10 BULLETIN 299, U. S. DEPAETMENT OF AGKICULTUEE. 
or the banks of watercourses. The seed is not durable and must find 
immediately favorable conditions for germination, especially moist- 
ure. This group has naturally a better chance of holding its own or 
even of increasing, like paper birch, through the interference of man, 
than any of the groups. Since this group has become the most widely 
distributed, it is natural that it should have produced more species 
than other groups in adapting itself to varied climatic conditions. 
The species vary, from those with smooth twigs and leaves, com- 
mon where climatic or soil conditions are favorable, to very pubes- 
cent forms where severe conditions prevail. (See botanical range 
map, Plate II.) 
The white ash group also has a wide geographic range, but less 
than green ash, because it seeds much less freely; the seeds are as a 
rule heavier and larger and less easily disseminated, and take much 
longer to germinate. On the other hand, their seed is more dura- 
ble, larger kerneled, and stouter, and adapted to somewhat more 
rugged conditions, so that it has a better chance of germinating and 
growing than green ash where soil conditions happen to be adverse 
to immediate growth. Trees of this group occur chiefly on uplands, 
especially in coves, on moist slopes and depressions, and along up- 
land watercourses. 
None of the species in the water or black ash groups have as wide 
a geographic range as does green or white (see PI. II), because their 
seed is heavier and less readily disseminated, and in the case of the 
black ash group seeding is less frequent. Black ash (F. nigra) is the 
wider distributed of the two and extends farther north than white 
ash, but not nearly so far south. Certain characteristics of trees of 
these groups, such as durability of seed of the black ash group and 
the wide flat wing of the water ash seed which by floating it increases 
its chance of finding a favorable spot for germination, enable them 
to perpetuate themselves on unfavorable sites to which they have 
been relegated by their nonaggressive character. The species of 
these groups, except blue ash (F. quadrangulata) , occur chiefly in 
swamps where conditions are poor for tree reproduction and growth. 
Blue ash is primarily a tree of rough and dry limestone hills, where 
conditions for reproduction are also somewhat severe and where 
acorn and other nut trees are the prevailing growth. These non- 
aggressive groups are likely to decrease continually in amount and 
importance. 
The shrub group is confined to a very limited area in the South- 
west, and may be classified as secondary chaparral species, though 
not considered desirable even for this kind of forest. 1 
!See Forest Service Bulletin No. 85, "Chaparral," by F. G. Plummer. 
