\begin{equation}
\Delta B   = B \times MIN(0.99,\frac{\Delta t}{a_c}\times (\frac{a}{a_c})^4)
\end{equation}
where $B$ is the biomass, $\Delta B$ is the fraction of biomass lost, $\Delta t$ is the times step (1 day)
and $a_c$ is the critical leaf age, a PFT-dependend parameter.
