soil water response

Different variables have been used as predictors of plant response to soil water availability in process-based models. Granier et al. (1999) and Landsberg and Waring (1997) are examples of using soil water content (SWC) to model soil water effects on vegetation. Hanson et al. (2001) and Schwalm and Ek (2004) used soil water potential (Ψ), whereas Running and Coughlan (1988) convert SWC directly to leaf water potential. The work of Hanson et al. (2004) presents an in depth model comparison of approaches with different complexity and resolution.

In iLand we use soil water potential (Ψ) as predictor of species-specific growth response to soil water availability. We follow Hanson et al. (2001, 2003) in defining the modifier as a linear function of Ψ between field capacity and a species-specific Ψ_min (Eq. 1)

\[\begin{aligned} f_{sw}=max(min(\frac{\Psi -\Psi _{min}}{\Psi _{fc}-\Psi _{min}}\: ;\: 1)\: ;\: 0) \end{aligned} \]

Eq. 1

with Ψ_fc=-0.015MPa.

Several empirical studies report species differences in response to and maximum Ψ (e.g., Auge et al. 1998, Saito et al. 2003, Wullschleger and Hanson 2003). As with shade tolerance, we use the homogenized drought tolerance ratings of Niinemets and Valladares (2006) and their corresponding approximate Ψ_min to derive species-specific Ψ_min values by interpolation.