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 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