Upper Treeline and Alpine Tundra - Mountains and Plains: The Ecology of Wyoming Landscapes - page 241

the response of tundra plants to nitrogen enrich-
ment is currently under investigation. University of
colorado ecologist timothy Seastedt and his associ-
ates have observed that willows are expanding their
distribution in the alpine zone, possibly because of
the combination of added nitrogen and climate warm-
ing. 43 this result might be expected, he suggests, in wet
areas that are protected from wind scour, that is, places
where organic matter accumulates as well as nitrogen-
enriched snow and water. the more exposed dry mead-
ows and fellfields would receive less nitrogen and would
be less likely to change. that's the hypothesis.

higher precipitation (see chapter 3). tree establishment
could be favored by both warmer temperatures and less
drought stress, but apparently such conditions have
not yet developed in some areas, such as in northern
colorado . 49 Moreover, with ongoing disturbances (such
as frost heaving, burrowing by small mammals, wind
abrasion, and herbivory), the chances of new tree seed-
ling establishment in the tundra are still low—and prob-
ably will be for many years to come. 50 A lpine ecologist
christian Körner concluded in 2012 that the advance
of treelines will always lag behind climate change by
at least 50-100 years, considering how long it takes for
new seedlings to become established and grow to tree
height in the alpine environment—even if it is slightly
warmer.
But not all alpine treelines are the same. For exam-
ple, new tree seedlings have been found above treeline
on relatively moist sites in the Front Range of colo-
rado. 51 Also, in the Uinta Mountains of Utah, photos
taken of the same place in 1870 and 2000 indicate a rise
in treeline of 180-550 feet. 52 in the same area, plants
other than trees have been observed moving upward in
elevation through seedling establishment. Such upward
migration might have occurred even without the cur-
rent warming trends, but vegetation response to warm-
ing clearly varies considerably.
As pertinent to the effect of warming on alpine plant
distribution is the paradox that earlier snowmelt in the
spring from warming increases the probability of frost-
related plant death or damage. Some alpine plants are
sensitive to cold temperatures after they begin growth,
which is delayed by snowcover. if the snow melts ear-
lier, frost damage is more likely. 53 Global warming that
causes higher mortality could retard the rate at which
treeline plants respond. Also, earlier snow-free dates
could lead to colder soils and more cryoturbation—two
factors that reduce the chance of seedling survival.
climate warming also will affect alpine animals that
are restricted to mountain tops. on mountains with less
land in the alpine zone, the populations of alpine species
are smaller; and if treelines advance upward because of
climate change, their populations will become smaller
still, increasing the chances of local extinction. Ani-
mals on one mountain may be able to travel to others
nearby if they can fly or if they are able to survive or

Advancing Treelines and Climate Change

it is well known that treelines have moved up and
down during the past 10,000 years (see chapter 2). 44 B ut,
if snowmelt is occurring earlier in the spring because
of climate warming, and glaciers are retreating, is the
alpine treeline moving upward throughout the region?
Answering this question has been the subject of con-
siderable research, especially since the mid-1990s. 45 the
results are mixed and offer an interesting example of
the complexities of interpreting the effects of climate
change. in general, the same results should not be
expected for alpine treelines everywhere.
Fundamental to treeline advance is the ability for
new tree seedlings to become established higher on the
mountain than their parent trees could, that is, higher
than the krummholz that already exists at treeline . 46 in
some places, newly established tree seedlings in the tun-
dra have been hard to find, suggesting that trees at this
elevation are not a good indicator of climate change . 47
How could this be? one study in Rocky Mountain
national Park concluded that the treelines are relictual—
they persist because adult trees tolerate the present-day
climate, even though the more delicate seedlings can-
not . 48 i in other words, the tree seedlings that founded
the tree islands of today became established at a time
when the climate was more favorable. numerous seed-
lings were found invading openings in the subalpine
forest below, especially in moist areas, but hardly any
were found in the drier openings at treeline or above.
in addition to warmer temperatures, one of the
predictions for climate change in the alpine zone is