448 BOTANICAL SURVEY OF DISMAL SWAMP REGION. 



ecologically than in their systematic forms, the humid area being 

 heavily forested, while the dry Lower Sonoran area is destitute of 

 forest in the strict, sense of the term. 



POSITION IN THE LIFE ZONES OF NORTH AMERICA. 



The southeastern corner of Virginia, including the Great Dismal 

 Swamp, constitutes the northeastern termination of the Austro- 

 riparian Area. Here this area covers but a limited tract which does 

 not greatly exceed the bounds of the Dismal Swamp region. It is the 

 low (de vat ion of this strip of sandy coastal plain and its neighborhood 

 to the ocean that permits the presence of Austroriparian flora, while 

 not far westward, with a comparatively only slight increase of alti- 

 tude, the Austroriparian element becomes subordinated to the Caro- 

 linian (Upper Austral), which prevails throughout the hilly middle 

 country or Piedmont region of Virginia, the Carol inas, and Georgia, 

 as well as in the greater part of Maryland, Delaware, and New Jer- 

 sey. This transition from the Lower to the Upper Austral zone is 

 probably induced as much by increasing distance from the sea, with 

 its tempering influence upon the climate, as by the relatively insig- 

 nificant increase in the elevation of the land. 



Farther southward the width of the Austorriparian belt- constantly 

 increases. In North Carolina it covers nearly one-third the total area 

 of the State, while fully half of South Carolina and Georgia and the 

 whole of Florida, excepting the extreme southern (tropical) portion, 

 belong to this zone. 1 



AUSTRORIPARIAN PLANTS REACHING THEIR NORTHERN LIMIT IN THE 

 DISMAL SWAMP REGION. 



Not a few species of plants which belong properly to the Lower 

 Austral zone range, in gradually decreasing number of forms and 

 individuals, north of the mouth of Chesapeake Hay. Nevertheless, as 

 a whole, or, better speaking, as the predominant floral element, the 

 Austroriparian flora finds there its northeastern limit. Conversely, 

 many of the species that inhabit the Dismal Swamp are most abundant 

 in and characteristic of the Upper Austral zone (Carolinian area), and 

 some even belong to the Transition zone. Hut the most conspicuous 

 and abundant species, such as contribute largely to the physiognomy 



1 The limits of this area are defined by Dr. Merriain (Bull. Div. Biol. Surv., U. S. 

 Dept. Agr. No. 10, p. 4.">) as follows: ''The Austroriparian area occupies the 

 greater part of the South Atlantic and Gulf States. Beginning near the mouth 

 of Chesapeake Bay, it crosses more than half of Virginia, North and South Caro- 

 lina, Georgia, Florida, Alabama, the whole of Mississippi and Louisiana, eastern 

 Texas, nearly all of Indian Territory, more than half of Arkansas, southern Mis- 

 souri, southern Illinois, the extreme southwestern corner of Indiana, and the 

 bottom lands of western Kentucky and Tennessee." It does not seem proper that 

 so much of Virginia and North Carolina should be included in the Lower Austral 

 zone. Certainly the flora of all but the most eastern portion of Virginia is pre- 

 dominantly Carolinian rather than Austroriparian. 



