Soil hydraulic conductivity determines the onset of water-limited evapotranspiration across scales

Terrestrial vegetation, central to water-energy-carbon interactions between land and atmosphere, such as evapotranspiration, is under severe pressure due to human disturbances and changing climate. Evapotranspiration switches from being energy to being water limited at critical soil water thresholds. Despite the importance of such soil water thresholds for terrestrial ecosystems, the key mechanisms and drivers (being them related to plants, soils or the atmosphere) controlling their values remain unclear at the ecosystem scale.

Soil water thresholds have recently been estimated from global networks of terrestrial flux measurements based on Eddy-Covariance method (FLUXNET). However, this approach does not allow to partition between soil evaporation and plant transpiration, which might have different thresholds. Therefore, we also estimated soil water thresholds from a complementary monitoring network based on sapflow measurements (SAPFLUXNET), which provides the actual flow velocity along the xylem being closely related to transpiration rate. Besides comparing the two measurements approach, we aimed to explain the key mechanisms controlling soil water thresholds.

We found that the two monitoring approaches provide similar values of soil water thresholds. These thresholds, expressed as either soil moisture θ_crit or soil matric potential ψ_crit, are function of soil texture globally. By applying a soil-plant hydraulic model (considering the key soil, plant, and atmospheric parameters) at plant and ecosystem scale, we show that at both scales, θ_crit and ψ_crit are determined by the abrupt decrease of soil hydraulic conductivity with decreasing soil moisture content, causing a loss in leaf water potential that triggers stomatal closure. For soils with a moderate decrease of hydraulic conductivity (loam), atmospheric conditions and vegetation properties become more relevant, resulting in a higher variability of soil water thresholds compared to sandy soils (sharpest decrease of hydraulic conductivity).

Overall, our results show that soil texture modulates land-atmosphere exchange globally across scales, biomes, and climates, highlighting the importance of soil water flow for predicting and understanding evapotranspiration dynamics.