by Abe Springer, School of Earth Sciences and Environmental Sustainability, Northern Arizona University
A new day is dawning in the practice of managing forests in the Southwestern United States. A recently inaugurated forest restoration program, the Four Forest Restoration Initiative (4FRI), seeks to improve forest health on portions of the Coconino, Kaibab, Apache-Sitgreaves, and Tonto National Forests, along the Mogollon Rim in Arizona. A collaborative effort to restore forest ecosystems, 4FRI was born out of a need to reduce the risk of catastrophic wildfire brought by past forest management practices.
As a result of more than a century of fire exclusion, grazing practices, and timber harvest activities, Northern Arizona forests are overgrown with thin, unhealthy trees that are prone to catastrophic wildfire and subsequent catastrophic flooding. 4FRI and other recent forest management actions are designed to support mechanical thinning of the over-dense forest followed by a more frequent fire regime. These thinning projects are designed to support sustainable forest industries that maintain local economies while making forests more resilient.
Significant interest exists in generating better understanding of the ecohydrology of past and present forest management, as well as planned forest restoration actions. Ecohydrology is a field of inquiry that examines how ecosystem management practices affect associated hydrologic systems, including streams, springs, and groundwater.
In Arizona, from the 1950s through the 1980s, projects related to the program “Water for Arizona” carefully examined the responses of experimental watersheds to various forest harvesting practices. Although management actions observed on these experimental watersheds were different from the 4FRI actions, they provide information about the types of hydrological responses we might expect from restoration treatments. Observations from the Wallow, Rodeo-Chedeski, Schultz, and other large catastrophic fires over the past 15 years have revealed the significant costs to hydrologic systems and downstream water users that may be avoided through forest restoration.
Average annual precipitation for the Ponderosa pine forests of Northern Arizona is near 20 inches per year, which is a threshold amount to produce small but measureable increases in runoff and recharge from forest thinning. Around 80 percent of all precipitation that falls in these forests returns to the atmosphere through evaporation and transpiration by vegetation. Of the remaining precipitation, about 15 percent runs off, and between 2 and 4 percent infiltrates to recharge aquifers that supply water to groundwater-dependent springs and streams.
In the Beaver Creek Experimental Watershed, which receives more than about 20 inches of precipitation per year, a small but measurable increase in stream discharge was observed after removing more than 30 percent of tree basal area. This increase in discharge depends on the climate after thinning and the follow-up treatments. If precipitation in the years following the thinning is below average, or prescribed burns are not used to maintain the thinner forest, hydrological benefits either are not observed or only last a few years. Also, the watershed headwaters experience most of the increased stream discharge benefits. These effects may not extend far enough downstream to have a significant impact on water supply.
Although none of the historic experimental watersheds were designed to observe groundwater effects, aquifers are likely to have a similar response as stream discharge to forest thinning. In years following forest thinning, there would be a small but measureable increase in groundwater recharge and storage if the climate conditions are favorable. Recent research at Northern Arizona University (NAU) supported by Salt River Project (SRP) initiated an innovative recharge measurement study on thinned and unthinned forest stands to confirm this hypothesis. The study is continuing to measure the response of recharge to variability in climate.
Imbedded in the Record of Decision for the Environmental Impact Statement of the 4FRI is a new study of observational watersheds to gauge hydrologic responses to low-, medium-, and high-intensity forest restoration thinning treatments. Observational watersheds, roughly 1,000 acres in size, are located within the Sycamore Creek and Upper Lake Mary watersheds of the first quarter of the overall 4FRI area to be approved for treatment. The study is designed to collect background data prior to forest thinning and then observe the response to thinning and follow-up maintenance burn treatments. NAU designed this study with support from SRP, the City of Flagstaff, the Rocky Mountain Research Station of the U.S. Forest Service, and the Kaibab and Coconino National Forests. Support for some of the instrumentation has been secured, some has been installed, but more support is needed to comprehensively measure all components of the water and energy budgets.
In the 2012 general election, Flagstaff voters were presented a bond issue to provide funding for accelerating forest restoration in two critical watersheds. The potential for catastrophic wildfire in the Shultz Creek watershed poses a severe flooding risk because of impacts on the Rio de Flag that runs through town. Wildfire in watersheds on Mormon Mountain poses risks to the operation and maintenance of Upper Lake Mary, the City’s surface water reservoir. Research by economists at NAU determined there was a strong and significant willingness to pay for watershed services from Shultz Creek and Mormon Mountain. Support of the bond issue by 73 percent of Flagstaff voters confirmed that citizens recognize the value of the watersheds to the health and vitality of the City. The Flagstaff Watershed Protection Program (FWPP) serves as a model for innovative new financing for forest restoration actions, which is receiving national attention.
Many challenges still exist to improve and sustain the services that forested watersheds provide. Additional innovations in financing are needed to accelerate actions necessary to restore the forests of Arizona before additional catastrophic wildfires can destroy them and cause significant harm to their watersheds. Also, to assess whether or not these forest actions have the anticipated hydrological impacts, it is imperative that we carefully measure hydrological responses. The actions taken by such initiatives as 4FRI and FWPP provide tremendous hope for better managing forested watershed services in the future.