Determining Soil–Tree–Grass Relationships in a California Oak Savanna Using Eco-Geophysics

Savanna ecosystems have long fascinated ecologists due to the codominance of trees and grasses. This codominance is often explained by the vertical juxtaposition of rooting depths leading to niche separation for water uptake. The tree–grass spatial patterns observed are often investigated within the competition, disturbance, and resource heterogeneity framework. Ecohydrologic studies are developing models to describe patterns based on this framework and the complex interplay between climate, soil moisture, and resource heterogeneity. A common impediment to unraveling the relative importance of each of these factors is a lack of detailed spatial and temporal data describing resource heterogeneity. In this research, we investigated the use of electromagnetic-induction (EMI) imaging as a way of determining soil spatial resource heterogeneity. The aim was to determine if EMI signal response, reflecting soil texture and moisture, can provide a rapid method for identifying soil spatial resource heterogeneity, thus providing a novel method to help differentiate between the importance of competition, disturbance, and resource heterogeneity in controlling tree–grass pattern development. We found a significant difference (t = 10.18, P = 1.57 × 10⁻²¹) between signal response under grass and trees. Higher bulk soil electrical conductivity (ECa) values occurred under grass (∼32 mS m⁻¹), indicating more clay, while lower ECa values (∼21 mS m⁻¹) occurred under trees where clay content was less. Significant ECa differences were also observed between the oak (Quercus spp.) and buckeye [Aesculus californica (Spach) Nutt.] tree communities, but not between the evergreen and deciduous oak communities. Soil ECa provides a rapid measurement method to identify soil resource heterogeneity in savanna tree–grass systems and could provide a vital tool for woodland restoration planning.