The soil matric potential where ecosystems get water-limited is independent of soil type and climate

Land-atmosphere exchange shifts from energy-limited to water-limited regime at a critical soil moisture, which marks a fundamental transition in the Earth system. Estimates of the critical threshold vary a lot across studies despite its importance for the mechanistic understanding of soil moisture limitation on transpiration and plant productivity.

We introduce a novel, model-based diagnostic approach — the Normalized Transpiration Deficit (NTD) method — and demonstrate that it yields results highly consistent with observational methods such as finding breakpoints in the evaporative fraction. Using a hydraulically-enabled version of the CABLE-POP land surface model, we conducted a factorial experiment across various soil textures, climate regimes, and plant hydraulic parameters. It suggests that the critical threshold occurs at a broadly similar soil matric potential (ψ_crit) across soil types, resulting in a quasi-linear relationship between the critical volumetric soil moisture (θ_crit) and sand content, as observed in earlier studies. The dependency of θ_crit on soil type vanished when it was normalised by field capacity, which yielded hence also a universal threshold of relative extractible water REW_crit, as found empirically for forest ecosystems.

Most of the variance of θ_crit, 86%, came from soil texture in the factorial experiment, while the variances of ψ_crit and REW_crit were largely explained by plant hydraulic traits, accounting for 87% and 77% of total variance, respectively. Within the plant hydraulic traits, the P50-values of stomatal conductance (ψ_50,l) and of xylem conductance (ψ_50,x) showed the strongest correlations with the critical thresholds, indicating that vulnerability to hydraulic dysfunction plays a key role in shaping plant responses to soil drying. There was, however, no direct effect of climate on any of the critical thresholds, i.e. the thresholds remained invariant across climates for given soil and vegetation types. This suggests that apparent climate dependencies reported in observational studies may be artifacts due to limited soil moisture ranges at each observational site, or they represent biological adaptation and acclimation that is currently not captured in our static model parameters.

These findings highlight the necessity of incorporating ecosystem-scale hydraulic regulation in biosphere models to reconcile divergent estimates of critical thresholds and to improve predictions of drought impacts on water and carbon fluxes.