PLANTS IN ARCTIC-ALPINE ENVIRONMENT — SHETLER 481 



face in small patches (pi. 4, fig. 2). These "frost boils," as they are 

 called, keep the surface soil churned up and unstable for plant growth. 



One of the most curious aspects of the Arctic environment is that 

 while water is almost universally evident on the Tundra in bogs, 

 pools, lakes, and streams, the climate is basically arid, and water is 

 frequently a limiting factor for plant growth. Mean annual precipi- 

 tation is extremely variable but seldom exceeds 20 inches, except for 

 certain localized zones especially in mountains. Probably more than 

 half of the vast Arctic gets 10 inches or less, and Barrow averaged only 

 a little over 4 inches a year for a 23-year period. This low figure 

 compares favorably with the annual rates for many warm temperate 

 desert situations. Summer rainfall can account for as much as half 

 of the annual precipitation in the Arctic, but snowfall commonly con- 

 tributes the greater portion. 



Under conditions of good drainage and rapid drying, such a low 

 precipitation would give rise to lifeless desert. What prevents this 

 in large part in the Arctic is the presence of permafrost, which keeps 

 surface water from percolating downward out of the root zone. Also, 

 the land is very flat and geologically youthful, and drainage patterns 

 are generally erratic and poorly developed. Most of the annual 

 precipitation accumulates in depressions, large and small, and stag- 

 nates, forming bogs, wet meadows, pools, and lakes in very large 

 numbers. On the other hand, evaporation and transpiration are rela- 

 tively high owing to frequent winds and generally low humidity, and 

 a plant must have a ready water supply in order to survive. The 

 abundant hummocks, terraces, ridges, and low slopes of the Tundra 

 often become extremely xeric after the spring thaw and brief initial 

 saturation period, and here only plants with adaptations not unlike 

 those required by desert plants can grow. As noted earlier, the higher 

 one goes into the Arctic the more desertlike the habitats become. 



Despite popular misconception, the Arctic is not generally deluged 

 each year by large deposits of snow. Average annual snowfall de- 

 creases as one moves northward on the continent, and in arctic regions 

 it is usually less than 40 inches. This is about equal to the minimum 

 average fall for the State of Pennsylvania and about half the muxi- 

 mum fall ! Insofar as the plants are concerned, then, the nature and 

 distribution of the snow, not the quantity, are the significant features. 

 In winter, except for depressions and leeward slopes, the mantle of 

 snow is very thin over most of the Tundra, and large areas get swept 

 completely bare by high winds. The plants are afforded little if any 

 insulation over these enormous areas and must be adapted to with- 

 stand the freezing winds that sweep the tundra slopes and plains. 

 The very cold temperatures cause the snow to be dry and gritty, which 

 in the wind does considerable damage to exposed plants by mechanical 

 abrasion. In summer, a light snow may fall at any time and linger 



