UNDERSTANDING SOIL MOISTURE DYNAMICS IN FORESTS RECOVERING FROM WILDFIRE USING ELECTRO-MAGNETIC INDUCTION

Increased wildfire activity in the western United States is a substantial environmental and societal problem expected to worsen with climate change; thus, it is important to better understand recovery of fire-prone landscapes and soils, especially after repeated fires. Three 0.25 ha plots with distinct burn histories (burned 2015 only, 2004 and 2008, 2008 only) and an unburned site, all of similar elevation, slope, and aspect, were established in Gifford Pinchot National Forest on the southern flank of Mt. Adams. Soil moisture and temperature were monitored at each plot (4 sensors placed at 10cm and 50cm depths) during fall 2016 and spring-summer 2017, sites’ topography and vegetation mapped, and monthly electro-magnetic induction (EMI) geophysical surveys (Dualem-1H, to 1.6 m depth) conducted in summer 2017. Samples of the top 30 cm of soil were collected across each site (n= 15/plot) using a response surface sampling design based on EMI surveys, to be sure to sample the range of soil conditions throughout the sites. Bulk soils were 15% or 8.7% organic matter by weight at unburned and burned sites, respectively, were all sandy loam or loamy sand, and of low salinity (average saturated paste extract conductivity 131.56 μS/cm). Variations in apparent electrical conductivity (ECa) from EMI surveys therefore related to variations in soil water content. Compared to the unburned control, burned plots had lower ECa (water content) in the top 30cm and higher ECa (water content) deeper in the profile. Average ECa below 80 cm was inversely related to time since last burn, suggesting less deep soil moisture storage after longer post-soil regrowth. The plot affected by both 2004 and 2008 fires was higher in ECa (water content) both at the surface and at depth than the plot affected by only the 2008 fire, suggesting repeated fire occurrences may have compounding effects on soil properties.