Limits of Bast Canadian Trees. 65 



Nevertheless the influence of other factors, presumably moisture 

 conditions, bears on the distribution of some of the species de- 

 cidedly. White and Red Pine, Hemlock and Sugar ^Maple show 

 this influence in most marked manner. 



The different stations, with their temperatures, were plotted on 

 a map, showing limits of forest trees. (C. F. S., Atlas of Canada, 

 No. 9, Limits, Forest Trees.) Lines were drawn connecting ap- 

 proximately points of equal temperature. These lines may be 

 called the tetrahoral isotherms. For the lines of 50° and 52° only 

 one station each was available. Hence, it was assumed that they 

 run more or less parallel to the limits of tree growth. 



These isotherms do not follow the parallels of latitude, but 

 push away up above them in the northwest toward MacKenzie 

 Basin. 



Local cold areas seem to exist in the following places : New- 

 foundland and the Gulf of St. Lawrence ; Bay of Fundy ; moun- 

 tainous region of the New England States. 



The results of this study allow the following relations in broad 

 lines to be stated. 



Black and White Spruce and larch (probably) follow approxi- 

 mately the line of 50°. The data are too scanty to be certain but 

 this line coincides with the contention of Mayr that this isotherm 

 forms the limit of tree growth and these species are at the limit. 

 Balsam Poplar, Canoe Birch and aspen probably follow approxi- 

 mately the line of 52°, S. E. as far as James Bay, beyond which 

 no data are available, but apparently this isotherm is maintained 

 by these species. 



Balsam Fir follows isotherm 54°, S. E. as far as south of 

 York Factory, in its western extension dropping to 55°. 



White Cedar (Thuja occidentalis) ranges from 53 to 54° as 

 its northern limit to 65° as its southern limit, except at the east 

 end of Lake Ontario, where it bends southwards. 



White Pine follows 52° from Newfoundland to Lake St. John, 

 then approximately follows a line between 55° to 60° to Lake 

 Winnipeg, where it turns south. 



Red Pine follows nearly the same line with a tendency to more 

 northern location. 



Sugar Maple runs on the Eastern end along 52°, dropping to 

 58° at the Western end (Lake of the Woods). 



