58 Black-welder, Birds of the Upper Yukon. [j a u n 



other factors. This ill-defined limit separates the two distinct 

 life zones of the region, — the Arctic-Alpine above and the Hudso- 

 nian below. Of the two, the latter is the larger in area and com- 

 prises all of the principal valleys. It is characterized by a dense 

 growth of spruce, in which the trees are usually of rather small size. 

 Near timber-line they are also of low stature. Along some of the 

 larger streams we found spruce trees more than two feet in diam- 

 eter, but they are by no means common. Alders, aspens, and wil- 

 lows form dense thickets along the streams and even around 

 hillside springs. The white birch grows along the bottoms of the 

 larger valleys, such as that of Beaver Creek. Although grass and 

 other forms of herbage spring up in many places on the south- 

 facing slopes, the prevailing cover of the ground is a thick carpet 

 of brownish moss and lichens, which is evidently a botanic com- 

 plex of many distinct species. This moss complex prevails not 

 only in the spruce forest, but almost everywhere that it can gain 

 foothold. Over certain large areas, especially where the slopes 

 are gentle or nearly flat, it forms what is locally known as "nigger- 

 head tundra," in which the tussocks apparently consist of certain 

 coarse bunch-grasses, half smothered by the thick carpet of moss 

 and lichens. Walking over this tundra is very fatiguing, owing to 

 the insecurity of foothold and the soft yielding nature of the turf. 

 Early in August dwarf blueberries are very abundant and char- 

 acteristic of the tundra. A little later a small prostrate variety 

 of cranberry ripens, especially in the more moist situations. 



The characteristic summer birds of the spruce forests are the 

 Hudsonian Chickadee, the Ruby-crowned Kinglet, the Robin, the 

 Slate-colored Junco, and the Alaskan Jay. A species of rabbit 

 is about the only mammal commonly seen, although there is evi- 

 dence that the moose, black bear, lynx, and other large mammals 

 are rather common. 



At timber-line the spruce becomes scattered and stimted, through 

 a narrow zone in which the dwarf birch and dwarf willow are 

 abundant. The former generally grows in dense thickets, which 

 are a serious obstacle to travel. Most of the alpine zone, however, 

 is characterized by the usual covering of mosses and various kinds 

 of grassy and flowering plants. In many places the gentler moun- 

 tain slopes are veritable flower gardens during June and July. 



