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

76 Chapter 3
organic matter (HOM) as a function of the N:C ratio of the litter (gLNC). However, the
equation developed by Pastor & Post (1985) contains a pole, i.e. the nitrogen mineralization
rate tends towards +∞ when the litter N:C ratio approaches 2.984%; moreover, the
data in Pastor et al. (1984) do not suggest strongly that there is a nonlinear relationship
between the litter N:C ratio and the nitrogen mineralization rate. Thus, for FORCLIM-S a
new, linear equation was developed from the data in Pastor et al. (1984): If there is litter
present in the soil, the N:C ratio of the litter (gLNC) and the amount of humus organic
matter (HOM) are used to calculate nitrogen mineralization; otherwise, a constant turnover
rate (kMin) of humus nitrogen is assumed (Eq. 3.57). In both cases actual evapotranspiration
(uAET) influences the turnover (gAETM, Pastor & Post 1985; Eq. 3.59); thus
it is assumed that uAET can be used to characterize the humidity as well as the temperature
of the organic soil layer:
∆HN
∆t
=
– MAX k 5 +
– kMin · gAETM · HN
k 6
gLNC , k 7 · gAETM · HOM
gLNC defined
(litter present)
gLNC not defined
(no litter present)
(3.57)
where
nLC
∑ LN c
c = 1
gLNC =
(3.58)
nLC
kCM · LOM c
gAETM = MIN
∑
c = 1
uAET
kAET – uAET , 1 (3.59)
nLC is the number of litter cohorts currently present in the soil of a patch, and kCM is a
parameter to convert litter organic matter to carbon. kAET is a parameter defining the
slope of the multiplier curve.
Both in LINKAGES and in FORCLIM-S, the turnover of humus organic matter (HOM) is
assumed to be proportional to the turnover of nitrogen (HN; Pastor & Post 1985):
∆HOM
∆t
= ∆HN
∆t
· HOM
HN
(3.60)