\begin{equation}
gdd_{crit}(i) =  GDD_{crit1,j} + GDD_{crit2,j} \times T_{l} + GDD_{crit3,j} \times T_{l}^{2}
\end{equation}
where GDD$_{crit1,2,3}$ are the PFT-dependent threshold parameters defined by the ::$pheno_crit$\%$gdd$, and $T_{l}$ is the long-term
reference temperature (::$tlong_ref$ - calculated in ::$stomate_constants$) in degrees C. i and j are the grid cell and PFT respectively.
