On the Ecology of Mountainous Forests in a Changing Climate: A ...

The forest model FORCLIM 73
3.3.2 FORCLIM-S: A model of the turnover of soil organic matter
Both LINKAGES (Pastor & Post 1985) and FORCLIM-S are formulated as difference equation
systems with a time step (∆t) of one year. The symbols used in FORCLIM-S are
listed in Tab. 3.7.
LITTER INPUT AND CREATION OF NEW LITTER COHORTS
For each of the six types of litter that are accumulated every year (Eq. 3.43–3.46), a litter
cohort is created unless the amount of litter for a type is zero. These new litter cohorts are
characterized by their initial organic matter content (LOM c,init , Eq. 3.47), which is required
to calculate litter lignin content (Eq. 3.50), and by their nitrogen content (LN c,init ,
Eq. 3.48), which is calculated by assuming an average nitrogen concentration depending
on the tissue type (Ellenberg 1986):
LOM c,init = uXL (3.47)
LN c,init = kInitN X · LOM c,init (3.48)
where LOM is litter organic matter, LN is litter nitrogen, X is a placeholder for F kLQ
(three types of foliage litter), T (twigs), R (fine roots), or W (stemwood), and kInitN X is
the initial nitrogen concentration of litter type X. The litter type subsequently influences
the calculation of litter decay rates (Eq. 3.49) and nitrogen leaching (Eq. 3.56).
LITTER DECAY AND NITROGEN IMMOBILIZATION
For foliage and root litter, the decomposition of the organic matter (LOM) is predicted
from the lignin to nitrogen ratio of the tissue (Melillo et al. 1982) and actual evapotranspiration
(uAET, Meentemeyer 1978). Stemwood and twigs are assumed to decay at constant
rates (Eq. 3.49). Pastor & Post (1985) derived these empirical relationships from a
large body of field data by regression analysis.
∆LOM c
∆t
=
– k 1 +k 2·uAET – (k 3 +k 4·uAET) · gLign c
· LOM
foliage
gNMR c
c & roots
– kLoss W · LOM c stemwood
– kLoss T · LOM c twigs
(3.49)