To find out how the forest ecosystem responds to landscape-scale forest restoration efforts, the Ecological Restoration Institute at Northern Arizona University and Salt River Project are teaming up in the nation’s largest forest restoration project ever attempted.

The half-million-dollar NAU-SRP study will involve computer modeling to understand what happens to hydrologic systems and other natural resources such as wildlife habitat and understory vegetation under different forest-restoration treatments over time. Treatments include thinning and prescribed burning to mimic the intensity, frequency and impact of natural fire. It also will include a systematic review of published literature on restoration research and compare pre-settlement and future forest scenarios, as well as impacts from crown fires on watersheds using fire-severity data from the June 2011 Wallow Fire.

Prior studies have shown that much of Arizona’s ponderosa pine forests have hundreds of trees more per acre than existed prior to 1900, affecting the natural processes of a healthy ecosystem. These overcrowded forests are providing fuel for unnaturally intense wildfires.

According to Wally Covington, principal scientist on the project and executive director for the ERI, this study will help provide a better understanding of how denser forests impact rerouting of snowfall, and the results can help land-managers make science-based decisions that benefit watersheds in the massive 2.4 million-acre forest-health project across the Mogollon Rim.

“The ability of Arizona’s forests to provide high-quality water has been compromised by degraded forest health,” Covington said. “In the last century, Arizona’s ponderosa pine forests have become denser because of fire suppression, grazing and past logging practices. Connecting tree canopies intercept snow, where it evaporates before reaching the ground. Anticipating there is less snow accumulation and snow retention, what is the impact on stream-flow runoff and groundwater recharge?”

While there are existing data about historical conditions and restoration treatment responses at scales of about 1,000 acres, the larger scale of this project is new territory.

“Wildfires are operating at the scale of 500,000 acres, so we need to broaden our understanding of ecosystem function and restoration at similar scales,” said Dave Huffman co-investigator and director of research and development for ERI. “Simulation modeling is one of our best tools to try to understand climate change and try to get ahead of these issues.

“Stand-replacing crown fires, like the Wallow Fire, burn up the vegetation and can make the soil hydrophobic, so it doesn’t absorb water for a period of time after the fire,” Huffman added. “As a result, monsoon rains in the high country following these intense wildfires can produce serious erosion, moving tons of soil, downed logs and ash downstream where they clog up lakes and reservoirs.”

Such sediment deposits create a number of issues, said co-investigator Abe Springer, a hydrogeologist with NAU’s School of Earth Sciences and Environmental Sustainability. “One immediate concern is for the health of aquatic organisms when there’s a change in water quality. Another is the sediment build-up in reservoirs. That can impact the water supply by decreasing the ability of the reservoir to store water.”

Arizona’s forested watersheds contribute nearly 90 percent of the state’s total stream flow and serve as important recharge areas for large regional aquifers, according to a report by the Water Resources Development Commission.

Findings from the NAU-SRP hydrologic study are expected to have far-reaching implications, helping land managers with large-scale forest-restoration projects across the Southwest.