Influence of oak trees on soil hydraulic characteristics: A spatial analysis in a Mediterranean silvo-pastoral ecosystem

Accurate prediction of soil water movement in complex environments requires detailed knowledge of the spatial and temporal variability of soil hydraulic properties, in particular hydraulic conductivity. The study presents the results of a field campaign documenting topsoil hydraulic properties in an oak-wood grassland (WG) in a Mediterranean ecosystem in Sardinia (Italy), a vulnerable ecosystem endangered by climatic and land-use changes. Emphasis is provided to the impact of the trees on soil hydraulic properties, through the assessment at the field complemented by a detailed investigation at the individual-tree spatial scale. Particle size distribution, organic matter, and dry bulk density were sampled, and infiltration was evaluated through Beerkan tests using single-ring automated infiltrometers. The data was processed with a recently proposed fractional wettability infiltration model to derive saturated hydraulic conductivity (Ks). Results revealed a complex spatial pattern of soil hydraulic properties strongly influenced by the tree presence. Mean Ks under canopy (U) was 376.6 mm h- 1, approximately 2.3 times higher than in adjacent open areas (O). Detailed measurements below and around an individual oak showed similar mean Ks values but also highlighted a spatial gradient in the Ks distribution, with the highest and more variable Ks values near the trunk and a progressive decline outward. Notably, the outermost under-canopy positions showed Ks values comparable to those detected in the first meters of the adjacent open area, suggesting that tree influence may extend beyond the canopy projection. The results suggest that the sampling strategies of the hydraulic soil properties should account for the fine-scale tree-induced heterogeneity for obtaining representative estimations. Incorporating this fine-scale variability into hydrological models can improve simulations of soil-water processes and enhance predictions of how land-use changes, for example induced by land abandonment, affect hydrological dynamics in tree-dominated Mediterranean ecosystems.