Upper Treeline and Alpine Tundra - Mountains and Plains: The Ecology of Wyoming Landscapes - page 234

periodically slides along a topographic depression that
extends into the subalpine forest, ending in the runout
zone. Depending on the amount of snow and other con-
ditions, avalanches can be gentle slides over short dis-
tances or thunderous events that break large trees in the
valley bottom. Sometimes several avalanches occur on
the same track in a single winter.
the vegetation of avalanche tracks is different from
the forest on either side or the alpine tundra above. in
the tundra, the track typically has sparse plant cover,
because the catchment area commonly has snow that
persists until midsummer, thereby shortening the grow-
ing season. Damage to small alpine plants is minimal,
because snow movement occurs within the snowpack
rather than at the soil-snow interface. Most physical
damage to plants occurs below the treeline, where tall,
woody plants are common. even there small trees and
flexible shrubs are not damaged. However, trees with
diameters greater than 4 inches usually break, because
boles of that size provide too much resistance to the
sliding snow. 14
thus, there is a vertical gradient in the plant growth
forms that is correlated with avalanche frequency. Small
plants and flexible shrubs dominate at the top, with trees
becoming more common lower on the track because of
longer periods of uninterrupted growth. the largest trees
occur at the bottom, which is rarely affected. Predictably,
the most severe disturbance occurs when trees are bro-
ken during large avalanches that extend to the runout
zone. Such disturbances occur every 50-100 years.

Fig. 14.5. White pine blister rust on a whitebark pine branch.
this exotic fungal parasite has killed a large portion of the
whitebark and limber pines in the Rocky Mountains and
is continuing to spread. the infection is fatal when the
parasite grows into the bark of the tree trunk. Photo by Anna
Schoettle.

cut cone-bearing branches. Grizzly bears can switch to
other food sources, but the loss of whitebark pine over
such a large area reduces habitat quality for this already
threatened species.
Millions of trees have been killed wherever five-
needle pines occur in north America, but the disease
currently is most severe in the Rocky Mountains. infec-
tion levels vary considerably, with the highest occurring
in close proximity to the rust's alternate hosts, which
include wild currants and gooseberries ( Ribes spp.) plus
a few other species . 16 Some populations of the trees
seem resistant to the disease, which gives conservation
biologists hope that future generations of the pines will
be more resistant. that seems to have happened around
the Great Lakes and farther to the east, where eastern
white pine is still common today, even though it was
often infected by the blister rust in the mid-1900s. A
complicating factor in western states, though, is that
whitebark and limber pine, even if generally resistant
to the rust, may nevertheless be sufficiently weakened
by the disease to be killed by the native mountain pine
beetle (see chapter 11). Research on this perplexing prob-
lem and the resilience of the ecosystem is ongoing. 17 As
at lower elevations, the nature of the ecosystem may
change considerably because of inadvertently intro-
duced invasive species.

White Pine Blister Rust at Treeline

A disturbing development is the invasion of an exotic
disease that kills whitebark pine—the common species
at treeline in the Greater Yellowstone Area and north-
ern Rocky Mountains. 15 Known as white pine blister rust
( Cronartium ribicola ), this fungal parasite also kills lim-
ber pine and bristlecone pine in the Southern Rockies
(fig. 14.5). All three of these pines tolerate severe envi-
ronments, and their peanut-sized seeds historically have
provided an important food supply for grizzly bears,
red squirrels, clark's nutcracker, and other animals (see
chapter 10). the bears are able to build up fat reserves
as they gorge themselves on the pine nuts, which some-
times are accessible on the ground because red squirrels