The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 219

homes, campgrounds, and along roads and trails to
prevent damage or injury when the dead trees fall. Un-
fortunately, cutting and burning beetle-killed trees after
the needles have turned red will do nothing to slow a
mountain pine beetle outbreak, because the new cohort
of beetles has already emerged from the tree by the time
the needles dry and begin to fall. Removing beetle-killed
trees is also questionable as a fire mitigation method,
because, as discussed, beetle-caused mortality does not
appear to increase fire hazar d. 82 A fter 20-50 years, when
the dead trees have fallen and the surviving small trees
are again forming a forest canopy, the heavy fuel load
will have the potential to support an intense fire. How-
ever, it is not known whether future fires in forests with
many beetle-killed trees will actually be more intense
than fires in a green forest where that has not happened.
Hence, at this time no clear guidelines exist for whether
trees killed by mountain pine beetle should be removed
from locations that are distant from homes, roadsides,
and other obviously vulnerable resources. Doing so may
be more expensive than can be justified, considering that
the next fire or beetle outbreak will probably be influ-
enced more by weather patterns than by fuel conditions.
Again, however, the spruce beetle presents a different
problem, and the removal of dead trees may be advisable
when they are a threat.

growing; other clones appear to be dead or have very
few sprouts . 84
At the same time that aspen appeared to be declin-
ing at low elevations, new aspen seedlings that became
established in burned forests after the 1988 Yellow-
stone fires were growing best at higher elevations—in
some places higher than the pre-1988 range of aspen
in Yellow stone. 85 t he same pattern has been seen after
recent fires in the canadian Rockies. in colorado's San
Juan Mountains, a comparison of historic photos from
the early 1900s with recent photos of the same land-
scape revealed new aspen clones at the upper treeline,
apparently the result of seedling establishment even
without any major disturbance by fire.86 . 86
the pattern of aspen mortality at lower elevations
and new recruitment at higher ones suggests that aspen
is already responding to the climate changes of the past
quarter-century. Such trends are likely to continue,
with some low-elevation clones dying as climate con-
ditions become less favorable there. Forest ecologists
and climate modelers have worked together to project
the future distribution of suitable habitat for aspen and
other plants in western north America. their approach
is to map the current range of aspen and to then
describe the extent of variation in climatic conditions
that exist throughout the current range, thereby creat-
ing a bioclimatic envelope. they then use climate models
to identify specific locations where suitable conditions
for aspen will be located in the future, assuming vari-
ous scenarios for climate change. For aspen, two major
expectations emerge from these initial efforts: (1) the
total extent of suitable habitat will likely decrease dur-
ing the twenty-first century, although aspen will persist
in small pockets throughout most of its current range;
and (2) new groves will most likely become established
at higher elevations (up to 1,000 feet higher than where
aspen grows now).
Similar studies have come to about the same conclu-
sion for most other Rocky Mountain trees . 87 notably,
because there is less land area at higher elevations, and
much of it lacks soil (such as on cliffs and talus slopes),
the area occupied by each species will be reduced. in
Wyoming, the amount of forested land could decline
from about 15 percent at the present time to 7-8 per-
cent. there could be exceptions to this trend for spe-
cific species. For example, Douglas-fir may be able to

Aspen Forests: Bellwether of Future Changes?

Quaking aspen is one of the most highly prized trees in
western north America, primarily for the biological and
aesthetic diversity it adds to the landscape. thus, it was
especially alarming when large aspen trees began dying
in 2003 and continued to do so through 2007, mostly
in colorado and Utah but also in Wyoming and other
western states (fig. 12.16) . 83 this widespread phenom-
enon has been called sudden aspen decline (or SAD).
For a time the cause of decline was unknown, but now
it appears that the severe drought from 2000 to 2002
coupled with unusually high temperatures was the trig-
gering factor. the greatest aspen decline has taken place
on south-facing slopes at lower elevations and on soil
types with low water-holding capacity, two observa-
tions that are consistent with the drought hypothesis.
the root systems and some small trees of many clones
survived and are now producing new sprouts that are