Impact of Climate Change on NA Forests Fire Courtesy of Tom Swetnam, U of Arizona, LTRR.

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