;o4 



NA TURE 



[January 25, 1894 



Soil. — From the above data it seems evident that the eftect of 

 extensive forests, more especially of those situated at high alti- 

 tudes, is, by cooling the air and reducing its capacity for retain- 

 ing aqueous vapour, to increase the precipitations SVhilst these 

 precipitations are taking place the crowns of the trees intercept 

 a large proportion of the total, and by breaking the violence of 

 the rainfall protect the soil from the danger of being washed 

 away during heavy storms. By the decomposition of fallen 

 leaves and twigs a .-trongly hygroscopic soil-covering is formed, 

 capable of imbibing and retaining moisture with sponge-like 

 capacity. Rapid evaporation of the soil- moisture is counteracted 

 through the protection afforded by the foliage against direct 

 insolati->ii during the day, and by the mechanical hindrance 

 offered to currents of wind. The crown of foliage likewise 

 prevents the soil cooling rapidly at night by radiation. The 

 hotter the summer, the more marked are these beneficial effects 

 of the woodlands. 



When, therefore, large tracts of country are denuded of 

 timber, increase of temperature during the days of summer, 

 rapid radiation of soil-warmth by night, diminished precipita- 

 tions (especially in the spring and summer), and unchecked 

 evaporation of moisture, due to complete insolation of the 

 soil by day and absence of any protection from winds, must 

 be the inevitable consequences. Examples of such actual re- 

 sults can be pointed out in many parts of continental Europe, 

 in Western Asia, and throughout India. In Great Britain and 

 Ireland the effects of the wholesale clearance of woodlands have 

 not been so marked, in consequence of the favourable influences 

 exerted on our climate by the Gulf Stream. 



In localities having no protective woodlandsheavy rains wash 

 away the surface-soil, torrents and freshets rush down the water- 

 courses with great violence, laden with detritus and discoloured 

 with the soil held in mechanical solution, whilst streams and 

 rivers often overflow their banks in consequence, devastating 

 large areas of low-lying tracts under cultivation. Forests, on the 

 other hand, tend to break the violence of the rainfall, and retain 

 for the time being about one fourth of the total amount on the 

 foliage and branches ; the roots of the trees and of the under- 

 growth help to bind the soil firmly ; the rainfall is retained by 

 the vegetable mould and by the spongy growth usually found on 

 the surface-soil, and thence gradually percolates to the deeper 

 layers, where it is held in reserve, to be finally parted with in 

 being utilised for the feeding of perennial streams having their 

 sources on the wooded slopes. 



Thus arose in the Alpine districts of Southern Europe the 

 necessity for maintaining ban-forests as a protection against 

 landslips, avalanches, &c. , and legal measures were early 

 adopted for safeguarding them in order to protect the lower 

 tracts from erosion of the soil when sodden with rainfall or melted 

 snow. 



7. As rega7-ds General Hygienic Effect on the Attnosphere. — 

 It is well known that on the one hand when large tracts of 

 forest are cleared for cultivation, especially in tropical and sub- 

 tropical countries, fever and ague are frequently the consequence, 

 and on the other that the planting up of notorious fever dis- 

 tricts, such as the Campagna di Roma, the Tuscan marshes, and 

 the Russian Steppes, has decidedly diminished the insalubrity of 

 these localities. But the causes are very probably rather due to 

 the degree of direct insolation of the soil, freely afforded in the 

 one case, and counteracted in the other, than to any hygienic 

 property inherent in tree-growth. In the latter case, too, stag- 

 nating surplus of soil-moisture may have been got rid of by 

 transpiration through the foliage, and this would of itself go far 

 towards removing causes of insalubrity, and improving the 

 climate. 



It is generally accepted that ozone kills miasma in the air, and 

 purifies it — at any rale impure air contains little or no ozone ; 

 the proportion of ozone is therefore usually taken as the measure 

 of atmospheric quality. The belief that the woodland air is, 

 like sea air, very rich in ozone has not yet been satisfactorily 

 proved. Experiments in Bavaria showed that in the forests the 

 percentage of ozone, though greater than that in the vicinity of 

 towns, was slightly less than in the open in the vicinity of forests, 

 and that there was no perceptible difference in this respect 

 between coniferous and deciduous forests. 



