MARYLAND WEATHER SERVICE 167 



enlarge and become more numerous, the young or dwarfed oaks 

 which have been biding their time now begin more rapid growth, 

 so that before many years there is a mixed pine and oak growth. 



As the humus content of the soil increases, the existence of cer- 

 tain other species is made possible, for example, hickories find a 

 suitable habitat, and less xerophytic species of oak are able to grow, 

 hence we find many forests in the oak-hickory stage. 



In the account of the Upland Forests a fourth association, namely 

 the Maple-Gum, was described. Since a Maple-Beech association 

 is known to succeed the Oak-Hickory in regions further north (vide 

 infra), observations were made on the origin of the Maple-Gum 

 formation. Although the Eed Maple occurs to a certain extent in 

 the border region between the uplands proper and the depressions 

 in which is found the assemblage named "Maple-Gum association," 

 the latter essentially belongs to the valleys, where according to the 

 writer's observations it immediately succeeds the pines as do the oaks 

 in higher situations. A good illustration of this occurs near Bowie, 

 Prince George's County. On this piece of rather low ground the 

 pines have begun to die out, and there is a plentiful undergrowth 

 of maple and sweet gum. Owing to the great degree of tolerance 

 possessed by the Red Maple, it is entirely probable that the seedlings 

 will live until further breaks occur in the pines, when a forest of 

 maple and gum will spring up. Owing to the exceedingly meso- 

 phytic conditions in the places where they occur, other trees such 

 as the Buttonwood, Tulip-tree, Beech and Black Gum find the neces- 

 sary shelter, so that this association comes to include a greater 

 variety of trees than any other. 



The succession of the lowlands, on the other hand, seems to be 

 from pine to a mixture of gum and oaks, with Sweet Gum as the 

 pioneer. After this tree has crowded out most of the pines with 

 the aid of such xerophytic oaks as the Spanish Oak, the more meso- 

 phytic oaks siich as the White, Black, AVillow and Swamp Oaks, 

 together with Maple and Black Gum, make their appearance. Hence 

 the series has probably been: pine, pine-gum, gum-oak. Most of 

 the coastal forests show a large admixture of pine, but it has been 

 pointed out that on the Sassafras loam there are but few pines, 

 with large numbers of various species of oaks, and even hickories 



