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

The forest model FORCLIM 75
where kNC in Eq. 3.52 & 3.55 is the amount of nitrogen immobilized per unit organic
matter that is respired. Pastor & Post (1985) used values of this parameter that were specific
for each litter type. Since the data base for European conditions did not permit to
derive specific values of kNC, simulation studies were conducted to explore the sensitivity
of FORCLIM-S to the value of kNC. The model was found to be little sensitive, and
therefore one value was used for all litter types in FORCLIM-S.
In the equation for the change in litter nitrogen content (LN, Eq. 3.54), gross nitrogen
immobilization and nitrogen leaching are distinguished; the equation thus represents net
nitrogen immobilization:
∆LN c
∆t
= gImmob c – gLeach c ≡ gNetImmob c (3.54)
where gImmob c is gross nitrogen immobilization (Eq. 3.55, Melillo et al. 1982),
gLeach c is the amount of nitrogen leaching from the litter (Eq. 3.56, Cole & Rapp 1981),
and gNetImmob c is the net immobilization rate of nitrogen. It should be noted that Pastor
& Post (1985, p. 92f.) did not subtract nitrogen leaching in the calculation of the change
of LN c (Eq. 3.54) although they used it to calculate net nitrogen immobilization; thus the
nitrogen balance in LINKAGES was disrupted.
gImmob c = – kNC · ∆LOM c
∆t
(3.55)
gLeach c =
kLeach · LN c foliage & roots
0 stemwood & twigs
(3.56)
A litter cohort is transferred to the humus compartment when its current nitrogen concentration
(gNMR c , Eq. 3.51) exceeds kCritN X , the critical nitrogen concentration of the
corresponding litter type X (Alexander 1977, Ellenberg 1986). In the simulation model,
this transfer is implemented as a discrete time approximation, i.e. LOM c and LN c are
added to the respective humus compartments (HOM and HN) and then are set to zero.
HUMUS DECAY AND NITROGEN MINERALIZATION
The turnover of humus nitrogen (HN) is calculated based on data recalculated from Pastor
et al. (1984). These authors determined the amount of nitrogen mineralized per unit of