KEEPING WARM 
row do reproduce by seeds, but these are 
usually successional species that invade 
the drained basins of shallow lakes. 
Within twenty-five to fifty years, these 
pioneers are replaced by other grass and 
sedge species, which produce seeds only 
occasionally but which, once estab- 
lished, can reproduce by rhizomes to 
form round clonal colonies. These sin- 
gle-genotype colonies eventually out- 
compete the pioneers and meet each 
other head-on. 
Water sedge, the ultimate competitor 
in cold, nutrient-poor tundras, produces 
seeds so rarely that it enters lake-bottom 
succession later than the other species, 
but its slow-growing, long-lived rhi- 
zomes and its phosphate-absorbing abil- 
ity enable it to dominate eventually. The 
other species are not eliminated; they 
simply must live with Carex aquatilis 
and share less of the space and nutrients. 
Reproduction by stolons (horizontal 
stems growing above the ground) and 
bulbils (small bulbs on flower stalks or 
in leaf axils) is a form of vegetative 
reproduction in the tundra confined 
mostly, but not entirely, to small dicoty- 
ledonous plants. Two species of saxi- 
frage, nodding saxifrage (Saxifraga cer- 
nua) and spider saxifrage (S. flagel- 
laris), are good examples. While it can 
produce flowers and seeds, nodding saxi- 
frage reproduces year after year primar- 
ily by bulbils in the axils of the leaves. 
Spider saxifrage can also produce flow- 
ers and seeds — but only once. As it 
flowers and produces new genotypes 
with its seeds, this species also sends out 
stolons, just as a strawberry does, with 
little plantlets on the ends. Since these 
plantlets on the ends of the stolons are 
genetically identical to the mother 
plant, which then dies, the successful 
genotype is replicated at the same time 
that sexual reproduction is producing 
new genotypes and increasing genetic 
diversity in the population. 
The seeds of sexually reproducing 
arctic and alpine plants may be dis- 
persed by wind, birds, or mammals. 
Most of the seeds are small. Some have 
tufts of hairs, others are hairless and 
shiny, and still others, such as those of 
Oxyria, have small wings. The wind is 
capable of pushing the seeds over the 
hard snow for long distances, and migra- 
tory birds may carry them even farther. 
Peter Raven of the Missouri Botanical 
Garden has suggested that the New 
Zealand alpine flora evolved, in part, 
from seeds carried by the stormy west- 
erly winds from the mountains of Aus- 
tralia and even from Patagonia. Long- 
distance dispersal by birds is very 
difficult to prove, but there is abundant 
evidence from the distribution patterns 
of certain species of birds and plants to 
suggest that such dispersal is frequent 
enough to account for some wide distri- 
butions, particularly bipolar ones. 
In considerations of the dispersal of 
arctic and alpine plants, the importance 
of the comings and goings of the great 
continental ice sheets cannot be overes- 
timated. When the continental glaciers 
advanced, arctic species moved slowly 
southward, forming a band of tundra 
across the northern plains of North 
America as their seeds were dispersed in 
front of the ice. Brainerd Mears, Jr., of 
the University of Wyoming has recently 
shown excellent geologic evidence that 
the present grasslands of Wyoming were 
tundra during full glacial times. 
Long mountain systems, such as the 
North and South American cordillera 
(consisting principally of the Rocky 
Mountains and the Andes), provide ex- 
cellent pathways between the Arctic 
and the subantarctic region of Patago- 
nia. During the Pleistocene, the continu- 
ing growth in elevation of large moun- 
tain ranges was crucial to the evolution 
and spread of cold-climate plants. When 
mountains reach sufficient height, they 
offer the challenge of open lands to 
those upward-migrating species that 
have enough genetic diversity to estab- 
lish successful populations above tim- 
berline. Once in an alpine situation, a 
species can adapt by ecotype formation 
and, with the aid of wind or birds, “is- 
land hop” along a mountain chain, 
jumping from peak to peak. Along the 
cordillera in the Northern Hemisphere, 
alpine species moved southward during 
glacial periods and northward during 
interglacials. 
The genus Saxifraga shows an in- 
triguing distribution of arctic and alpine 
species. More than 300 species exist in 
the Northern Hemisphere; only one of 
these, tufted saxifrage ( Saxifraga 
cespitosa), extends into the Southern 
Hemisphere. Juliana Mulroy of Denison 
University has studied the ecology of 
plants of the S. cespitosa complex from 
the High Arctic down the cordillera to 
Tierra del Fuego. The species extends 
from the Arctic down the Rocky Moun- 
tains to San Francisco Peaks, Arizona. 
Although not common anywhere along 
this route, S. cespitosa can be found 
here and there on some isolated peaks 
and ranges. Then, after a 4,000-mile gap 
Tufted saxifrage grows from the High 
Arctic down the Rockies to Arizona 
and along the Andes from Ecuador to 
Cape Horn. Long ago, migrating birds 
may have dispersed its seeds from 
Alaska as far south as Argentina. 
Fred Bruemmer 
86 
