Impacts of Climate Change and Land Use  on the Southwestern United States

Impacts of climate change on life and ecosystems

Forest Fires and Drought in the U.S. Southwest

by
Mark W. Patterson
Department of Geography and Regional Development
The University of Arizona


 

 

Drought and fires

Forests in the southwest have recently experienced fires of unprecedented intensity and extent, which has increased the vulnerability of people using forest resources. To some degree this is a result of forest management practices that have allowed decades of dead wood (fuels) to accumulate, and thus increase the potential for large, destructive fires. But climate variability, in the form of prolonged periods of drought, has also left forests in tinder dry conditions, and thus more susceptible to more intense fires. As public agencies entrusted with the management of forests, change management practices to those which recognizes the role of fire in maintaining forest health and suppression is de-emphasized, the impacts of climate variability on fire management will become more pronounced.


Climate variability in the southwest is often characterized by long periods of drought. The Palmer Drought Severity Index (PDSI), commonly used to monitor regional climatic conditions, is a long term measure of dryness or wetness of a region. As a long term measure, single precipitation events seldom drastically affect the PDSI. The PDSI is based on the relative dryness or wetness of an area, so that areas with differing water budgets may be compared. There are 11 divisions of this index, ranging from extreme drought (<-4.0) to extreme moist spell (>4.0). These divisions are illustrated in the table at right. At the national scale, the southwest is most susceptible to drought events. The map below shows a 45-year composite of the PDSI of the U.S. with the southwest region scoring the lowest (driest).

Drought conditions do influence the occurrence of forest fire events in the southwest. Data collected from tree-ring analyses were used to reconstruct the PDSI and fire events. As shown in Figure 1, the relationship between the PDSI and the areal extent of forest fires, there is evidence of a strong inverse relationship between the two. That is to say, larger fires tend to occur when the PDSI is lower.

Using data from dendrochronology studies again, Figure 2 displays a fire frequency time line for the southwest. For the first 200 years, forest fires occurred frequently until the early 1900s when fire suppression was actively engaged.



Who is vulnerable?

Recent large fires, such as the Rattlesnake Fire in the Coronado National Forest (AZ) (28,000 acres) and the Coffee Pot Fire in the Sandia National Forest (NM) (22,000 acres), have draw attention to the vulnerability of people. Those vulnerable are the people who own homes in and adjacent to forested land and the recreational users such as hikers, campers, boaters, bird-watchers and hunters. Both fire management practices and climate variability affect these groups. Since fuels have accumulated on the forest floor as a result of suppression efforts, the potential for large, destructive fires have increased. Forest fires often translate into a loss of homes and recreational opportunities. Moreover, seasonal climate variability in the form of drought conditions sometimes results in the restriction of camp fires, which often detracts from the camping experience, or in worse scenarios, results in the closure of the forest.

Humans are not the only group vulnerable to climate variability and fires. The forests are home to many flora and fauna, some of which are on the endangered species list. Such species include the Mt. Graham Red Squirrel and the Mexican Spotted Owl. Drought and fire events can destroy wildlife habitat, which in turn may severely impact animal population dynamics.

 

Ecosystem Management

Public resource agencies are currently shifting their fire policies from complete suppression to a recognition that fire is an integral component of the landscape. This shift in policy direction falls under the rubric of ecosystem management. While debates continue over just what ecosystem management is, resource managers agree there is a need to try to return forests to those conditions that existed prior to suppression activities, when forests were "more healthy". Pre-suppression forests experienced fires more frequently, but these fires were less destructive. They served as a means keeping fuels from accumulating on the forest floor and maintaining low stand density. Ironically, fire is seen as a means accomplishing this goal. Prescribed natural fires (PNF) (fires started by nature and permitted to burn) and management ignited fires (MIF) (fires intentionally started by agency personnel and permitted to burn) are two ways in which fire is being re-introduced into the forest ecosystem. For example, on June 24, 1997 the USFS had a 20,000 acre MIF in the Peloncillo Mountains in southwest New Mexico. But these types of fire are dependent on climatic conditions, as factors such as the number of days since the last precipitation event, the relative humidity and wind speed and direction must be taken into account before lighting MIFs or suppressing potential PNFs.

Ultimately, forests in the southwest can be returned to pre-suppression conditions, but resource managers must realize that climatic conditions are different today than they were 100 years ago. This means that a forest, in which fire is not suppressed today, will be different than a forest of a century ago. Climatic variability has altered the forest equilibrium. If we set the goal of returning forests to past conditions, climatic variability must be accounted for, and this will require additional inputs. Is society will to pay for these inputs? As resource agencies struggle with identifying, defining and implementing an ecosystem management plan several issues will surface.

 


Figures


Figure 1.


Figure 2.


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