Soil hydraulic conductivity defines minimum Root:Shoot surface ratio in moisture-limited environments

In drying soils, root water uptake is limited by the low soil hydraulic conductance. The magnitude of this conductance drop and its temporal dynamics are function of soil texture, soil water status, root hydraulic architecture, atmospheric demand and canopy conductance.  Under dry climates, in order to survive, plants can adapt their carbon allocation by maximizing their root:shoot surface ratio, thereby decreasing their transpiration surface while increasing their root surface.

Thanks to a simple soil-plant hydraulic model, we show that soil hydraulic conductivity controls the minimum root:shoot surface ratio. A meta-analysis of shoot:root surface ratio is combined with a database of soil hydraulic properties to demonstrate how the minimum root:shoot surface value changes with soil conductivity across soil textural classes for dry biomes. We discuss the mechanisms by which plants can control their carbon allocation in such conditions and investigate the sensitivity of this minimum root:shoot surface ratio to future shifts in evaporative demand.