HYDROLOGIC RESPONSES OF A HIGH-ELEVATION SPRING-FED MEADOW TO FOREST RESTORATION

Over a century of preferentially managing forests in the Western U.S. for timber production has altered the forest ecosystems and dependent hydrological processes. Fire suppression in Ponderosa pine forests caused significant increases in tree density and encroachment of trees into meadows. The resultant increases in overstory evapotranspiration and snow sublimation dramatically decreased snowmelt recharge to both shallow and deep aquifers. Shallow aquifers supporting ephemeral springs are perched in volcanic and colluvial deposits over 1,000 m above the regional aquifer. The spring-fed meadow in Hart Prairie, Arizona, hosts the world’s largest Bebb willow community dependent upon the availability of the shallow, perched groundwater recharged from winter snowmelt. Ponderosa pines encroaching into the prairie and other factors have prevented new Bebb willow recruitment for over a century. Mechanical thinning and prescribed burning in 2013 returned the Hart Prairie springs catchment area closer to its pre-colonial ecosystem conditions. A flume was installed in 1995 to measure runoff from the spring catchment area. The historic flume discharge data were organized, daily discharge and volumetric information was calculated using R-Studio software, and the resulting data sets were evaluated for the number of flowing days annually. After the forest thinning in Hart Prairie, the number of flowing days increased indicating a possible increase in groundwater recharge correlating with a decrease in canopy cover and evapotranspiration. Preliminary rainfall-runoff analyses from the flume indicate an increase in aquifer recharge and runoff post forest treatment which might provide conditions sufficient to recruit and establish new Bebb willow trees. More years of post-forest thinning response are needed to determine if the hydrologic response to forest thinning is statistically significant.