DISTRIBUTION OF VEGETATION IN THE UNITED STATES. 



I. METHODS USED IN SECURING AND PRESENTING THE 



DISTRIBUTIONAL DATA. 



The botanical data on which we have based the correlations that 

 are to be discussed in the succeeding pages are presented in carto- 

 graphic form in plates 1 to 33. A detailed map of the vegetation of 

 the United States (plate 1) has been executed as a basis for our cor- 

 relations of climate with the vegetational areas of the country as a 

 whole.^ The features of the map will be discussed in a succeeding sec- 

 tion, together with a general account of the vegetation of the 18 sub- 

 divisions which it recognizes. This map is of very uneven merit for 

 the different parts of the United States, owing to the fact that there 

 is an abundance of literature for certain portions of the country, while 

 there are very few descriptive treatments or maps of the vegetation for 

 other portions. As it stands, however, this map is somewhat too 

 detailed for use in correlation with the climatological data that we have 

 been able to secure. For this reason we have deemed it desirable to 

 make a generalized map based upon the detailed one (plate 2). The 

 latter contains 18 vegetational areas, whereas in the former the number 

 has been reduced to 9 by a combination of the areas which are most 

 similar in character. Even after this is done there are some of the 

 vegetational areas of the United States for which we have only a very 

 small number of climatological stations. 



In order to investigate the correlations between climatic conditions 

 and the distribution of certain common growth-forms, a series of 7 

 maps has been drawn, showing the cumulative occurrence of these 

 forms (plates 3 to 9). The maps have been prepared by the method 

 used by Transeau in his investigation of the forest-centers of the 

 eastern United States.^ It consists merely in indicating on a single map 

 the distributional limits of all of the plants involved. The area in 

 which all of them are found together represents the region of maximum 

 development of the particular group which has been selected. In some 

 cases these maps have been drawn for all of the species of a particular 

 growth-form, whereas in other cases they have been drawn for a repre- 

 sentative group of the most common species of a particular growth- 

 form. These maps are of value, not only in showing the center of 

 development of a particular form or group of species and in showing 

 the extreme limits of the form, but also in showing the manner in 

 which the abundance of the given form shades off in different direc- 

 tions from the center. 



^Shreve, Forrest, A map of the vegetation of the United States, Geographical Rev. 3: 119- 

 125. With map. 1917. This map is reproduced here as our plate 1. 

 'Transeau, E. N., Forest Centers of Eastern America, Am. Nat., 39: S75-S89. 1905. 



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