Climate Change and the Rocky Mountains - Impact of Global Changes on Mountains: Responses and Adaptation - page 440

massive insect outbreaks. In the past several decades, wildland fi re in the
Rocky Mountains has increased in frequency and scale, particularly in
Colorado, Wyoming and Montana where signifi cantly fuel loads have had
longer warm seasons to dry out (Westerling et al. 2006). Although part of the
more active fi re seasons and potential for future increases in fi res are due
to past forest management and fi re suppression policies, longer growing
seasons have contributed tree growth and additional fuel as well. Fires are
now four times more frequent and burn more than six times the annual
average area of past years (Westerling et al. 2006). The average fi re season
now lasts 78 days longer than the long-term mean. By 2050 the area burned
by wildland fi res is expected to double in the northern U.S. Rocky Mountains,
especially in August, due to higher temperatures (Yue et al. 2013). Increased
fi re frequency also means that a larger percentage of the mountain landscape
has had recent fi res and this affects numerous ecosystem dynamics such as
soil instability, debris fl ows, canopy interception of snow, changed albedo
and stream turbidity. With hotter and longer summers projected in the
next decades, Rocky Mountain ecosystems are likely to become more fi re
dominated and cause signifi cant social disruption and fi nancial costs. But
larger and more frequent fi res will also transform many mountain areas
because the current dominant vegetation will not necessarily re-establish
after fi res, even decades later. Many forests can persist for centuries after
the climatic conditions under which they were established have changed.
Climate warming that has already occurred in the Rocky Mountains means
that once a catastrophic wildland fi re has burned an area suffi ciently to
remove most of the vegetation, the formerly dominant tree species may not
be able to re-establish and species adapted to warmer conditions and more
frequent fi re will be the benefi ciaries. These landscape scale transformations
will, in many cases, constitute thresholds that have been crossed and the
new vegetation communities and dynamics will be permanent from a
human perspective.
Although episodic forest insect outbreaks are a common feature of
mountain ecosystems in the Rocky Mountains, the signifi cantly larger
scale and length of recent outbreaks has exceeded the norm. Logan et al.
(2010) and others have tied warmer winters to larger overwintering beetle
populations, faster lifecycles for some species, and more prevalent drought-
stressed trees less able to defend against beetle infestation. The mountain
pine beetle killed between 2-3.3 million hectares of mostly lodgepole pine
forests in the western USA in a 13-year outbreak. In the Rocky Mountains
it appears that beetle species have benefi tted the most from fewer extreme
cold events during winter but other forest insects such as spruce budworm
have also increased in prevalence.