The woodland air was found to contain most ozone in winter, 

 which shows that its production could not be due to any 

 chemical action of the foliage, for there are no leaves on decidu- 

 ous trees at that season, whilst conifers transpire merely, and do 

 not assimilate. Jt also indicates that the excess is probably due 



NO. 1265, VOL. 49] 



to the comparative freedom of air in the forest from the smoke, 

 carbonic acid, and many other impurities with which air in the 

 vicinity of towns is contaminated and defiled, and to the with- 

 drawal of enormous supplies of oxygen from the air which 

 takes place for the support of animal life at all populous centres. 

 Thus whilst in general the quantity of carbonic acid in the 

 atmosphere is somewhat under four volumes in 10,000, that is 

 the normal amount in London air ; but in thick fogs this pro- 

 portion is frequently doubled, and has been known to be more 

 than trebled, or even to exceed 14 volumes in the city. 



Sunlight, however, has the power of decomposing carbonic 

 acid in the presence of chlorophyll, the green colouring matter 

 contained in foliage, the carbon being-absorbed by the plant 

 for its growth, and the oxygen set free. During darkness a 

 contrary action takes place, oxygen being consumed by the 

 foliage, and carbonic acid given off. As, however, particularly 

 in the case of deciduous trees which are in leaf only Irom April 

 till October, the hours of light far exceed in number those of 

 darkness, the general hygienic effect of trees in cities and towns 

 — apart from their invaluable sesthetic influence — tends decidedly 

 towards the purification of the atmosphere from excess of 

 carbonic acid. 



Ozone again is an allotropic modification of oxygen obtainable 

 by passing a series of electrical discharges through it; hence 

 it is more than probable that in forests in exposed localities, 

 more especially those at high altitudes, where storms and elec- 

 trical disturbances of the air are most frequent, a greater 

 quantity of ozone must be generated in the atmosphere than in 

 localities less subject to such powerful ozonising influences. 



Ebermayer, undoubtedly one of the greatest authorities on 

 this subject, says^ ; — 



" In the middle of the great 'ozone-factory,' which we must 

 consider the forest to be, neither more oxygen nor less carbonic 

 acid is off'ered to mankind for breathing than over large un- 

 wooded areas." 



At another part of the same article he also adds- : — 



" From the hygienic standpoint it is worthy of notice that, 

 according to my examinations, the air in and immediately above 

 the crowns, then that in the immediate vicinity of the forests, 

 has more ozone than that in the interior of the forests, where a 

 portion of the ozone is consumed by the decomposing foliage 

 lying on the ground." 



It appears, therefore, to be his matured opinion at present 

 that whilst more ozone is found in forests than in the open 

 — which the Austrian students of the subject deny, or at any 

 rate are not yet prepared to admit without further observations 

 and proofs — yet the decomposing matter covering the soil 

 consumes the surplus, and often more than that, so that no 

 difterence can be established in favour of the forest air. In this 

 withdrawal of ozone in excessive quantities from the air by 

 decomposing vegetable matter, the unhealthiness of tropical 

 jungles, and the prevalence of malaria at all the lower elevations 

 witnin the tropics usually covered by woodlands, seem easily 

 explainable. 



According to Endres and to Fernow ^ it is claimed that 

 forests tend to resist the spread of epidemics, and to offer 

 a bar to the progress of diseases like cholera and yellow fever. 



Regarding the Sanitary Injltience of Forests, the latter states 

 {op. cit. p, 21) as his summary that "(i) the claimed influence 

 of greater purity of the air due to greater oxygen and ozone 

 production does not seem to be significant ; (2) the protection 

 against sun and wind, and consequent absence of extreme con- 

 ditions, may be considered favourable ; (3) the soil connections 

 of the lorest are unfavourable to the production and existence of 

 pathogenic microbes, especially those of the cholera and yellow 

 fever, and the comparative absence of wind and dust, in which 

 such microbes are carried in the air, may be considered as the 

 principal claim for the hygienic significance of the forest." 



Fortunately there are not many infectious diseases the germs of 

 which can be carried by water ; as yet only two are known with 

 certainty, cholera and enteric fever. When outbreaks of these 

 diseases occur in tropical countries, the infectious power of the 

 germs is favoured by warmth and moisture ; moreover, when 

 epidemic, these diseases usually break out in thickly populated 

 towns and similar localities, where it is impossible to submit 



1 " Hygienische Bedeutung der Waldluft und des Waldbodens " in vol. 

 xiii. of •' Forschungen auf dem Gebiete der Agricultur-Physik," edited by 

 Prof. WoUny, 1890, p. 429. 



2 Op. cit., p. 435. 



3 " Forest Influences," p. 172, 1S93. 



