474 BOTANICAL GAZETTE [DECEMBER 
characteristically deciduous hardwood area. It is true that these 
broad outlines have been obscured in many places by large tracts 
being covered with pioneer forms, such as Populus and Betula, as 
a result of ‘‘burns”’ (10, 11, 15). These regional forest limitations 
cannot be explained except upon some basis of soil differences, 
such as have been described as determining the local limitations 
of the forest types of Algonquin Park. The evidence clearly 
indicates that the slowly weathering rock of the Laurentian Plateau 
has been a barrier against’ migration of the hardwood forest, which, 
however, has been able to establish outposts where favorable soil 
conditions have been found. In brief, the development of a soil, 
particularly with reference to its humus content, may act as a 
limiting factor regionally as well as locally. 
Light factor 
It is generally accepted that seedlings of some tree species 
grow only where there is abundance of light, while others grow best 
under shade conditions (26). FROTHINGHAM (12) has classified the 
trees of the northern hardwood forest upon the basis of light 
tolerance. The seedlings of pioneer species are necessarily light 
tolerant in contrast with those species forming the climax forest, 
which are shade tolerant; seedlings of Pinus Banksiana and P. 
Sirobus thrive only in direct sunlight, which is also the case with 
seedlings of Abies balsamea and Picea canadensis, although to a 
less marked degree. On the other hand, the seedlings of Acer 
and Fagus grow best in the dense shade of mature trees; Tsuga 
canadensis is an example of a conifer which is similar in this respect. 
Because of the specificity of the range of tree species with respect 
to intensity of light, certain forms cannot be pioneers, while others 
are eliminated from forests which have been well established, except 
where destructive agencies such as cause windfalls and erosion are 
at work. To this extent the intensity of light may act as a limiting 
factor in tree distribution. 
Time factor 
The time factor deserves a most important place in any Con- 
sideration of the distribution of forest trees, and it is of particular 
_ significance in connection with the forests of Ontario. Time as 4 
