. Brigham Young University science bulletin. Biology -- Periodicals. Biological Series, Vol. 14, No. 2 of Northern Yukon VEGETATION 15 The general aspect of northern Yukon is dominated by vegetation of low relief, , tun- dra. The hills and valleys have the look of a well-tended golf course (Fig. 11). The vegeta- tion is not unifonn, however, it varies in cover and in composition. The major vegetative con- trol appears to be the amount of moisture that is available. Substrate differences appear to influ- ence the water supplv directlv, and thus the plant cover indirectly. Th
. Brigham Young University science bulletin. Biology -- Periodicals. Biological Series, Vol. 14, No. 2 of Northern Yukon VEGETATION 15 The general aspect of northern Yukon is dominated by vegetation of low relief, , tun- dra. The hills and valleys have the look of a well-tended golf course (Fig. 11). The vegeta- tion is not unifonn, however, it varies in cover and in composition. The major vegetative con- trol appears to be the amount of moisture that is available. Substrate differences appear to influ- ence the water supplv directlv, and thus the plant cover indirectly. The limestone, shale, sandstone, and siliceous conglomerates, slates, and schists of the lower, rounded mountains gen- erally lack the more lush vegetation of the ba- , wet meadows, and stream courses of lower elevations. Rock stripes, boulder patches, and rock outcrops appear at first glance to be barren of higher plants. They are vegetated by a sparse cover of plants widely spaced. Even the lowlands display a mosaic pattern in the green mantle of the land. Frost-heaved patches of gravel of ir- regular shape produce a varied pattern in the tundra. The swales where water drains slowly over the surface are bright green compared to the slopes alongside them. Polygonal patterns ap- pear here and there in the tundra, apparently always associated with poorly drained sites. The lower trenches of most polygons are moist or have standing water in them and frequently are more lushly vegetated than the surrounding area. Tall vegetation is present only along the streams, \\'here willows and an occasional cottonwood stand a few feet above the surface. Southward along the upper reaches of the drainage of the Old Crow and rarely in the headwaters of the streams which drain to the Arctic Sea, there oc- curs a depauperate assemblage of the Boreal Forest, chiefly along stream courses, and along some south facing slopes. This forest extends to about 69° 15' N'^ along the Firth River, at ap-
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