414 



CONSERVATION 



est products, except to point out that 

 their value maintains forests in areas 

 where agriculture nii.qht be pursued if 

 the forest products were not more val- 

 uable than the possible agricultural 

 products. The distribution of forests, 

 considered with reference to their prod- 

 ucts, is wholly one of cash returns. On 

 this basis alone, however, the percent- 

 age area now left us cannot econom- 

 ically be reduced until the increasing 

 population begins to feel a scarcity of 

 grain. The present price of lumber, 

 the search for substitutes, and the com- 

 ing timber famine sufficiently prove 

 this. 



Forests protect all civilized activities 

 that depend on a regular supply of 

 clear water or that are liable to injury 

 from irregular and excessive flood run- 

 off. They are not the only protecting 

 agency. We have discussed the very 

 great importance of engineering and 

 good tillage in this respect. But 

 wherever they grow forests do protect, 

 and they do grow and protect where 

 no other agent economically can. 



This truth is challenged, for we in- 

 herit the habit of the ax and the saw. 

 and the efi^ects of our reckless use of 

 these tools are not yet so conspicuous 

 as to be clearly distinguishable in the 

 gross result of natural and artificial 

 activities. The challenge cannot stand. 

 however, against any fair consideration 

 of the relations of forests and water in 

 contrast to those of bare slopes and 

 water. 



In the opening pages of this paper 

 we considered those phases of water 

 circulation that are known as precipi- 

 tation, ground water, evaporation, and 

 surface flow. Forests affect each one 

 of these, and each will be discussed in 

 turn in the order named, so that we 

 may clearly distinguish the several as- 

 pects of controverted points. 



Forests in relation to precipitation. — 

 Precipitation consists of rainfall, and 

 in cold climates of snowfall, also, and 

 these two kinds of precipitation are 



differently affected by forests. Con- 

 cerning forests and rainfall, unquali- 

 fied statements have led to exaggera- 

 tion and contradiction by which the 

 real facts are obscured. When a vast 

 plain like the upper Alississippi \''alley 

 from St. Louis to the Great Lakes is 

 cleared and brought under cultivation, 

 no notable change in the amount of 

 precipitation is likely to follow, for the 

 air currents which sweep over it come 

 from the Pacific and from the warm 

 humid atmosphere of the Gulf. They 

 are loaded with moisture and are driven 

 north toward cooler zones. Rain falls 

 from them inevitably, in spite of any 

 minor effect of radiation from the 

 fields of Illinois or Ohio. Were the 

 wide surface stripped of green and 

 soil and left a region of rock and sand, 

 as central Asia was,'^ then local radia- 

 tion would doubtless increase the irre- 

 gularity and violence of rains, but the 

 air rising from our fields of grain, 

 though not as cool as that above a for- 

 est, is not so heated as to materially 

 affect the powerful rain-bearing cur- 

 rents. Moreover, the general evenness 

 of altitudes and uniformity of radiation 

 from the green covering over a wide 

 area tends toward regularity of condi- 

 tions and gradual rather than violent 

 changes. The ocean of green, whether 

 of trees or grain, ameliorates the clim- 

 ate in like manner, though not in like 

 degree, as does an ocean of water. 

 Hence it is reasonable to expect that 

 the total precipitation and the run-off, 

 as indicated by stream gauges on rivers 

 fed from this region, will not show 

 any decided change as a result of de- 

 forestation. This appears to be the 

 fact, though careful observations of 

 rainfall and run-off have been too re- 

 cently begun and are made at stations 

 too far apart to give satisfactory data 

 for comparisons. 



Deforestation of a mountain range 

 presents a different case, since in the 

 long run it results in extensive erosion 

 and bare surfaces. Leaving this last 



'Pumpelly, Raphael: Relations of Secular Rock Disintegration. Am. Jour. Set., vol. iS, 

 Feb., 1879. 



