Moss grows according to the equation
$B(t) = B(t-1) + P(t)$
where $B(t)$ is the moss biomass (kgC) at time step $t$, $B(t-1)$ is the biomass
at the previous time step, and $P(t)$ is the moss productivity at the time step $t$
Moss productivity is calculated as
$P(t) = S_lA_{max}f_{light}B(t-1) - B(t-1)q$
where $S_l$ is the moss specific leaf area, $A_{max}$ is the moss maximum productivity,
$f_{light}$ is a growth response factor, and $q$ is a respiration and turnover parameter.
$f_{light}$ is calculated as
$f_{light} = \frac{exp(-0.7(LAI_{forest}+LAI_{moss}))-l_{comp}}{l_{sat}-l_{comp}}$
where $LAI_{forest}$ is the leaf area index of the forest, $LAI_{moss} = B\times S_l$
is the leaf area index of the moss, $l_{sat}$ is the light saturations point, and
$l_{comp}$ is the light compensation point.