Awada et al. (2026) Influence of oak trees on soil hydraulic characteristics: A spatial analysis in a Mediterranean silvo-pastoral ecosystem

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-01-05
• Authors: Hassan Awada, Simone Di Prima, Pier Paolo Roggero, Mario Pirastru
• DOI: 10.1016/j.jhydrol.2026.134922

Research Groups

• Department of Agricultural Sciences, University of Sassari, Italy
• National Biodiversity Future Center (NBFC), Palermo, Italy
• Université de Lyon, UMR5023 Ecologie des Hydrosystèmes Naturels et Anthropisés, CNRS, ENTPE, Université Lyon 1, Vaulx-en-Velin, France
• Desertification Research Group, NRD, University of Sassari, Italy

Short Summary

This study investigates the spatial variability of topsoil hydraulic properties in a Mediterranean oak-wood grassland, revealing that oak trees significantly enhance saturated hydraulic conductivity (Ks) under their canopy compared to open areas. The research highlights a fine-scale spatial gradient of Ks around individual trees, emphasizing the need for detailed sampling strategies to accurately represent hydrological processes in such heterogeneous ecosystems.

Objective

• To evaluate how isolated oak trees affect the spatial variability of soil properties, particularly saturated hydraulic conductivity (Ks), at both field and individual-tree scales in a Mediterranean oak-wood grassland in Sardinia, Italy.
• To inform the design of monitoring schemes for soil hydraulic properties to obtain representative estimations in heterogeneous, tree-influenced environments.

Study Configuration

• Spatial Scale:
  • Field scale: 6.5-hectare Mediterranean wooded grassland in northeastern Sardinia, Italy (Coordinates: 9.2849806 E, 40.8185496 N, WGS 84). Measurements were taken at 12 positions (6 under tree canopy, 6 in open areas).
  • Individual-tree scale: Detailed assessment under a single isolated oak tree and its adjacent open space, with measurements at 11 positions along a transect from the trunk outwards.
• Temporal Scale: Field campaign documenting topsoil hydraulic properties. Data acquisition for infiltration tests was conducted until a stationary infiltration regime was achieved.

Methodology and Data

• Models used:
  • Fractional Wettability (FW) infiltration model (Di Prima et al., 2025) for modeling cumulative infiltration curves and estimating saturated hydraulic conductivity (Ks).
  • Transient infiltration equation (Lassabatere et al., 2006) for the wettable soil fraction.
  • Equation (Di Prima et al., 2021) for the water-repellent soil fraction.
  • Linear regression models for analyzing relationships between Ks, organic matter (OM), and bulk density (BD).
• Data sources:
  • Field measurements: Automated single-ring Beerkan infiltration tests (56 at field scale, 33 at single-tree scale) using 0.15 m diameter rings and approximately 0.0026 m³ of water. Initial soil moisture content was also determined.
  • Soil sampling: Undisturbed soil cores (0.05 m height × 0.05 m diameter) for dry bulk density (BD) at depths of 0.025–0.075 m, 0.125–0.175 m, and 0.225–0.275 m. Disturbed soil samples for particle size distribution (PSD) and organic matter (OM) content at depths of 0–0.15 m and 0.15–0.30 m.
  • Laboratory analysis: GSA −Particle Size Analyser for PSD; LECO CHN 628 for organic carbon content (converted to OM using a factor of 1.72).
  • Statistical analysis: Shapiro–Wilk test for normality, Mann–Whitney U test for comparing differences between treatments (p < 0.05).

Main Results

• Saturated Hydraulic Conductivity (Ks):
  • At the field scale, mean Ks under tree canopy (U) was 1.05 × 10⁻⁴ m s⁻¹, approximately 2.3 times higher than in adjacent open areas (O) (4.49 × 10⁻⁵ m s⁻¹).
  • At the single-tree scale, mean Ks under canopy was 1.04 × 10⁻⁴ m s⁻¹, while in open positions, it averaged 4.61 × 10⁻⁵ m s⁻¹.
  • Ks values generally decreased with increasing distance from the tree trunk, with the highest and most variable Ks values observed near the trunk.
  • The hydrological influence of the tree was suggested to extend beyond the canopy projection, as outermost under-canopy Ks values were comparable to those in the first meters of the adjacent open area.
• Soil Properties:
  • Bulk density (BD) was identified as the main driver of tree effects, with reduced BD under trees (4.3 % less at field scale, 9.2 % less at single-tree scale) compared to open areas. BD showed a clear trend of increasing with distance from the tree trunk.
  • Ks showed a moderate negative relationship with BD (R² = 0.53) at the single-tree scale.
  • Organic matter (OM) was generally higher under trees, peaking near the trunk, but differences between U and O were not statistically significant at the field scale. At the single-tree scale, OM was 14.4 % higher under the tree.
  • Particle size distribution showed significant differences in sand (p = 0.0247) and fine silt content (p = 0.0411) between U and O positions at the field scale.
• Infiltration Dynamics: Cumulative infiltration curves exhibited diverse shapes (concave-to-linear and convex-to-linear), reflecting spatial heterogeneity in soil structure and localized water repellency, which were effectively captured by the Fractional Wettability (FW) model.

Contributions

• Provides a comprehensive understanding of the spatial variability of saturated hydraulic conductivity in Mediterranean oak-wood grasslands, highlighting the critical role of trees in shaping soil hydraulic behavior.
• Quantifies the significant enhancement of Ks under tree canopies and reveals a pronounced spatial gradient of Ks around individual trees, extending beyond the canopy projection.
• Emphasizes the necessity for sampling strategies of hydraulic properties in tree-influenced environments to explicitly account for fine-scale tree-induced heterogeneity, rather than relying on random measurements.
• Demonstrates that incorporating this fine-scale variability into hydrological models can substantially improve simulations of soil-water processes and predictions of land-use change impacts in tree-dominated Mediterranean ecosystems.
• Validates the effectiveness of the Fractional Wettability (FW) infiltration model in characterizing complex infiltration dynamics in soils with mixed wettable and water-repellent conditions.

Funding

• National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 – Call for tender no. 3138 of 16 December 2021, rectified by Decree no. 3175 of 18 December 2021 of Italian Ministry of University and Research funded by the European Union – NextGenerationEU; Award Number: Project code CN_00000033, Concession Decree no. 1034 of 17 June 2022 adopted by the Italian Ministry of University and Research, CUP J83C22000870007, Project title “National Biodiversity Future Center − NBFC”.
• PRIMA S1 2021 Salam-MED project (Sustainable land and water management in Mediterranean Drylands), coordinated by Pier Paolo Roggero (Grant number: 2123; CUP number: J83C21000200006).
• PRIN 2022 PNRR—MILETO (P2022587PM), funded by the European Union—Next Generation EU (supported Simone Di Prima).

Citation

@article{Awada2026Influence,
  author = {Awada, Hassan and Prima, Simone Di and Roggero, Pier Paolo and Pirastru, Mario},
  title = {Influence of oak trees on soil hydraulic characteristics: A spatial analysis in a Mediterranean silvo-pastoral ecosystem},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.134922},
  url = {https://doi.org/10.1016/j.jhydrol.2026.134922}
}