24: The New York State College of Forestry 
est level as one may say — which in ecological terms is a type 
of plant association that presents the highest degree of mesophy- 
tism of which the region as a whole is capable (10), but 
whose forward movement is locally controlled by soil condi- 
tions (edaphic factors) in this case the complex of conditions 
which a peat bed offers when its surface has been built up 
to the mezn level of the water table. When, in tracing this 
vegetation succession, one speaks of invasion and of the sup- 
pression of one species or type by another, a condition of 
antagonism is of course implied which is scarcely the status 
of a normally developing organism to which the phenomena 
of plant succession have been compared. It is true neverthe- 
less that each association tends to bring about changes in the 
substratum which, however difficult to detect and measure, 
throw the balance against the present occupants and in favor 
of certain other species. We may say that the defense breaks 
down and the attack succeeds by reason primarily of conditions 
created by the defense itself. The sharp line of demarka- 
tion between pure shrub and an advanced stage of invasion 
by conifers shown in Figure 10 would suggest that this is the 
boundary line of a burn. The normal encroachment of conifers 
upon the shrub stand is clearly shown in Figure 16 where the 
aggressiveness of black spruce is especially obvious. This 
species is here, as generally in Adirondack bogs, the foremost 
conifer in the invasion. Very often, if not generally, the forest 
stage is a pure stand of black spruce. This condition is clearly 
related to its aggressive growth habits as well as to its toler- 
ance of bog conditions. This aggressiveness expresses itself in 
the rapid growth and wide reach of the lower branches in pro- 
portion to the main axis which enables it to overcome the 
shrub species. The rapid upward growth of sphagnum among 
the wider reaching branches creates a wet blanket about them 
which induces prolific root development (4) (8) and so as 
in the case of shrubs previously described, a colony is formed. 
In the course of time, the growth of the several branches each 
with its own root system may result in a clump of spruce 
trees. The compactness of growth in this low spreading habit, 
