Download Impact of Climate Change on W Forests

Impact of Climate
Change on NA Forests
Fire
Courtesy of Tom Swetnam, U of Arizona, LTRR
Fires and Ecosystem Health
 Fire is a natural part most western forest
ecosystems
 It plays an important role in maintaining the
health of forest ecosystems
 In fact, many forest types are dependent
upon a particular frequency and intensity of
fire for survival

Ponderosa Pine forests are an example of a
fire-dependent ecosystem
Fire regime of Ponderosa Pine Forests
 Under “natural” conditions,
ponderosa pine forests burn every
2-12 years
 These frequent, low-intensity fires
(surface fires) serve to:





Clean out understory thus reducing
fuel load
Enhance nutrient cycling
Promote germination of ponderosa
pine seeds
Maintained open forest structure
Maintained mountain meadows
Courtesy of Tom Swetnam, U of Arizona, LTRR
Fire regime of ponderosa pine
 How do we know the natural fire frequency
for Ponderosa Pine?

Dendrochronology (study of tree-rings!)
Sugar pine with fire scars labeled.
© A.C. Caprio
Causes of Fire in Ponderosa Pine Forests
 The cause of natural fires in Ponderosa Pine
is usually lightning strikes
 However, many of the frequent periodic fires
are also thought to be human-caused
Native American Fire Management
 Almost every Native American tribe has engaged in
active fire management of forests
 The purpose of these fires was to enhance their
quality of life within the environment

Based on personal or secondary accounts of Native
American fire management, fires were burned to
 Establish or keep diverse habitats such as meadows
interspersed with forests
 The forests productivity is greatest at the interface of
meadows and forests
Native American Fire Management
 Purposeful fires set by Native Americans differ from
natural fires by



the seasonality of burning
 Fires were set in early spring or summer, or in the fall
after the hunt and berry picking season was over
 Never in mid-summer when the forest was most
vulnerable to catastrophic wildfires
frequency of burning certain areas
 Selected areas were burned every year, every other
year or as long as a five years
the intensity of pattern and overall ignition patterns
 Frequent fires ensured low-intensity fires which served
to maintain the health and biodiversity of forests
Post-European Fire Management
Practices


The effects of forest management in the past
100 years have placed western dry forests at
risk of large, high-intensity fires.
Multiple interrelated factors are:



Logging large trees
Fire suppression
Livestock grazing
Effects of logging
 Effects of logging


Logging operations have historically
removed the largest, most fire resistant
trees
The young trees that replace the cut trees
are highly susceptible to fires and serve as
fire ladders leading to crown fires
Effects of fire suppression
 Fire suppression has led to


High density fires with high fuel loads
Infrequent, high intensity fires
 High temperature fires are destructive to
forest ecosystems


Leads to crown fires which kill mature trees
Causes soil sterilization which kills all seeds
and organisms so that forest regeneration
takes decades
Effects of grazing
 Livestock grazing on public lands has
severely reduced the amount of grasses
 This leads to

More severe fires since they are only able
to burn with a significant build-up of woody
debris
Climate Change and Fire
 The following have been observed and
predicted:
 Increased wildfire frequency and intensity


Drier soils
Tree mortality due to global warming type
drought and/insect infestation will increase fire
severity
 Longer fire season due to longer growing
season
Climate-fire linkages
 Synchronous large fires in the Southwest
over three centuries have been associated
with the high-SO phase (dry phase) and
deficient spring precipitation
 This implies that seasonal climate, and not
just fire weather, determines burning of
vegetation on a subcontinental scale.
 With a drier, warmer climate forecasted for
the SW, this means more frequent large fires
Climate-fire linkages
 Conversely,

Large-scale severe
fires can accelerate
global warming
since carbon is
released into the
atmosphere

Especially if tree
mortality is due to
fire since stored
carbon is released
Courtesy of MODIS Rapid Response Project at NASA/gsfc
Forest Restoration
 What can be done?
 Reintroduce fire as a natural component of the
ecosystem through controlled burns
 Reduce the number of small trees




This will decrease risk of severe fires
Decrease competition with mature trees increasing
drought-related stress and mortality
Small trees comprise ~90% of trees in the southwest.
Larger trees are relatively rare after 100 years of
logging in forests
 Stop or reduce grazing in our forests
 Enhancing habitat for imperiled or endangered
species