FORESTRY 185 
culture differ widely. Inthe one it is the individual tree, planted for 
fruit or ornament, with which we have to deal; in the other it is a mass 
of trees, planted for cover or for timber. = 
The relation of fcrestry to agriculture is that both have to do with the 
soil for the production of a crop; in the one case, however, the crop is 
utilized from year to year, in the other only after an accumulation of 
many years’ growth. The planting of the crop must, therefore, be done 
with a knowledge of the changing conditions through many years, the 
changing relations of the different parts of the crop, and the behavior of 
the trees toward each other during their growth. 
There is another relation of forestry to both horticulture and agricul- 
ture which pertains most specially to the prairies and plains of our west- 
ern country. Itisits protecting influence against injurious winds and 
its capacity to produce climatic amelioration. 
I need not, before you, discuss at length this first influence. Everybody 
living on the plains and prairies knows the effects of the windbreak or 
can test it readily. That the compacter mass ofa grove or forest-belt is 
even more efficacious in breaking the force of the wind—and thereby its 
evaporative power—and fora longer distance. can also be readily under- 
stood. But it seems not so readily understood in what manner climatic 
amelioration beyond that of a windbreak may be expected from a forest- 
belt, and a few words are necessary to elucidate this. 
I cannot insist urgently enough upon the proposition that climatic ef- 
fects must be mass effects; tnat only masses, dense and large, of foliage, 
large areas covered with tree growth, can have appreciable effects upon 
the local climatic conditions of the area lying to the leeward ofthem. For 
such climatic effects are produced in the main by difference of temperature, 
and thereby of humidity, in large air columns within and without the 
forest growth. 
It is well known that the temperature of the air is derived by radiation 
from the ground, that the sun first heats the soil, and the soil heats the 
air. 
If we were to cover therefore, a large area of ground with a blanket 
some distance from the ground, or else with a dense leaf canopy, thus 
withdrawing it from the rays of the sun, the soil under this protection 
would not warm as readily as that outside; in consequence the air below 
the cover would remain cooler, and with the impeded evaporation it 
would retain more moisture. The effect of such a cooler and moister 
area would naturally extend some distance above and beyond the protec- 
tive cover, and any winds or currents of air traveling through this cooler 
and moister stratum must necessarily be modified in their temperature 
and humidity conditions, and impart these modifications to the open area 
beyond. 
That this difference in temperature and humidity under a forest-cover 
actually exists has been well established by experience not only, but by 
careful, systematic observations for many years at stations specially 
established for the purpose, a discussion of the records of which will pres- 
ently be published by the Forestry Division. 
It stands to reason, that if the soil area thus withdrawn from the in- 
fluence of the sun be small as compared with the surrounding open 
country, the amount or volume of air with such a difference of tempera-— 
