Upper Treeline and Alpine Tundra - Mountains and Plains: The Ecology of Wyoming Landscapes - page 235

Surviving in the Alpine Tundra

Annual plant growth of the tundra has been estimated
at 40 g/m 2 /year on dry, windswept sites and 300 g/m 2 /
year on more protected, relatively moist sites. 22 Such
growth rates, although low compared to subalpine for-
ests, are reasonably high, considering that the growing
season is only 30-75 days long.
Predictably, some unusual adaptations have evolved
to allow alpine plant growth. 23 F or example, all alpine
plants are capable of photosynthesis and growth at cold
temperatures, and many are drought tolerant. 24 ever-
green and wintergreen leaves are common, enabling
growth early and late in the season, and even while
still covered with up to 20 inches of snow. Root systems
tend to be shallow, probably to facilitate nutrient uptake
as decomposition in the surface soils occurs, and the
roots usually have mycorrhizae that greatly increase the
surface area available for nutrient acquisition. Alpine
plants also seem well adapted for mitigating the adverse
effects of a short growing season. For example, alpine
pennycress—a diminutive herb—grows from upper
treeline down into the foothills, but the higher popu-
lations complete their growth and reproduction more
rapidly than those at lower elevations . 25
Another plant adaptation is the cushion plant
growth form, with branches and leaves that grow in
dense mats close to the soil surface (for example, moss
campion, alpine forget-me-not, and alpine phlox). Soil
temperatures during the day can be surprisingly warm
because of the intense sunlight at high elevations. the
dense branches absorb heat from the soil—warmed by
the sun—and they slow the movement of air through
the plant, thereby slowing the rate of cooling. Dark color
and epidermal hairs also help maintain warmer leaf and
bud temperatures in this otherwise cool or even cold
environment. Some insects depend on the more moder-
ate microclimate of the cushion plants . 26
Reproduction of alpine plants is largely vegetative,
that is, sprouting from roots and rhizomes, because of
the difficulties associated with seedling establishment
in such a rigorous environment. nevertheless, many
plants have evolved special adaptations for seed pro-
duction. Some plants have unusually large and showy
flowers, presumably to attract scarce insect pollinators
during the short warm periods when the insects are
active. Flowers with large petals serve as windbreaks
for insects and are thought to provide a warmer micro-

Above treeline, many of the shrubs and herbs that sur-
vive are also found in the Arctic. Both kinds of tundra
have cool, short growing seasons. However, there are
prominent differences: the Arctic has long summer days
with little diurnal temperature fluctuation, low ultra-
violet and visible light intensities, and permafrost over
much of the landscape. in sharp contrast, alpine zones
have high intensities of ultraviolet and visible light,
shorter summer photoperiods, and great diurnal tem-
perature extremes in the summer—often reaching more
than 68°F during the day and dropping below freezing
at night. Also, permafrost is rare in the alpine tundra. 18
Wind and cold are clearly important aspects of the
alpine environment, but drought is another factor
affecting plant survival . 19 evapotranspiration is more
rapid because of lower atmospheric pressures at high ele-
vations, which increases the probability of plant water
stress. Also, water uptake by roots is slower when the
soil is cold. Various investigators have noted the similar-
ity of desert and alpine plants in terms of leaf size and
other structural features. nutrients can be limiting as
well, which is partially compensated for by the presence
of mycorrhizae and nutrient resorption prior to leaf fall.
notably, most of the root system, which has two to ten
times more biomass than the stems and leaves above-
ground, is in the top 4 inches of the soil, where nutrient
availability is highest and the temperature is relatively
warm during summer.
Arguably, the alpine tundra has the most severe
conditions for plant growth to be found anywhere in
the region: extreme temperature fluctuations during a
24-hour period (which is very difficult for most plants
to tolerate); frequent freezing and thawing; rapid rates
of drying (away from melting snowdrifts); nutrient limi-
tations; and a cool, short growing season. Still, several
hundred plant species tolerate this environment in
Wyoming alone. 20
the predominant growth forms of alpine plants are
perennial herbs (including forbs, grasses, and sedges)
and short woody or semi-woody shrubs. Most biomass
is belowground. Annuals are rare because seedling
establishment is difficult in the alpine environment. 21
Lichens are common, but they are not a dominant fea-
ture of alpine vegetation, as they are in the arctic tundra.