INFLUENCES OF WIND AND SNOW ON NORTHERN TREE-LINE ENVIRONMENTS AT CHURCHILL, MANITOBA, CANADA

The influences of wind and snow distribution at the tree line near Chu rchill, Manitoba, were studied. Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experimen ts with white spruce branchlets. For white spruce, the abrasion zone w as evidently within 80 cm above the snow surface, and needle removal b y abrasion was strongly influenced by branch age. Removal by abrasion of similar-aged needles was highest in new branches and declined with branch age up to 7 or 8 years, and then increased again with older bra nches. Trees that grew 80 cm in height in 7 or 8 years were successful in passing through the abrasion zone. In forest-tundra environments t he matted forms of trees were snow covered early in winter and receive d little abrasion. Sparsely needled islands of trees were largely cove red with snow at the base. More densely needled trees and some trees w ithin woodlands were less ''posed to abrasion. The blockage of air flo w resulted in high-speed jets, which cleared out snow within a ''horse shoe-shaped vortex'' around the base of the tree. Both in much of the woodlands and in the open forest, snow abrasion was minimal or non-exi stent and snow load on the branches (qali) becomes the prevalent stres s. During winter, a large proportion of the snowfall was initially blo wn off the, exposed surfaces of Hudson Bay and the coastal tundra regi ons, into the woodlands, and eventually across the tree line and into the open forest. As the woodlands filled up with snow in mid-winter, t he rate of snow accumulation in the forest increased from double to tr iple the snowfall. Variations in the rate of accumulation occurred wit h wind speed and direction. During May, snowmelt began on exposed tund ra first and usually ran off the frozen surface. Snowmelt occurred abo ut three weeks later in the open forest and was characterized there by less runoff, as the water more readily permeated the thawing peat. Th e late snow beds are characteristic of the forest-tundra woodlands and were usually gone by mid-July. The woodlands were snow free for 1.5-2 months during the year, while the open forest was snow free for about 3 months and the tundra was largely snow free for 6 months or more.