480. BOTANICAL GAZETTE [DECEMBER 
The encroachment of the deciduous hardwood forest of Ontario 
upon the coniferous forest is accompanied by competition. The 
progressive changes in such conditions as humus content of the soil 
and light intensity are such as to increasingly favor the former asso- 
ciation to the detriment of the latter. Abies, for instance, grows 
readily on good soil, but it cannot tolerate the shade of an Acer 
forest. The competition becomes too great; in other words, the 
changes in environmental factors have been such that the mean of 
the range of tolerance of Acer more closely approximates existing 
conditions than that of Abies. The factor of competition plays its 
chief réle in the so-called transition areas, where the specific ranges 
of tolerance of the species concerned all include existing conditions 
although unequally. That species dominates, other things being 
equal, whose mean of tolerance more nearly approximates environ- 
mental conditions. 
Specific ranges of tolerance 
The specific ranges of tolerance of some of the dominant forest 
species of Ontario, together with their relation to limiting factors, 
will be considered. Many of the data are represented diagram- 
matically in the accompanying diagrams (figs. 3-6). These dia- 
grams summarize data collected regarding the specific ranges of 
tolerance of a number of forest species. In preparing the tempera- 
ture diagrams (fig. 3), for example, other factors have been elimi- 
nated by selecting data respecting localities where other conditions 
have been favorable; in this way the maxima and minima have 
been determined. The diagrams are relative rather than quanti- 
tative, hence they suggest a field of research which would supply 
absolute numbers. When the maxima and minima have been — 
determined, the means are represented by the mid-points of the 
lines joining these extremes. The radii of the circles of which the 
lines joining the extremes are diameters represent the magnitudes 
of the specific ranges of tolerance. The comparative areas of dis- 
tribution as determined by the several limiting factors are repre- 
sented, theoretically, by circles whose centers are the means of 
their ranges of tolerance and whose radii are the lines representing 
those ranges. 
