broad-leaved trees, is also very liable to injury; and next to it young Oak standards in copse suffer most when their crowns are still more or less covered with dead foliage. Damage is usually greater in 30- to 50-year-old crops than in younger or older woods; while isolated park or avenue trees, standards in copse, and trees at the edges of compartments, are of course more exposed to danger than those growing in close canopy inside the woods.

Prevention is impossible. Mixed conifer woods are, however, less subject to damage than pure Scots Pine.

Remedy of Damage is as for that caused by wind and snow.

(6) Rime or Hoar-frost, which also forms an ice-coating on twigs and branches, does all the more harm when immediately preceding snowfall. A coating of rime then enables the snow to settle in larger quantities on the foliage and twigs, and when this thaws slightly and freezes again a thick coat of ice is formed. While therefore in itself injurious, the greatest danger from hoar-frost is the more extensive damage from snow and ice that may arise indirectly through it.

Prevention. The only practicable measures consist in forming mixed woods rather than pure crops of the brittle Scots Pine in misty tracts, in maintaining a good shelter-belt or fringe along all exposed edges, and sometimes also in heavy thinnings.

Remedy of Damage is the same as for snow and ice.

5. Lightning very often strikes trees, and frequently kills them outright; but the total amount of damage thus done to woodlands is fortunately trivial, as the forester can do nothing to prevent its occurrence.. From their isolated position, all park, field, hedgerow, and avenue trees are more apt to be struck than those growing in close-canopied woods.

When lightning strikes a tree it in many cases merely destroys a strip of bark about an inch broad, and follows the run of the fibres. In trees of tortuous growth (like many Horse-Chestnuts) it therefore makes a spiral mark. Conifers struck by lightning soon die; but Oak and most other broad-leaved trees heal the wound by cicatrisation, and continue growing. In other cases, however, the lightningstruck trees shed their bark almost entirely, and are often completely split or even splintered. Lightning has sometimes been seen to spring from one tree to another; and it is a remarkable fact that in Pine-woods, when a tree has been struck and killed by lightning, many outwardly uninjured stems gradually die off round about it.

Dry, inwardly unsound, and rotten trees are sometimes set fire to and burned down by flashes of lightning; but it can hardly be said that there is any practical danger of lightning causing fires in woodlands.

From their usually standing either isolated or towering above other trees, Italian Poplar and Oak are more frequently struck than any other trees. In the close canopy of woods Spruce and Pine are more often struck than most other trees, but Beech very seldom, owing no doubt to its rounded crown not offering any prominent point upon which the electricity can discharge itself.

In Alpine districts the tops of young Spruce sometimes die off extensively. It has been maintained (v. Tubeuf) that this is due to slight electric discharges between the treetops and clouds enveloping them during the winter period of rest from active vegetation; but the damage has also been asserted (Möller) to be due to the attacks of a small moth, Grapholitha pinicolana, destructive to Spruce and Silver Fir foliage in Central Europe (v. Tubeuf and Möller controversy, 1904).

6. Atmospheric Impurities.1-Smoke from houses and lamps in cities, from factories and smelting-works, and even from railway-engines when frequently passing through wooded valleys, is always more or less injurious to the growth of trees and of woodlands subjected to their influence. The injury is mainly due to the sulphurous acid 2 contained in the smoke, which changes the natural colour in the foliage, and causes the death of many poles and trees. Similar injury is also, though not on so large a scale, caused by hydrochloric acid gas evolved in making soda, and by arsenious or nitrous acids.

The action of smoke is both mechanical and chemical. Its mechanical action excludes sunlight from the green cells of the leaf, and hinders assimilation; and in proportion as the light is excluded, the plant suffers. The smoke in the air reduces the intensity of the sunlight, and the superficial coating of soot over the leaf-surface clogs the pores. Dust acts similarly, but affects less the intensity of the light, and is therefore less injurious. Rough-leaved trees suffer most, as the soot and dust are less easily removed by wind and rain. The chemical action of smoke is mainly or entirely due to poisonous gases. The carbon monoxide liberated during the incomplete combustion of coal is not of itself directly injurious to plants. Arsenious acid, sometimes present in coal-smoke, is also not

1 See also remarks on Trees in Towns, in vol. i. p. 503.

2 Sulphuric anhydride (SO, often called sulphurous acid) on combining with water forms sulphurous acid (H2SO,), and this oxydises into sulphuric acid (H2SO).

specially injurious. Fluoric acid may seriously injure trees near manufactories producing fertilisers containing soluble phosphoric acid. But by far the most injurious gas found in smoke is sulphurous acid. Even one part in 50,000 of the air is decidedly injurious to plants. The injurious gases penetrate the leaves by osmosis over the entire surface, and not through the stomata. They directly poison the leaves, and indirectly destroy the balance between imbibition and transpiration, the sulphurous acid desiccating and destroying the leaf-tissues.

Sulphuric anhydride gas is imbibed by osmosis through both leaf-surfaces, and becomes converted into sulphuric acid by moisture and oxidation. But when rain washes sulphurous acid into the soil, the roots of plants are not injuriously affected, as the water soon converts the sulphurous into sulphuric acid, Fig. 185.

