THE ATLANTIC FOREST REGION 373 



The existence of a forest on the Atlantic side of North America 

 is a result of several natural conditions, chief among which is a copious 

 rainfall. The average yearly precipitation east of the Mississippi 

 Valley amounts to some fifty or sixty inches, increasing towards the 

 coast and the Gulf border. This insures an abundant water supply 

 in the subsoil — the stratum into which the roots of forest trees delve 

 in their search for moisture. Soils, too, play their part in the foresting 

 of a land. An underlying layer of clay holds the water, which collects 

 above it in the permeable sands and loam of the subsoil where the tree 

 roots interlace in a vast network. The varied nature of soils over 

 wide regions determines, within certain limits of temperature, the 

 character of tree growth. This explains in part the preponderance 

 of pines on a sandy soil where the water passes more or less rapidly 

 through the root area. Pines are physiologically dry trees as com- 

 pared with the broad-leaved, deciduous species; their tough and narrow 

 needle-like leaves do not so readily favor the transpiration process — 

 the freeing of the water which has ascended through their vessels from 

 the roots. What ground water enters the transpiration current is, 

 therefore, not too easily lost to the tree through its leaves. The case 

 of the broad-leaved trees is different, for their roots tap soils more or 

 less constantly moist and the ascending transpiration current is quickly 

 relieved by the broad expanse of leafage which they present to the air. 



Temperature is unquestionably the controlling feature in the north- 

 ward and southward distribution of trees. Along the Atlantic sea- 

 board the effective temperatures in tree dispersal are related, in a 

 general way, to the " lay of the land." In the same latitude various 

 species belonging to a more northern habitat appear in the highland 

 districts, while many southern forms are more or less abundant in the 

 lowlands. Along its inland border the coastal plain, in many places, 

 ends in a low rise of land, or " upland terrace," from the top of which 

 one sees the flat expanse of the plain over many miles. Back of the 

 observer lies the rolling country of the Piedmont district (the " up- 

 lands" of the early settlers and farming people), a landscape of hills 

 and valleys stretching away to the eastern border of the Blue Ridge. 

 South of the valley of the Delaware this terrace feature marks, in a 

 very general way, the limits of certain northern and southern trees. 

 The sweet gum or liquidambar of the southern region is abundant on 

 the coastal plain in southeastern Pennsylvania, but is of rare occur- 

 rence on the uplands. The sheltered nature and rich alluvial soils 

 of river bottoms extend the ranges of some of the more southern trees 

 beyond this limit, and the same sweet gum is found growing in the 

 valley of the Connecticut. In like manner the valleys of the Hudson, 

 the Delaware and the Susquehanna are each tinged with a more south- 

 ern tree life than are the surrounding uplands along their course. 

 As a reverse of this picture, certain trees of a more northerly distribu- 



