NATIVH WOODY PLANTS OF THE UNITED STATES 19 
factors of the environment. They take no cognizance of local physio- 
graphic areas, which may themselves cause a change in soil, tempera- 
ture, or rainfall. Major physiographic areas only are recognized. In 
other words, the map is generalized. 
Vegetation areas, as they can be delineated from the notes of old 
surveyors and travelers, are more or less understood, but growth areas 
must be better and better known as we proceed to apply ecological 
principles to land use. Anyone thoroughly familiar with local con- 
ditions in a smaller area, such as a county or even a State, can point 
at once to places on Mulford’s map where the conditions are not as 
represented. Mountain tops in region 11, for instance, should all be 
characterized as 14, with the growing conditions of which they more 
nearly coincide. But the expression of more or less average condi- 
tions over the whole country is fairly accurate. 
In the following characterizations of conditions in the growth 
regions, three main items are included for each: (1) The climate, 
expressed in terms of Thornthwaite’s climatic provinces (570), (2) 
the length of the growing season, that is, the average number of days 
between the last killing frost in the spring and the first killing frost 
in the fall, and (3) the average annual number of days with snow 
cover. All three items have a considerable influence upon the growth 
of plants. 
THORNTHWAITE’S CLIMATIC PROVINCES 
Consideration of the map showing climatic regions superimposed 
on plant-growth region (fig. 2, in pocket) or of Thornthwaite’s orig- 
inal work (570) will show the climatic provinces to be characterized. 
by three factors; humidity, temperature, and seasonal distribution 
of precipitation. The combination of these three forms a climatic 
province. 
The five major humidity provinces are characterized as A (wet), 
B (humid), C (subhumid), D (semiarid), and E (arid). To deter- 
mine them, the procedure was as follows: The total monthly precipi- 
tation divided by the total evaporation was determined for every 
station whose temperature and precipitation records extended 
through a range of 10 years or more, The sum of the 12 monthly 
P_E ratios was called the P—E index, or the precipitation effective- 
ness of the station. Through field observation and study of vegeta- 
tion maps and monographs the five major humidity provinces were 
recognized and defined. Where the P-E index was 128 and above, 
the humidity province was designated as A (wet). Similarly, P-E 
index 64-127 defines B (humid); 32-63, C (subhumid); 16-31, D 
(semiarid) ; and less than 16, E (arid). Characteristic vegetation 
for A is rain forest; for B, forest; for C, grassland; for D, steppe; 
and for H, desert. 
Four subtypes were recognized : 
r—moisture abundant at all seasons. 
S=moisture deficient in summer. 
w=moisture deficient in winter. 
d=moisture deficient at all seasons. 
The determination of the four subtypes depends upon the relation 
existing between the P—E index and the sum of the P-E ratios of 
summer or winter, depending upon which is greater. 
