468 BOTANICAL GAZETTE [DECEMBER 
respect to such species. The general conclusion that temperature 
is usually, if not universally, the determinant of northern limits 
has resulted from making general statements based upon selected 
and favorable instances which are specific rather than general. 
There is abundant evidence that while temperature acts as a 
limiting factor in many instances, it is by no means the only factor 
controlling even the northern limits of tree species. This is amply 
demonstrated by the data recorded on the accompanying map 
(fig. 1). Many of the lines indicating the northern limits of tree 
species intersect; this cannot be accounted for on a temperature 
basis. Isotherms do not intersect nor do lines indicating the 
length of the growing season. The northern limit of Pinus Banksi- 
ana at 100° W. long. traverses a region the yearly isotherm of which 
is 25° F.; at 80° W. long. the isotherm which the northern limit 
traverses is 32.5° F.; at 75° W. long. it is 20° F.; and at 7o° W. 
long. it reaches the 32.5° F. isotherm. The isotherms correspond- 
ing to the northern limits of Ulmus americana at various regions are 
at 100° W. long. 27.5° F.; at 95° W. long. 32.5° F.; at 80° W. long. 
30 F.; at 75° W. long. 40° F.;"and at 70° W. long. 32.5° F., a 
remarkable range of variation. The looping of the lines repre- 
senting the limits of such species as Picea canadensis, Populus bal- 
samifera, and Populus tremuloides, as shown in the Labrador region 
(northern Quebec), is significant, particularly in the case of 
Picea canadensis, in contrast with the closely related Picea mariana. 
The northward deviation of the limits for Betula lutea, Acer sac- 
charum, Tsuga canadensis, and Quercus rubra at 80° W. long., 4 
point where the isotherm swings southward, cannot be explained 
on a temperature basis. The anomalous tree distribution in the 
Saugenay region is another case in point. Any idea of the possi- 
bility of explaining the western limits on a temperature basis has 
long been discarded. It is evident that in the instances men- 
tioned something other than temperature must be the limiting 
factor. 
Water factor 
Water as a factor in the determination of tree distribution has 
received considerable recognition. Cowzes (8) says, “On the whole 
there has been a general tendency to overestimate the influence of 
