2011 - Cooperative Institute for Research in Environmental Sciences ...

Carol Wessman
with graduate students
Impacts of Multiple Disturbances and
Their Interactions in Subalpine Landscapes:
Blowdown, Logging, Fire and Beetle Kill
FUNDING: CIRES, CU-BOULDER ECOLOGY & EVOLUTIONARY
BIOLOGY, NATIONAL SCIENCE FOUNDATION
Climate change and
human activity are rapidly
changing disturbance regimes,
likely leading to forest
disturbances of greater
frequency, extent, intensity
and variety. Future disturbance
interactions may be
unprecedented and unpredictable.
Cascading largescale
disturbances, such
as drought, insects and
fire, will interact to shape
future forest landscapes
and ecosystem services by
altering successional rates
and pathways.
Colorado’s Routt National
Forest has experienced
several catastrophic
disturbances over a short period: record windthrow in 1997;
salvage logging from 1999–2001; and regional wildfire in
2002. Epidemic spruce-beetle and mountain-pine-beetle
infestations are widespread. Historical records of such disturbance
complexes are rare and lack information on processes
important to ecosystem recovery.
The objective of a 2010 study was to determine if disturbance
history in subalpine forest influenced characteristics
of a subsequent disturbance—possibly creating a novel
disturbance with effects significantly different from what
would be expected from the final disturbance alone (fireonly).
Ninety-nine study plots were established on a gradient
of disturbance-interaction severities. Fire-only areas provided
a baseline fire response, and salvage logging served as an
experimental treatment of pre-fire fuel reduction. Fire-effects
modeling indicated that the combination of severe blowdown
and fire created an uncharacteristically long-lived fire;
geographic information system (GIS) analyses demonstrated
an increase in patch size of areas experiencing both severe
blowdown and fire (thus, requiring long distances for seed
dispersal) in contrast to fire alone. These two characteristics
directly impact the two major fire-resilience mechanisms of
subalpine forest: cone serotiny and seed dispersal. Consequently,
increasing blowdown severity prior to the fire was
significantly correlated with decreasing conifer regeneration,
and resilience was crippled in areas where atypical fire characteristics
resulted from the disturbance interactions. Due to
the ability of alternate cover types (such as aspen and grassland)
to exclude seedling establishment, substantial future
recruitment is unlikely, leading to long-term changes in the
spatial heterogeneity of regional composition and function.
58 CIRES Annual Report 2011
Ph.D. student and CIRES Graduate Fellow Brian Buma (left) and MA
student Adam Markovits in an undisturbed subalpine forest, Routt
National Forest, Colorado.
PHOTOS BY CAROL WESSMAN
Severely blown-down forest created high-fuel loads going into the 2002
fire. The interaction of the blowdown and fire disturbances damaged
forest-resilience mechanisms, resulting in little to no conifer regeneration
eight years after the fire.