120 



which they can be convinced is by a scientific demonstration based upon careful 

 measurements, experiments and observations. The last few years have, therefore,, 

 been diligently used by some investigators, who do not intend to prove any theories 

 but to simply find out the truth. Out of the large amount of material which has 

 been accumulated I have tried to summarize what we really do know of the in- 

 fluence that the forest may exercise upon the climate. 



I say " may " exercise, because whether it does or does not depends upon certain 

 conditions, just as it depends upon certain conditions whether a cloud full of 

 moisture will precipitate the same or is dissolved without precipitation. This must 

 be fully understood. 



The influence which a forest can possibly exercise is, in the first place, mainly 

 due to its nature as a cover. I^ow, whether we place our cover before or behind the 

 wind the eflect will be different. The position, therefore, towards other climatic 

 factors is of importance. We may have a cover made of mosquito netting or one 

 made of wool ; the effect of either, of course, will be different. Similarly so with a 

 forest ; the density, its age or height, and the extent of forest is of consequence. The 

 composition, too, must be of some moment, for a coniferous wood acts as a cover all 

 the year round, while a deciduous-leaved one loses much of its protective features 

 during the winter. 



We see, therefore, that it woul4 not be wise to speak of the forest in general 

 terms as doing this or that, but we can only speak of a forest under certain condi- 

 tions, and therefore in certain localities as influential, while in other localities, under 

 different conditions, the influence may not be felt or may be just the opposite of the 

 one experienced in the first case. The influence may be beneficial and it may be 

 objectionable. Thus, a large forest against the north-west may temper the cold 

 blizzard, while towards the north-east it may cut off desirable rain-winds. In one 

 locality it may be desirable to have a shelter against a burning south-wester, while 

 in another the effects of the warm wind are needed to raise the otherwise low 

 temperature. 



It may also be conceded at once that cosmic influe^nces, such as mountains and 

 large water surfaces exercise, are mightier than forest influences, and that the latter 

 are probably only of local importance in modifying climatic conditions, and the 

 extent of their influence is conflned to their nearest surroundings. Yet these very 

 surroundings are the ones in which each of us lives, and since we cannot change the 

 cosmic climate we should be satisfied to modify it locally, if that can be done. 



In the first place, we know, from actual measurement and comparison, that the 

 climatic conditions in the interior of and above a contiguous forest area are different 

 from those of a large open field in the neighbourhood. This difference is due to the 

 protection of the soil from direct sunshine and to the increase of the area which 

 radiates heat, to the evaporation of the water from the leaves and the mechanical 

 obstacles presented to the circulation of the atmosphere — the winds. In 

 consequence, we find that the forest shows a reduced range of temperature — that is to 

 say, neither the extremes of heat nor of cold are as great in a forest as without the 

 forest. It is cooler in summer and warmer in winter ; it is a mitigator of extremes in 

 temperature ; it has a more uniform temperature. 



2. This mitigating difference is found still greater in the temperatures of the 

 soil, the range in the forest soil being ten to fifteen degrees less than in the field soil. 



3. The forest increases the humidity of the air in its interior, especially during 

 the dry months, and altogether the moisture conditions of the forest air are more 

 uniform. 



4. The soil in the forest loses six to ten times less water by evaporation, and 

 therefore remains more evenly moist than the field. 



5. That the forest interior is less exposed to the action of winds will be readily 

 conceded ; hence, shifting sands, such as found along sea shores, would be kept quiet 

 and of course evaporation greatly reduced. 



6. The air above a forest is generally cooler and relatively moister than it would 

 be without the forest cover. 



