Hydraulic Properties of a Rock-Soil-Root System: Insights From Fraxinus ornus L. Saplings Growing on Different Carbonate Rocks

Abstract

Drought impacts trees in varied temporal and spatial patterns, suggesting that heterogeneity of below-ground water stores influences the fate of trees under water stress. Karst ecosystems rely on shallow soil overlying bedrock that can store available water in primary pores. A contribution of rock moisture to tree water status has been previously demonstrated, but actual mechanisms and rates of rock-to-root water delivery remain unknown. We report accurate measurements of hydraulic properties of two rock types (Breccia and Dolostone), of typical Karst red soil, and of roots of a common Karst tree species grown under different rock-soil combinations. Experimental data were used to build a water exchange model that supported the hypothesis that roots can extract water from porous and highly conductive rocks (Breccia), but not from more compact ones (Dolostone), especially when plants grow in rocky substrate or experience water stress, and thus have low root hydraulic conductivity and low rates of water extraction from rocks. Our data support the hypothesis that rocks represent important water stores for plants growing in rock-dominated habitats. Heterogeneous rock properties translate into different rates of water delivery to root systems, underlying complex patterns of tree mortality under severe drought stress.

Keywords: bedrock; drought; plant hydraulics; root.

Figure 1

Schematic representation of the rock‐soil‐root system used in the model, with a total area of 9 mm², a root diameter of 1 mm, and a rock content of 40%. The model evaluates two scenarios: (1) root in contact with rock and (2) root not in contact with rock. The numerical solver employs the bimodal van Genuchten‐Mualem formulation (Durner et al. 1999) to assess water exchange efficiency across different spatial configurations of the rock‐soil‐root system.

Figure 2

Median values, 25% and 75% percentiles of (A) rock hydraulic conductivity (k _rock) and (B) primary porosity for Breccia (N = 5) and Dolostone (N = 5). Different letters indicate statistically significant differences between the two rock types (p < 0.05, Wilcoxon rank sum test). Corresponding p‐values are reported.

Figure 3

3D Micro‐CT reconstructions of Breccia (A) and Dolostone (B) rock samples, with corresponding parameters derived from 3D image analysis (see the upper table). Values are presented as mean ± SD (N = 3). Statistically significant differences between groups (p < 0.05, t‐test) are indicated by different letters. Pore volumes are shown in black, and the rock matrix is depicted in grey. The acronyms (B) and (D) in the upper table represent Breccia and Dolostone, respectively.

Figure 4

Boxplots of root hydraulic conductivity (A, k _root) and leaf‐area normalised root hydraulic conductance (B, K _root) measured in saplings exposed to different experimental conditions: Soil Well Watered (SWW), Soil Stressed (SS), Breccia Well Watered (BWW), Breccia Stressed (BS), Dolostone Well Watered (DWW), and Dolostone Stressed (DS). Light grey boxes represent well‐watered samples, and dark grey ones represent water‐stressed samples. Stressed samples had N = 6, and well‐watered samples had N = 7. Different letters indicate statistically significant differences among groups (p < 0.05). Kruskal‐Wallis followed by Dunn's pairwise test was conducted on all treatments for both (A) and (B).

Figure 5

Log transformed unsaturated hydraulic conductivity (k) as a function of water potential ($\psi$) in Soil, Breccia, Dolostone, and in the Root. Water potential values considered in the model ranged from 0 to −1.5 MPa. The dotted line at 4.2 correspond to a water potential of −1.5 MPa.

Figure 6

Maximum water potential of soil and rock as a function of simulation time in a system with Dolostone or Breccia, for both root‐rock contact and non‐contact scenarios. The model used the bimodal van Genuchten‐Mualem formulation (Durner et al. 1999) to assess how soil and rock hydraulic properties, as well as root‐rock contact, influence water transfer to roots. Rocks occupied ~40% of the volume, with soil filling the rest. A 100 × 100 µm mesh size was used, and simulations explored the dynamics of water potential over time, ranging from 0 to −1.5 MPa. Boundary conditions included zero‐flux at the sides and fixed water potential at the root endodermis.

Figure 7

Average water content as a function of the simulation time in the model system with Breccia or with Dolostone. Vertical lines indicate the simulation time corresponding to the 25% decrease of initial water content for both rock types and for the root‐rock in contact (solid line) and root‐rock not in contact (dotted line) scenarios.