292 SCOTTISH GEOGRAPHICAL MAGAZINE. 



5. Local heat and moisture derived from lakes and rivers are bene- 

 ficial to the growth of our northern trees, and in many localities they 

 even determine the presence of some species. The white spruce, balsam 

 poplar, etc. attain a greater size on the islands and along the banks of 

 the large rivers flowing northward than at a distance from their influence. 

 Along the North Saskatchewan, which flows through a dry region, the 

 negundo, green ash, grey elm, white birch, and black alder grow only on 

 the banks of the river. Everywhere in the white cedar region, as we 

 approach its northern limit, these trees are confined to a border along the 

 rivers and around the lakes. The most northerly outposts of the black 

 ash occur at rapids in north-flowing streams, where the overhanging 

 trees can enjoy the heat escaping from the water. In such advanced 

 situations the tender leaves and shoots are apt to be nipped by the late 

 spring frosts. Fruit is grown with more success on the southern sides of 

 the larger lakes than elsewhere, as the air which moves from the north- 

 ward off the water prevents the occurrence of summer frosts. 



6. The effect of general elevation above the sea is well illustrated in 

 the large area between James Bay and Lake Superior, in which no elm 

 or black ash occur, whereas these trees grow to a good size on the Albany 

 and the branches of Moose river to the northward. 



7. Local elevation, and consequent dryness and warmth, as compared 

 with adjacent low and cold lands, account for the growth of such trees as 

 the sugar maple, ironwood, basswood, hemlock, etc. in many places north 

 of the Ottawa and elsewhere at considerable distances beyond their last 

 appearances in the river valleys. For the same reason, thrifty forests of 

 yellow birch cover the hills in large districts north of Lake Huron. 



8. Local depressions have a favourable influence in carrying the tree- 

 lines northward. Lake Temiscaming, at the great bend of the Ottawa, 

 lies in a north and south valley several hundred feet below the level of 

 the surrounding plateau, and here the white oak grows fully fifty miles 

 beyond its general range, while the limits of several other species 

 protrude some distance up the valley. Lake St. John, about 150 

 miles north of Quebec, is only slightly elevated above the sea-level, and 

 the forest around it has a more southern character than that of the 

 neighbouring high lands ; but this may be partly due to the occurrence of 

 nearly horizontal Silurian rocks. The basswood grows here apparently 

 as an outlier, and the lines of the red oak, sugar maple, yellow birch, 

 and red pine curve northward so as to include the lake. 



9. Diseases and insect pests have occasionally some effect on forest 

 growth in Canada. A few years ago most of the spruce-trees over a 

 large tract in New Brunswick and the Gasp6 peninsula died of a disease 

 which was accompanied by a white fungus upon the roots. About the 

 year 1889 an imported saw-fly commenced to attack the larches in the 

 region to the south-east of James Bay, and, being in a new country with- 

 out natural enemies, it spread with wonderful rapidity. By the year 

 1895 it had killed most of the larch- trees all the way from Kupert's 

 House to within 100 miles of Ottawa city, and from these points to the 

 centre of the Labrador peninsula, a distance of some 500 miles each 

 way. 



