tlic severe climate. In tlie extreme Xortli tliere is 

 fcrf<cliial snow and ice. a polar desert ; vegetation is 

 practically absent, and animal life is restricted to 

 marine forms along the ocean coast. The slight de- 

 velopment of tundra in the antarctic is of the lichen- 

 moss barrens ; most of the antarctic continent is cov- 

 ered with ice (Lindsev 1940). 



■ilpinr tundra 



Tundra extends into the tropics on the high 

 Western mountains and into New Kngland on a few 

 of the higher Appalachian peaks. This alpine tundra 

 consists chiefly of grasses and sedges without con- 

 spicuous development of l'>icaceae or the great 

 masses of foliose lichens and mosses found in parts 

 of the arctic (Cox 1933, Daubenmire 1943. Hay- 

 ward 1945). About ^7 per cent of 170 vascular spe- 

 cies collected in the alpine tundra of the Colorado 

 Rockies also occur in the arctic, and about half of 

 these are circumpolar in distribution. Most of the 

 remaining species are endemic to North America, 

 and many species are uniquely endemic to the Rockies 

 (Holm 1927). The taxonomic composition of alpine 

 vegetation varies greatly from place to place, but 

 most of the plants are perennials. The dw^arfness of 

 the shoots in proportion to the flowers and fruits that 

 they bear is very striking. As one ascends the moun- 

 tain slopes, the grass tundra gives way to lichen- 

 moss barrens, and then to perpetual snow and ice. 

 On the downslope side, there is often bush tundra 

 and at tree-line the trees are dwarf and misshapen 

 {krumhoh) from the wind and cold. Flowering 

 herbs are often abundant and conspicuous. 



The occurrence of krumholz is evidence that trees 

 have extended up the slopes of mountains as far as 

 they are able under present climatic conditions 

 (Griggs 1946). The alpine tree-line is usually very 

 irregular. Outlying trees may occur at some distance 

 in advance of the forest proper if they can secure the 

 protection of an embankment, or find other suitable 

 microhabitats. In some mountain areas trees advance 

 to higher altitudes on ridges than in valleys because 

 snow accumulates to greater depths in the valleys and 

 takes longer to melt. 



Origin 



The origin of the tundra flora is uncertain 

 (Raup 1941), but the species involved may be segre- 

 gates from serai stages, especially bogs, of the Arcto- 

 tertiary flora that were tolerant of arctic and alpine 

 environments. With the cooling of the continent and 

 the coming of the glaciers we may suppose that these 

 species were left behind when the rest of the flora 



! n, „ kocky Mountain National Park, Colorado. 



Sub-alpine forest below, alpine tundra above. 



retreated southward. During Pleistocene glaciation, 

 these species survived in the Alaska refugium (Hul- 

 ten 1937), in possibly unglaciated islands of the 

 Arctic Archipelago, and along the southern margin 

 of the glacier. Pollen profiles indicate that climate 

 and vegetation in the Alaska refugium were not 

 greatly different then than now (Livingstone 1955). 

 During the warm interglacial periods, tundra was 

 probably limited to far northern regions and high 

 niountains, with forests covering much of what is 

 now the low arctic. 



When the glaciers retreated in post-Pleistocene 

 time, there appears to have been a period when many 

 tundra species were continuous in distribution from 

 the arctic plains onto the mountain slopes in the 

 northern Rockies. Probably at this time also arctic 

 species were able to disperse farthest southward, as 

 alpine tundra occurred more extensively at lower ele- 

 vations, and intervening forests were less extensive 

 (Daubenmire 1943). As the forests dispersed north- 

 ward through the valleys and lowlands and then 

 gradually up onto the mountains, alpine vegetation 

 retreated to the higher elevations and became sepa- 

 rated from the arctic tundra proper. This northward 

 dispersal of coniferous forest was especially rapid 

 during the thermal maximum period, but in some 

 localities, as in Alaska, it is not yet complete (Griggs 

 1936). On the other hand, the tree-line may be re- 

 treating southward at the present time in other areas 

 (Raup 1941). The coming of the forests interrupted 

 the complete colonization of alpine slopes by arctic 

 species, but the forests brought a new element into 

 the mountain flora derived from refugia south of the 

 glacier (Raup 1947). Since the coniferous forest 

 contained serai grassy stages with species intruding 

 from the grassland biome, some of these species also 

 penetrated the alpine vegetation and became part of 

 it. Furthermore, tundra and grassland probably came 

 into direct contact during the glaciation periods, so 

 there is intermingling of tundra and grassland species 

 in arctic as well as alpine regions (Hay ward 1945). 



Tundra biome 317 



