Differences in soil water retention properties and plant available water below trees and grasses in a Mediterranean savanna

Feedback loops between plants and soil shape and stabilize plant communities. In savanna-like landscapes, which are common in arid and semi-arid regions, trees and grasses coexist at close spatial scales. These different growth forms can influence soil formation and properties within just a few meters of each other.

In this study, we investigate soil hydraulic properties in an extensively managed Holm oak savanna-like ecosystem (Dehesa) in central Spain by comparing soils beneath trees and in adjacent open grass areas. We analyze saturated hydraulic conductivity, soil water characteristic curves, derived parameters such as field capacity and permanent wilting point, and associated soil texture and organic carbon content. In addition, we analyze a 10-year time series of in situ soil water content and micrometeorological variables within microhabitats to determine whether differences in the static properties also translate into water availability differences within the ecosystem.

On average, the topsoil below trees contained 6.2% more pore space within the range of plant-available water than the topsoil below open grass areas. This was associated with, and likely driven by, higher levels of organic carbon beneath the trees. There was no significant difference in clay content between the two microhabitats. 

However, field observations of soil moisture showed high heterogeneity, with the soil beneath the trees not remaining significantly wetter than in the open area despite the higher storage capacity and reduced radiative energy input due to shading. Data from two eddy covariance towers showed that, unlike grasses, trees sustain transpiration throughout the year, suggesting enhanced water uptake near the trunk.

Together, these results illustrate how different vegetation types affect the same soil just a few metres apart. They also show that, although trees increase soil water storage capacity, it remains unclear whether this positive effect is offset by the large amounts of water extracted by trees and higher interception losses, ultimately leading to the soil being similarly dry beneath trees as in the open area during the Mediterranean summer.