Influences of Wind and Snow on Northern Tree-Line Environments at Churchill, Manitoba, Canada

Abstract

The influences of wind and snow distribution at the tree line near Churchill, Manitoba, were studied. Wind profiles above the snow surface, snow crystal size and quantity were examined during abrasion experiments with white spruce branchlets. For white spruce, the abrasion zone was evidently within 80 cm above the snow surface, and needle removal by 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 In forest-tundra environments the matted forms of trees were snow covered early in winter and received little abrasion. Sparsely needled islands of trees were largely covered with snow at the base. More densely needled trees and some trees within woodlands were less exposed to abrasion. The blockage of air flow resulted in high-speed jets, which cleared out snow within a "horseshoe-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-existent and snow load on the branches (qali) becomes the prevalent stress. During winter, a large proportion of the snowfall was initially blown off the exposed surfaces of Hudson Bay and the coastal tundra regions, into the woodlands, and eventually across the tree line and into the open forest. As the woodlands filled up with snow in midwinter , the rate of snow accumulation in the forest increased from double to triple the snowfall. Variations in the rate of accumulation occurred with wind speed and direction. During May, snowmelt began on exposed tundra first and usually ran off the frozen surface. Snowmelt occurred about three weeks later in the open forest and was characterized there by less runoff, as the water more readily permeated the thawing peat. The 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.

C-Band SAR Imagery for Snow-Cover Monitoring at Treeline, Churchill, Manitoba, Canada

“…We conducted snow surveys at these sites during the two winters following our initial vegetation survey. These snow surveys revealed significant spatial heterogeneity in the snow cover conditions, which is similar to the observations reported in [19] and [20]. We noted a significant difference between open and closed areas and the transition zone in between.…”

“…We know from [19] that falling snow near Churchill is largely eroded and blown southward off the exposed surfaces of Hudson Bay and the adjacent lowlands into the woodlands and eventually across the treeline and into the open forest. The accumulation rates at treeline can be as high as three times the measured rate of snowfall, depending on the wind speed and direction.…”

“…Recent simulations of ecological sensitivity to climate change have shown that, with the significant warming experienced at the high latitudes of the northern hemisphere, the northern and southern ecotones of the boreal forest will be the areas of greatest change in the future [18]. Because snow is an essential component of the Arctic treeline ecosystems [19][20][21], it is crucial to develop the operational capability to map the seasonal and spatial distribution of snow cover and its properties in these areas.…”

A Survey of Lichens and Bryophytes in White Spruce, <em>Picea glauca</em>, Tree Islands on a Calcareous Beach Ridge in Northeastern Manitoba

“…Despite the declaration that density of the spruce islands may be too low to moderate the severity of the environment (Scott et al 1993), some species such as Alectoria nigricans, Candellariella effluorescens, and Protopannaria pezizoides were found in specific microhabitats within the island of spruce trees. Alectoria nigricans was found on a twig in the shaded interior parts of a spruce island.…”

Die Entwicklung eines Fichtenwalds mit Weißfichte und Flechten entlang der schnell auftauchenden Küste der Hudson Bay im subarktischen Québec

“…Dans ce contexte, l'effet du climat sur la structure et la répartition des forêts apparaît déterminant. À sa limite nord, l'épinette blanche adopte des formes anormales (port asymétrique, formes polycaules, troncs irréguliers) sous l'effet de la fréquente mortalité d'axes en réponse à l'exposition au vent et au froid (Payette, 1974 ;Scott et al, 1987Scott et al, , 1993. Ces formes sont moins fréquentes chez les arbres apparus au XX e siècle, soit dans des conditions climatiques moins rigoureuses.…”