DISTRIBUTION OF VEGETATION IN UNITED STATES. 53 



THE COMMONEST EVERGREEN NEEDLE-LEAVED TREES OF THE SOUTHEASTERN 



UNITED STATES. (PLATE 6.) 



The four evergreen needle-leaved trees which are most widespread 

 and most dominant in the Southeastern Mesophytic Forest are Pinus 

 echinata, P. tceda, P. palustris, and P. cariboea. The ranges of these 4 

 pines have been superposed on a single map (plate 6). Pinus echinata 

 possesses the most northerly range of this group, and it and Pinus 

 tceda exceed the distribution of the southeastern evergreen formation 

 itself. The three most widely distributed species of this group reach 

 their western limit at about the ninety-sixth meridian. The distribu- 

 tion of P. palustris is closely coincident with that of the southeastern 

 evergreen formation, while that of P. caribcea lies entirely within that 

 formation. These four species are all found in southern Georgia and 

 northern Florida and the extreme southern portions of Alabama and 

 Mississippi. The region of maximum occurrence of this group lies, 

 therefore, in the heart of the southeastern evergreen area. 



THE COMMONEST EVERGREEN NEEDLE-LEAVED TREES OF THE NORTHEASTERN 



UNITED STATES. (PLATE 7.) 



The ranges of the 4 evergreen needle-leaved trees which are most 

 generally dominant in the eastern section of the Northern Mesophytic 

 Evergreen Forest are plotted together and are shown in plate 7. These 

 trees are Pinus strobus, Tsuga canadensis, Abies balsamea, and Pinus 

 divaricata. 



The region of cumulative occurrence of these trees corresponds 

 closely with the distribution of the evergreen forest formation. The 

 southernmost extension of this group is found in the case of Tsuga 

 and the northernmost in the case of Pinus divaricata. All four of these 

 trees are found together in northern New England, northern New 

 York, and in Michigan, Wisconsin, and Minnesota. 



The range of climatic conditions has been determined separately 

 for each of these trees, owing to the fact that their regions of cumulative 

 occurrence correspond so closely with the eastern section of the Northern 

 Mesophytic Evergreen Forest. The same has been done with respect 

 to the dominant trees of the Southeastern Evergreen Forest. 



THE ECOLOGICAL DISTRIBUTION OF PINUS T/EDA. (PLATE 8.) 



It is rarely that data are available on the relative abundance of a 

 plant within its area of geographical distribution. Owing to the excel- 

 lent work of Mohr,i we are able to use both the geographical and 

 ecological distribution of the loblolly pine (Pinus tceda). The map 

 prepared by Mohr has been reproduced in plate 8 and shows three 

 areas of varying abundance in addition to the region of scattered 



^Mohr, Charles, Timber Pines of the southern United States, U. S. Dept. of Agric, Bur. For. 

 Bull. 3, 1896. 



