MARYLAND WEATHEB SERVICE 157 



This table includes the trees occurring most frequently in the 

 region, and represents the result of a large number of observations. 

 The consideration of an example will make clear the method of 

 estimation employed. In the column "Upland loam" it will be seen 

 that the White Oak is marked 100 ; this means that it occurs in all 

 (i.e. 100%) of the forest tracts situated on loam on the uplands 

 of the district (as far as they were examined) ; Scrub Pine is 

 marked SS, indicating that it was found in 88% of the tracts ex- 

 amined ; similarly the Buttonwood was found in only 9% of the 

 "Upland loam" forests which were visited. The table represents 

 merely an approximation to the relative abundance of the various 

 species, but even as such is highly instructive. For instance, it will 

 be seen that while the Loblolly Pine occurs in 76% of the forests 

 on "Meadow," it occurs on only 35% of the forests on "Upland 

 sand," while for the Scrub Pine the numbers are respectively 86 

 and 100. It is fair to infer, therefore, as stated above, that the 

 Loblolly Pine is the less xerophytic species, and will thrive under 

 poorer drainage conditions. 



Pine-Oak Association. — As was stated above, pines of an age of 

 forty to fifty years are usually accompanied by oaks, in proportions 

 varying greatly according to circumstances. The most common oak 

 in the upland formations is undoubtedly the White Oak. Three 

 other oaks are also widely distributed and numerous as to individ- 

 uals, the Black Oak, the Spanish Oak, and the Post Oak. In the 

 third rank is the Black Jack Oak, which is most abundant in the 

 most xerophytic tracts of the area, such as the Susquehanna clay 

 near Bowie, Prince George's County. Forests consisting largely 

 of these oaks and one or both of the pines are to be found every- 

 where through the district, and are by far the commonest sort of 

 forest. As will be subsequently discussed, the Pine-Oak associa- 

 tion is believed to represent a sort of adolescent stage in the develop- 

 ment of the forests, being preceded by the Pine association and fol- 

 lowed by one in which pines are absent. The struggle for su- 

 premacy between the oaks and pines is apparent in many places. 

 Other oaks than those mentioned may appear in this association, 

 but they are characteristic rather of later stages of forest develop- 