Dew and rain do

which combines harmlessly with alkalies contained in the soil. not affect the absorption of the sulphurous gas, but only hasten the injurious action of the acid on the foliage, because they quickly transform the sulphurous acid on the leaf-surface into sulphuric acid. The needles of conifers first turn yellow or red at the tip, and sometimes show a sharply-defined line between the damaged and the still healthy parts, until the whole needle is poisoned, changes colour, and dies (Fig. 185). In broad-leaved trees the action of the sulphuric acid on young foliage that is just developing gives the leaves a mottled appearance, and covers them with light and dark reddish-brown spots (Fig. 186), which may gradually increase until the whole of the green tissue is poisoned and the leaf is killed. When the injury is due to nitric or hydrochloric acid, the leaves have always discoloured edges (Fig. 187).

Evergreen coniferous trees, owing to the long persistence of their needles, are far more sensitive than broad-leaved trees to the injurious action of all kinds of

atmospheric impurities. And the longer their foliage persists, the less power have they to resist the poisonous action. The trees that are most injured are Silver Fir (and probably also Douglas Fir), then Spruce, then Scots and other Pines, while the deciduous Larch is far more susceptible than any of the broad-leaved trees, which, owing to their annual change of foliage, are altogether less sensitive. Among broad-leaved trees in woodlands, Elm, Maple, Sycamore, Oak, Ash, Poplar, Lime, and Mountain Ash, are less damaged than Birch, Alder, and Hornbeam, while Beech is usually the most sensitive of all. Agricultural crops and vegetables suffer far less than shrubs and trees from atmospheric impurities.

In woodlands the damage is always greatest in the vicinity of factories, and especially where narrow valleys make the air-currents set in particular directions; and most damage is done when the atmosphere is damp and foggy, and wherever the smoke is blown on the woods mainly in one direction. The injurious influence of smoke from chemical works and similar factories may be plainly felt to a distance of 2 to 3 miles. The first signs of injury consist in a general sickly look, then in discoloration of foliage, death of twigs and branches, stag-headedness, and the dying off of trees here and there. The leaf-canopy next gets partially interrupted, and as the nuisance continues the crown of foliage gradually becomes thinner and more broken, until large blanks are made by the trees being killed. Young pole - woods from 15 to 30 years old suffer most in this way; and the beating-up of blanks or replantation of woods destroyed is an extremely difficult operation. Pine pole-woods are very liable to attacks of the fungus Lophodermium pinastri in smoky localities (see p. 161).

Prevention and Remedy.-In Germany, the owners of factories are liable to pay compensation for injury done by smoke from their works, so that they use every endeavour (without much success as yet) to mitigate the nuisance.1 Endeavours to catch the sulphurous acid contained in the smoke and to convert it into sulphuric acid for commercial purposes have been found impracticable, as only about one-quarter to one-third of it can be withdrawn from the air; while tall chimneys conducting the impurities into higher layers of air merely widen the area affected. Somewhat similar protection is in Britain afforded by the Alkali Acts, under which steps may be taken to enforce prevention of the escape of hydrochloric acid from gas in injurious quantities from works.

The only practical measures of any real use are to try and grow broad-leaved crops instead of conifers in smoky localities, to maintain thick shelter-belts of hardy species in the direction from which factory-smoke comes, and to make casual falls of mature wood here and there annually or periodically, combined with natural regeneration, rather than clear falls and replantation. But where woods or plantations have been destroyed by the atmospheric impurities, sowing or planting of smoke-poisoned blanks is next to useless. Coppice-woods are least liable to injury. But unfortunately the market for small wood is now so poor that such crops are seldom profitable in Britain.

1 The latest invention of this sort is a fuel-saving and smokeless-combustion process patented by Messrs Ganz & Co., Vienna. It has been favourably reported on to the Austrian Government by a specially appointed Commission (in May 1905), but details concerning it have not yet been published.

Note on Spruce (see p. 198).-When growing on glacier boulder-drift and in very stony, rocky alpine tracts (e.g., Chamonix, 3000-5000 feet), Spruce resists storm-winds quite as well as Larch or Scots Pine, and is as little liable to windfall as either of these, despite the fact of the Larch having no foliage in winter. But Spruce is there shorter and more conical and tapering in the stem, relatively thicker and more strongly buttressed near the roots, more sparsely branched, and distinctly less thickly foliaged and less shade-bearing than throughout the hill-tracts of Central and Southern Germany.

THE MANAGEMENT AND VALUATION

OF WOODLANDS

CHAP.

I. THE THEORETICAL PRINCIPLES OF WOODLAND MANAGEMENT.

II. THE MEASUREMENT OF TIMBER-CROPS.

III. THE FORMATION OF WORKING-PLANS, OR THE PRACTICAL APPLICATION OF THE THEORETICAL PRINCIPLES.

IV. BOOK-KEEPING ON WOODLAND ESTATES.

V. THE VALUATION OF WOODLANDS.