Vapor pressure deficit is used in iLand as proxy for atmospheric humidity. It serves as the mediator between atmospheric conditions and transpiration via stomatal conductance, and acts as a direct modifier on primary production. We follow Landsberg and Waring (1997) in applying the same response function for both production and stomatal conductance (Eq. 1)

\[\begin{aligned} f_{D,d}=e^{-k\cdot D} \end{aligned} \] | Eq. 1 |

with *d* a index for day of the year, *k* an empirical constant, and *D *the atmospheric vapor pressure deficit in kPa. The model is sensitive to this modifier, both with regard to water cycle feedbacks and direct effects on *NPP*. It is thus noteworthy that Mäkelä et al. (2008), conducting a empirical parametrization of Eq. 1 over sites in Europe, obtained strongly divergent parameters for *k *compared to Landsberg and Waring (1997). A possible explanation lies in differences in temporal resolution, with daily (Mäkelä et al. 2008) vs. monthly (Landsberg and Waring 1997) responses.

In iLand Eq. 1 is calculated on a daily basis. It is used as daily modifier on stomatal conductance in the water cycle simulations and averaged to obtain the *f _{vpd}* modifier for the monthly calculation of utilizable radiation and production.

Seidl, R., Rammer, W., Scheller, R.M., Spies, T.A. 2012. An individual-based process model to simulate landscape-scale forest ecosystem dynamics. Ecol. Model. 231, 87-100.