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Climate Trend and Forest Fires
by Brian Petersen
 The past decade
has seen an unusual number of large, stand replacing fires in the western
forests of the United States. They are unusual in that tree ring data from
the time period 1500-1900 shows a trend of low intensity fires occurring on
a frequent basis that helped to maintain open stands and preclude large
stand replacing fires.
Forest
fires in western ponderosa pine forests have increased in size and severity
and have garnered substantial media and political attention. The US now
spends over $1.5 billion annually on fire management and has adopted the
Healthy Forest Restoration Act (HFRA) in an effort to curb fires. Many
people believe that decades of fire suppression have created this current
crisis. The HFRA prescribes thinning as the management tool to reduce fire
loads, reduce catastrophic fires, and to reestablish ecological balance.
However, a
group of researchers analyzing post fire related deposits, or debris flows,
have proposed that fire suppression alone may not be the only culprit for
the rise in catastrophic fires this century. Establishing a chronological
record of erosion and debris flows over the past 8,000 years has uncovered a
climactic pattern to fire history.
 Using charcoal
data found in these debris flows, the researchers correlated such flows with
climate records. They found that colder periods were characterized by low
intensity fires and small debris flows, whereas warmer periods experienced
more catastrophic fires, followed by large debris flows. Cooler, wetter
conditions maintain moisture in the canopy of trees and produce understory
growth that lead to low intensity fires and maintenance of open stands.
Warmer, drier conditions, particularly extended periods of drought, cause
canopy moisture to decrease to critically low levels and increase the
likelihood of catastrophic fire.
During the
Medieval Climatic Anomaly (1050-650 years before present) warm temperatures
led to multi-decade droughts, severe fire conditions and large debris flow
events. In contrast, the Little Ice Age (from 1500-1900) produced high
frequencies of low intensity burns. Current management strategies to reduce
catastrophic fire, including the HFRA, use the Little Ice Age conditions as
the baseline. Climate change this past century has created much drier
conditions and a higher frequency of catastrophic fires that thinning will
not likely be able to reduce.
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