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

The forest model FORCLIM 53
decay of each year's litter is tracked through time, thus mimicking many litterbag studies.
When the critical nitrogen concentration (Alexander 1977) is reached, the litter is transferred
to a common “humus” compartment, and nitrogen mineralization starts. The
amount of nitrogen available for plant growth (uAvN) is calculated as the difference between
the nitrogen mineralized from the humus pool and the immobilization demand of
the litter cohorts (gImmob, Fig. 3.3).
Pastor et al. (1984) and Pastor & Post (1985) found good correlations between litter
decay rates, actual evapotranspiration (uAET), litter lignin content (gLign), and the
nitrogen to mass ratio of the litter (gNMR); these indices are used to formulate litter decay
rates in LINKAGES and FORCLIM (Fig. 3.3). The leaching of nitrate from nitrogen-rich
litter is taken into account (Cole & Rapp 1981) as well as a constant atmospheric deposition
rate of soluble nitrogen compounds. The more recalcitrant litter types (twigs and
wood) are assumed to decay at a constant rate. The hypothesized effects of canopy openings
on litter and humus decay rates as incorporated in LINKAGES were not included in
the FORCLIM model (Fig. 3.3).
Most of the litterbag studies to date were conducted under boreal conditions and/or in
America; only few data are available for central European conditions and species (e.g.
Berg & Staaf 1981, Ellenberg 1986, Lüscher 1991). Thus it was necessary to collapse
the 17 litter types distinguished in LINKAGES to three types of leaf litter (fast, medium,
and slow decay), twig litter, root litter, and stemwood litter.
At least two weaknesses remain in the LINKAGES as well as the FORCLIM-S model: First,
there is no compartment with a very slow turnover rate of soil organic matter. It is well
known that part of the organic matter is “protected” or “stabilized” and very recalcitrant to
decay (e.g. Parton et al. 1987, Verberne et al. 1990); neither the LINKAGES nor the
FORCLIM model simulate these processes. Second, Pastor & Post (1985) had to use the
N:C ratio of the litter to formulate the mineralization rate of the humus compartment;
while this approach was phenomenologically correct (Pastor et al. 1984), it represents an
empirical, not a causal relationship. Lüscher (1991) investigated the dynamics of the
forest floor and the humus of many soils in Switzerland; however, this data base does not
allow to reformulate the rate of nitrogen mineralization from the humus as a function of
abiotic variables on a more mechanistic basis. Thus, the data from Pastor et al. (1984)
had to be used again in FORCLIM (Fig. 3.3).