PLANTS EST ARCTIC-ALPINE ENVIRONMENT — SHETLER 479 



The arctic-alpine environment in general scarcely has what might 

 be called an annual safe period, insofar as frosts are concerned. There 

 may be an annual period during which on the average frosts do not 

 occur, but an occasional frost may descend upon the plants anywhere 

 at any time even in the dead of surmner. Over an 18-year period the 

 average frost-free period at Barrow was July 4-21, for an average 

 growing season oil7 days a year ! Doubtlessly, the effective growing 

 season is considerably longer, yet we get some idea of just how trun- 

 cated the Arctic summer can be for plants. On the basis of the 

 few comparative data available, we can safely conclude, I believe, 

 that the growing season is no longer than 50 days throughout a large 

 part of the Arctic, and it is often much shorter. Still, this can be an 

 adequate season for the arctic plants, which are physiologically 

 adapted for getting the reproductive cycle over with a rush. The 

 ubiquitous purple saxifrage {Saxifraga oppositojolia L., pi. 5), for 

 example, can begin growth and set good seeds within a month (Porsild, 

 1951). As for the Alpine, we know even less about the growing 

 season from systematic weather records, but we do know from casual 

 observations alone that late-lying snows reduce the effective growing 

 season in many alpine situations to approximately that of the Arctic. 



Subsurface temperatures in most of the arctic region are continu- 

 ously below freezing from one year to the next, and the ground is 

 permanently frozen. This soil frost, called "permafrost," extends from 

 the surface or near it, depending on the season, downward several 

 feet to hundreds of feet ; in places the permafrost zone is more than 

 1,000 feet deep. Each summer the top few inches to 2 or 3 feet — in 

 alluvial sites to 6 or 8 feet — thaw, and the average annual depth of 

 thaw establishes the nature of the vegetation that can colonize the area. 

 Permafrost is impenetrable to plant roots, and the annual frost-free 

 layer must serve as the effect root zone where water and mineral ex- 

 change can occur and roots can take anchor. This zone is usually 

 very narrow, and deep-rooting species are harshly selected out, which 

 surely is one important reason why trees cannot establish themselves 

 on the Tundra. Deeper thawing in alluvial sites, where the soil is 

 coarser, explains perhaps why taller shrubs and certain typically 

 deeper rooted plants, such as some legumes, are often found here. 



The formation of "soil polygons" is one of the most celebrated 

 geomorphic phenomena in the Arctic. Their occurrence is widespread 

 in permafrost areas, and despite much study their development is still 

 a matter of controversy. From a high vantage point, as from a plane, 

 they are most conspicuous, imparting to the land surface a polygonal 

 pattern like giant mud cracks (pi. 4). They can be described as a 

 series of mounds or dishlike impressions, usually at least 25 feet or 

 more on a side, separated from each other by a reticulum of more or 



