1917] MOORE—CONIFEROUS FORESTS I5r 
(Pinus Strobus). It occurs on almost any site, even bare rock, 
provided there is moisture. 
2. The white pine association, composed of nearly pure white 
pine, isnot abundant. The association on Mount Desert, although | 
predominantly white pine, contains a strong admixture of red 
spruce and cedar, and sometimes of red pine (Pinus resinosa). It 
occurs on somewhat drier sites than the spruce association. 
3. The cedar association does not form as pure stands as the 
two preceding ones. Although cedar predominates pa NeIE 
there are generally considerable proportions of fir, spruce, an 
white pine, with red maple (Acer rubrum) and paper birch te 
papyrifera). It occupies the moist flats. 
4. The pitch pine (Pinus rigida) association, generally sharply 
separated from all others, is composed of pure pitch pine, or some- 
times pitch pine and a little red pine. It occupies mostly the dry 
rocky southern exposures. On rocky flats not exposed to full 
_ isolation, white pine, fir, and spruce are creeping in under the pitch 
pine; on these flats, if not elsewhere, the pitch pine appears to be 
a pioneer association. 
5. The grey birch-aspen (Betula populifolia Populus tremuloid 
and y, following fires, andi is 
ae sooner or later by the original coniferous forest. : 
A striking feature of these forests, a ninsaaos common to many 
spruce forests in the west as well as in th e 
of fir reproduction under the spruce, even when the parent stand 
is nearly pure spruce. It is unnecessary to go into the many 
hypotheses advanced to explain this. Perhaps the most wide- 
spread theory, and the one tried out in this investigation, is that _ 
the accumulation of acid in the soil under the spruce is detrimental . : : 
_ to spruce and favorable ne fir. . 
_@) shows that certain p or a racy th soi. ‘Could 2 s 
