290 BOTANICAL GAZETTE [MAY 
I. FACTORS. 
The factors to be taken into account in attempting to explain 
the relations that exist in different plant associations are so com- 
plex that it will be well to consider them in detail. Since the 
object of this paper is to answer some of the questions involved 
in the development of forests, special emphasis will be laid 
upon those factors which appear to be related to tree growth. 
In order to understand the distribution of trees the subject must 
be approached from both the negative and affirmative stand- 
points. In other words, the question is, why are there forests on 
certain physiographic formations and none on those which lie 
close by? Also within the forest itself there predominates now 
one and now another tree type. Insome places the coniferous 
forest is prominent, in others the maple-beech-hemlock type is — 
the chief feature. Indeed, if enough regions are studied an 
indefinite number of combinations may be observed. Thus not 
only must the presence or absence of trees be explained, but also 
where trees are present a reason must be given for the dominance 
of any particular kind of forest. If these questions can be 
answered satisfactorily, some light will be thrown on the origin 
and development of forests. In the answer three sets of factors 
are involved, climatic, ecological, and historical. 
A. Climatic factors. The greater part of the eastern half of 
the United States is a potential forest. Here the two great 
climatic factors, temperature and moisture, are favorable to the 
development of forest trees. When a climate makes possible 
the development of any predominant type of vegetation that 
type is called a climatic formation* To be specific, the eastern 
half of the United States has a forest formation. But if a bird’s- 
eye view of any portion of this formation be obtained, there 
will be found within it groups of other plant types. These are 
designated by Warming? plant societies. Within this vast forest 
a SCHIMPER, A. F. W.: Pflanzengeographie auf physiologischer Grundlage- 
I . 
* WARMING, E.: Plantesamfund. 1895. German edition, translated by Knob- 
lauch. 1896. 

