TIME-LAPSE ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT) REVEALS SEASONAL SOIL-WATER DYNAMICS IN AN EVERGREEN SHRUB-GRASSLAND ECOTONE

Coyote brush (Baccharis pilularis DC), a native evergreen shrub in many California plant communities, is increasingly targeted for removal in grassland restoration projects. Yet little is known about the implications of vegetation type on the soil-water dynamics which influence key fluxes like groundwater recharge, weathering rates, and carbon uptake and release. Electrical resistivity tomography (ERT) has previously been used to study soil hydraulics in forest, grassland, and cropland settings, and root structures in greenhouse settings. The current study is the first known to use ERT to image seasonal soil-water dynamics in a coastal prairie setting across well-established shrub-grassland ecotones. The study site, west of Santa Cruz (36.9711°, –122.0869°), is on a coastal marine terrace with a 2–9% slope. The soil, Pinto-Watsonville loam, is a ≥90,000 year-old mollisol developed from coastal sediments. The profile is bioturbated to ~60 cm and includes a clay-rich B horizon with maximum clay content at ~1 m. The climate is Mediterranean, i.e., warm dry summers and mild wet winters. 2-D electrical resistivity imaging was conducted eight times in 2015 on three 12-m transects spanning replicate ecotones. Each transect consisted of 48 semi-permanent electrodes spaced at 0.25 m, with 24 under shrub canopy and 24 in open grassland. A Syscal Pro (Iris Instruments, Orleans, France)* measured apparent resistivity using a standard Wenner configuration (225 quadrupoles). Cross-section images were generated using RES2DINV (Geotomo Inc.). Auger-based water content during the study period confirmed that electrical resistivity changes reflected soil water changes. Seasonal dry-down of soil beneath grass began about ten weeks earlier than beneath shrubs. Drying beneath grasses terminated abruptly at seed production and was restricted to the top 40 cm. Dry-down beneath shrubs occurred later but continued through the study period to the full study depth (~110 cm). Soil-water beneath grasses underwent greater seasonal variation and was spatially more uniform. Soils beneath shrubs remained wetter into the summer dry period and drier once rains began. Shrub soil-water was heterogeneous, varying with distance from root crowns, i.e., proximity to shrub roots. (*Trade names for identification purposes only.)