The Vegetation of North America 319 



The deciduous forest formation. Limited on the north by the 

 northern evergreen forest and on the south by the Gulf coastal 

 plain, a great forest of broadleafed deciduous trees extends from 

 the Atlantic coast westward to the great plains of central Ne- 

 braska, Kansas, and Texas. This is the oldest forest on the 

 continent and has occupied much of this region for several million 

 years. Sometimes it was far more extensive, sometimes it was 

 more restricted ; but it has been practically continuous since the 

 Cretaceous period of the earth's history. 



This forest is dominated by oaks, hickories, elms, ashes, maples, 

 chestnut, beech, sycamore, cottonwood, and tulip. It attains 

 its best development on the mountain slopes in North Carolina 

 and Tennessee and the lower Ohio River Valley. Under the 

 most favorable conditions this forest attains a height of 150 feet, 

 and some of its trees develop trunks 6 to 14 feet in diameter. 

 In summer they spread an enormous area of green foliage; in 

 winter the above-ground shoots consist only of cork-covered 

 trunks and branches. On uplands and on the poorer soils the 

 oak, chestnut, and oak-hickory forest types dominate. On the 

 richer uplands sugar maple, beech, and tulip trees, with various 

 other mesophytic species, occur. In the river valleys elm, ash, 

 soft maple, birch, and sycamore make up the forest covering. 

 Under the larger trees dogwood, redbud, sourwood, and numerous 

 other shrubs decorate the second levels. On the ground are 

 flowering plants that bloom before the trees have set their leaves. 

 The autumn coloration is a notable feature each year at the close 

 of the vegetative season. 



The deciduous forest region is characterized by short, cold 

 winters, with some snow, usually averaging less than 2 feet, and 

 a frostless season of from 5 months at the north to 8 or 9 months 

 at the south. The rainfall varies f-rom 40 to 50 inches eastward 

 and diminishes westward. Generally it exceeds the annual rate 

 of evaporation ; in the mountains it may be twice as great. The 

 average relative humidity is less than in the northern evergreen 



