DISTRIBUTION OF VEGETATION IN UNITED STATES. 33 



We have also recognized as alpine summits all of the areas lying 

 above timber-line, and as Swamps and Marshes the areas with saturated 

 soil in the Atlantic Coastal Plain. 



In selecting names for the vegetational areas thus distinguished we 

 have felt it desirable to use designations which have to do with the 

 ecological character of the plants found in these areas. We have 

 avoided the use of such words as coniferous, which alludes to a mor- 

 phological feature, and have also avoided using any designations which 

 would imply that the vegetation of a particular area is controlled by a 

 particular physical condition, as, for example, the use of such terms 

 as ''monsoon forest" or "pinelands of the oolitic limestone." We are 

 well aware that the names that we have used are neither brief nor 

 convenient, but they have been selected with the cautions that have 

 been mentioned, for purposes of ecological consistency. 



An effort has been made to draw all of the vegetational boundaries 

 on the map shown as plate 1 with relation to the original plant covering. 

 This primeval condition has been so greatly disturbed over large 

 portions of the country, particularly in the Northeast, that it is now 

 very difficult to be certain as to the limits between the virgin forest 

 formations. Our map is therefore undoubtedly much less accurate 

 for the northeastern portion of the United States than it is for the 

 Western States, in which the vegetational differences are more marked, 

 the country is less disturbed by human activities, and the published 

 ecological descriptions of vegetation are much fuller and more accurate. 



The following paragraphs serve to give a brief characterization of 

 each of the vegetational areas found in our detailed map, plate 1 : 



California Microphyll Desert. — The southernmost part of Nevada 

 and the interior portion of California form an area in which the vege- 

 tation is closely related to that of the Great Basin by reason of the fact 

 that the dominant plants in each are microphyllous shrubs. In the 

 California Desert the creosote-bush {Covillea tridentata) and the sand- 

 bur {Franseria dumosa) are the most common plants and the dominant 

 ones over extensive areas. A large number of deciduous shrubs are 

 found, large semisucculents occur throughout the more elevated por- 

 tions of the desert {Yucca hrevifolia, Y. arhorescens) , and stem-suc- 

 culents (Opuntia spp.) are found in small numbers. In both the Great 

 Basin and the California deserts the plants which perennate by under- 

 ground parts are very rare and grasses are uncommon, while short- 

 lived annuals are abundant in the spring months. 



Great Basin Microphyll Desert. — This desert occupies the floor and 

 low mountains of the Great Basin from southern Washington to 

 southern Nevada and eastward to Colorado, lying at elevations of 

 1,000 to 5,000 feet. Throughout practically the whole of this area 

 the vegetation is dominated by a single species, the sagebrush {Arte- 

 misia tridentata), a small-leaved evergreen shrub which sometimes 



