Woody Encroachment Intensifies Deep Soil Drying at Daily, Seasonal, and Decadal Scales

The conversion of grasslands to shrublands, known as woody encroachment, has increased vegetation water use, particularly in mesic systems. However, declines in soil moisture due to woody encroachment have not been extensively explored. This study examines the impacts of woody encroachment on the depth and degree of soil drying in a mesic tallgrass prairie in Kansas, USA. We compared soil drying beneath roughleaf dogwood (Cornus drummondii) and non-encroached tallgrass prairie using half-hourly measurements of soil moisture from 2021 to 2024 (event scale), electrical resistivity in June and October of 2022 (seasonal scale), and neutron probe measurements collected monthly between 1984 and 2021(decadal scale). Across all time scales, we found increased soil drying beneath shrubs compared to grasses, particularly in deeper layers. Soil moisture declined up to 20% more at 60 cm depth beneath shrubs compared to grasses after individual storm events, whereas declines at 15 cm were similar beneath shrubs and grasses. Electrical resistivity imaging suggested double the depth and degree of seasonal soil drying beneath shrubs compared to grasses. Over nearly four decades, woody-encroached catchments experienced a greater degree of soil drying than grass-dominated catchments, especially at depths below 1 m. The intensification of soil drying, combined with larger storms in mesic grasslands, is likely to have consequences for deep carbon storage and export and for groundwater recharge in woody-encroached grasslands.