NATIVE WOODY PLANTS OF THE UNITED STATES 1Q 



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 isi 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 Mulf ord'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 E, 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. 



