4 BULLETIN 285, U. S. DEPARTMENT OF AGKICULTUBE. 



feet in the southern ranges. Above these altitudes the hardwoods 

 give place in large measure to spruce and fir. On northerly slopes 

 the climate suitable for northern hardwoods is often at several 

 hundred feet lower altitude than on southerly exposures. 



The soils in the northern hardwood zone are, as a rule, loamy 

 sands, the result of the decay of granite, quartzites, and siliceous 

 gneisses. In the eastern mountains they are partly glacial and 

 partly residual in origin, thin, and of low agricultural value. In 

 the Lake States and through much of the northeast they were depos- 

 ited by the glaciers in moraines and glacial hills or laid down in beds 

 of varying thickness by glacial streams. Here the hardwoods occupy, 

 for the most part, the water-assorted loams and clays or the unas- 

 sorted morainal tills, rich in clay, but also thrive on light, sandy 

 soils in localities subject to prevailing moist winds, as in western 

 lower Michigan. In the Appalachians, south of the limit to which 

 the glaciers extended, the soils result entirely from the decomposi- 

 tion of the native rocks. Where schists prevail, fertile loams are the 

 products of decomposition, and these may reach some depth in the 

 coves and broader valleys. 



The climatic factors which determine the distribution of forests 

 are moisture and temperature. These differ in relative importance 

 according to the nature of the region. In temperate semiarid regions 

 the determining factor is moisture; in temperate humid regions it is 

 temperature. The northern forest region is distinctly humid, and 

 the composition of the forests is therefore influenced chiefly by tem- 

 perature. Its western limits, however, are fixed chiefly by moisture 

 factors. 



The growing season is approximately five months, from May to 

 September, inclusive. 1 The duration of the season varies within 

 the region, and is shortest in the north and at high altitudes. This 

 factor has undoubtedly a large influence upon the composition of 

 the northern hardwood forests, which is not so much a matter of 

 the sensitiveness of the species to extremes of temperature as it is 

 of optimum temperature. 



How moisture and temperature affect the different species in the 

 complexity of the forest environment is still so little known that no 

 positive information can be given. The best that can be done is to 

 compare the available climatic data from observation stations within 

 the northern forest region with corresponding data from stations 

 just outside. Table 1 accordingly gives the average monthly tem- 

 perature and precipitation during the growing season for adjacent 

 parts of the northern and southern hardwood regions. Similar 

 data for April and October arc also given, together with the annual 

 precipitation and depth of snow. 



i Raphael Zon, " Meteorological Observations for Purposes of Botanical Geography, Agriculture, and 

 Foresn; . ! »ept. of Agriculture Monthly Weather Review, vol. 4.', No. 4, April, 1014; map shows division 

 of the United States on basis of periods of vegetative growth and rest. 



