Page:Popular Science Monthly Volume 75.djvu/377

Rh 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 compared 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 northward and southward distribution of trees. Along the Atlantic seaboard 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 "uplands" 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 occurrence 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 southern tree life than are the surrounding uplands along their course. As a reverse of this picture, certain trees of a more northerly