FORESTS AND CLIMATE 3*5 



the end, the last and the least important of all, modifying slightly, per- 

 haps, the total effect of all the other controls, comes the surface-covering 

 of the earth. This may be snow, or grass, or sand, or lava. Here be- 

 longs the forest, a special kind of surface covering. 



On the other hand, the forests. What do we mean by forests ? Do 

 we mean the vast, dense tropical forests of the Amazon, or a grove of 

 trees on a New England farm ? Have we in mind evergreen or decidu- 

 ous trees, or both? Are the forest trees tall or scrubby? Is their 

 height uniform or varying? Is there undergrowth or is the forest 

 clean ? Are we considering the forested slope of a steep and lofty moun- 

 tain or the trees in a valley bottom ; a tropical or an extra-tropical for- 

 est ; a region of heavy or one of moderate rainfall ; of much or of little 

 cloud? Clearly, a complex problem is here before us. No wonder that 

 so much diversity of opinion exists with regard to it. Few of those who 

 discuss the question are at all aware of its extent or complexity. They 

 see only one or two small aspects of it, and upon a very insufficient, and 

 often inaccurate, knowledge they base broad and misleading generaliza- 

 tions. 



In a matter of such general interest it is most important to proceed 

 carefully, and to see clearly just what we do, and what we do not know. 

 That is the purpose of the present paper : to set forth, as the writer sees 

 it, the status of the " forest and climate " discussion in the light of the 

 available facts. It may be added, parenthetically, that it is only com- 

 paratively recently that a scientific study of the subject has been pos- 

 sible. 



The Historical Method of Treatment: its Unreliable Eesults 



The favorite method of attacking the problem of forest influences 

 has been the historical method. Probably the large majority of those 

 who believe in such influences are affected, consciously or unconsciously, 

 by the use of historical arguments. A certain region, we hear, was once 

 forested. There are now few or no traces. " People " say that the cli- 

 mate there has " changed." Hence, the disappearance of the forests 

 must have produced the change of climate. This is not an unfair illus- 

 tration of the historical argument. Sometimes, of course, simple hear- 

 say, and general impressions, are replaced by actual records of the 

 change in area covered by trees, and by rainfall observations (extending 

 over a relatively short period), or by rough accounts of the depth of 

 water in rivers and streams. But, at best, this method of treatment is 

 very unreliable. All the elements in the discussion are uncertain : the 

 early forest conditions; the supposed "change" of climate; the ac- 

 curacy of any available meteorological observations. Granted that a 

 "change" of climate has actually taken place, was the so-called 

 " change " the cause, or the effect, of the change in forest cover ? And 

 may not the "change" have been the result of the well-known oscilla- 

 tions of the climatic pendulum, which bring periods of wetter and then 



