Vol 'i9W ] Blackwelder, Birds of the Upper Yukon. 59 



In the summer this zone is inhabited by caribou, in roving groups or 

 individuals, a few little bands of Dall's White Sheep, the Ptarmi- 

 gans, Longspurs, Wheatears, Gray-crowned Rosy Finches, Pipits, 

 and Duck Hawks. Neither reptiles nor amphibians were observed, 

 and even insect life, with the exception of the ubiquitous mosquito, 

 appears to be rather scarce. 



The Yukon Flats are a plain covered with alternating swamps, 

 lakes and forests, and permeated by meandering rivers tributary 

 to the Yukon. 



The luxuriant verdure of the Flats seems to pass through a 

 definite ecologic cycle in which three stages are tolerably distinct. 

 The newly abandoned gravel and sand bars are soon overgrown 

 with dense willows, and the silt banks by horse-tail (Equisetum) 

 reeds and tall grasses. When no longer subject to frequent over- 

 flow, the poplars, chiefly the Balm-of-Gilead and a species of 

 aspen, grow up among the willows, smother them and form dense 

 thickets. During the latter part of this stage, on fairly well 

 drained land, the white paper-birch develops and may grow to a 

 diameter of more than a foot. In the shade provided by the pop- 

 lar and birch thickets, the spruce is slowly seeded and, eventually 

 growing to a greater height, exterminates both of them by its deep 

 shade. The permanent forest has then become a solid stand of 

 spruce, in which the trees range up to about two feet in diameter. 

 The characteristic small growth in its dark recesses consists of 

 alders, the red currant, certain ferns, and a thin carpet of lichens 

 and mosses. The bayous and swamps have a distinctive flora of 

 reeds, grasses, and especially mosses, which apparently prevent 

 the growth of trees even when the swamp has become solidly filled. 

 The blueberries and the low-bush cranberries are locally plentiful 

 on these moss-covered swamp flats and on such as have not been 

 appropriated by the forests. The berries are, however, very much 

 less characteristic of the Flats than of the hilly region to the south. 



Although there is a definite tendency for the permanent spruce 

 forest to spread over the entire area, actual observations show that 

 it is decidedly patchy in its distribution, and in some areas covers 

 only a small proportion of the ground, in comparison to the swamp, 

 willow, and poplar floras. This is apparently due in part to forest 

 fires, but chiefly to the annual inroads of the meandering streams. 



