64 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [F0I.VI. 



and here and there unites with, the Pacific forest. Eastward, in northern British 

 territory, it makes a narrow junction with northwestward prolongations of the broad 

 Atlantic forest. 



So much for these forests as a whole, their position, their limits. Before we glanc© 

 at their distinguishing features and component trees, I should here answer the ques- 

 tion, why they occupy the positions they do ; why so curtailed and separated at the 

 south, so much more diffused at the north, but still so strongly divided into eastern 

 and western ? Yet I must not consume time with the rudiments of physical geography 

 and meteorology. It goes without saying that trees are nourished by moisture. They 

 starve with dryness and they starve with cold. A tree is a sensitive thing. With its 

 great spread of foliage, its vast amount of surface which it cannot diminish or change, 

 except by losing that whereby it lives, it is completely and helplessly exposed to every 

 atmospheric change ; or at least its resources for adaptation are very limited, and it 

 cannot flee for shelter. Bat trees are social, and their gregarious habits give a certain 

 mutual support. A tree by itself is doomed, where a forest, once establishedj is com- 

 parativelv secure. 



Trees vary as widely as do other plants in their constitution; but none can with- 

 stand a certain amount of cold and other exposure, nor make head against a certain 

 shortness of summer. Our high northern regions are therefore treeless, and so are the 

 summits of high mountains in lower latitudes. As we ascend them we walk at first 

 under Spruces and Fir-trees or Birches; at 6,000 feet on the White Mountains of New 

 Hampshire, at 11,000 or 12,000 feet on the Colorado Rocky Mountains, we walk through 

 or upon them ; sometimes upon dwarfed and depressed individuals of the same sjtecies 

 that made the canopy below. These depressed trees retain their hold on life only in 

 virtue of being covered all winter by snow. At still higher altitude the species are 

 wholly different, and for the most part these humble airline plants of our temjierate 

 zone — which we cannot call trees, because they are only a foot or two or a span ortwa 

 high — are the same as those of the arctic zone, of Northern Labrador, and of Green- 

 land. The arctic and the alpine regions are equally un wooded from cold. 



As the opposite extreme, under opposite conditions, look to equatorial America, on 

 the Atlantic side, for the widest and most luxuriant forest-tract in the world, where 

 winter is unknown, and a shower of rain falls almost every afternoon. The size of the 

 Amazon and Orinoco — brimming throughout the year — testifies to the abundance of 

 rain and its equable distribution. 



The other side of the Andes, mostly farther south, shows the absolute contrast, in 

 the want of rain and absence of forest ; happily it is a narrow tract. The same is true 

 of great tracts either side of the equatorial regions, the only district where great des- 

 erts reach the ocean. 



It is also true of great coiitinental interiors out of the equatorial belt, except where 

 cloud-compelling mountain chains coerce a certain deposition of moisture from air 

 which could give none to the heated plains below. So the broad interior of our coun- 

 try is forestless from dryness in our latitude, as the high northern zone is forestless- 

 from cold. 



Regions with distributed rain are naturally forest-clad. Regions with scanty rain, 

 and at one' season, are forestless or sijarsely wooded, except they have some favoring 

 compensations. Rainless regions are desert. 



The Atlantic United States in the zone of variable weather and distributed rains, 

 and with the Gulf of Mexico as a caldron for brewing rain, and no continental expanse 

 between that great caldron and the Pacific, crossed by a prevalent southwest wind in 

 summer, is greatly favored for summer as well as winter rain. 



And so this forest region of ours, with annual rainfall of 50 inches on the Lower 

 Mississippi, 52 inches in all the country east of it bordering the Gulf of Mexico, 45 to 

 41 in all the proper Atlantic district from East Florida to Maine, and the whole region 

 drained by the Ohio — diminished only to 34 inches on the whole Upper Mississippi and 

 Great Lake region — with this amount of rain, fairly distributed over the year, and the 

 greater part not in the winter, our forest is well accounted for. 



