Plant-available nitrogen (N_av) is used in response functions following Pastor and Post (1985) to derive the soil nutrient modifier f_N (cf. Landsberg and Warings' (1997) fertility rating). We re-calculate the three N response classes of Pastor and Post (1985) as (Eq. 1)

\[\begin{aligned} f_{N}=max(1-e^{a_{N}\cdot (N_{av}-b_{N})}\: ;\: 0) \end{aligned} \]

Eq. 1

with a_N (-0.045, -0.055, -0.065) and b_N (10, 25, 40) empirical parameters and the respective values for the three classes (tolerant, intermediate, intolerant of low N availability). This approach to derive growth response to N has been successfully applied in a number of gap model models, including LINKAGES (Wullschleger et al. 2003), ForClim (Bugmann 1996) and (in modified form) PICUS (Seidl et al. 2005).