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

The forest model FORCLIM 51
Disturbances extrinsic to the forest patch, such as forest fires and windthrow, provide a
third source of mortality, which is episodic (Shugart 1984). This mortality is included in
the FORCLIM model using a simple approach: All the trees currently present on the patch
are killed if such a disturbance occurs. Other sources of tree mortality, such as forest
management practices like thinning and logging, are disregarded in FORCLIM.
3.2.2 Soil submodel: Turnover of soil organic matter
Nitrogen is one of the major plant nutrients, and its availability limits plant growth in
many terrestrial ecosystems (Kimmins 1987). The nitrogen cycle in forests is intimately
coupled with the carbon cycle (Shaver et al. 1992): The amount of organic matter returned
to the soil depends on primary productivity, which is limited by nitrogen availability
(Waring & Schlesinger 1985, Lyr et al. 1992). In turn, nitrogen availability is largely
determined by nitrogen mineralization, the conversion of organic nitrogen to ammonium
with concomitant release of CO 2 (Alexander 1977); nitrogen mineralization itself depends
on climate and on the type of carbon compounds with which the nitrogen is associated
(Mellilo et al. 1982, McClaugherty et al. 1985). Thus in an analysis of the turnover of
soil organic matter at the ecosystem scale, the couplings between the carbon and nitrogen
cycles should be considered explicitly (Pastor & Post 1985).
The basic paradigm for most decomposition models developed to date was formulated by
Jenny et al. (1949); Olson (1963) formalized it in a simple exponential-decay model.
However, the parameters of this model are specific for each soil, depending on climate
and the type of litter returned to the soil. Thus, it was a logical step to relate decay rates to
environmental parameters such as temperature and precipitation (or a combination of
these), and to simple chemical indices of substrate quality (e.g. Meentemeyer 1978,
Melillo et al. 1982). Several models of the carbon cycle were constructed for forests
(Aber & Melillo 1982, Weinstein et al. 1982, Pastor & Post 1985, Aber et al. 1991), and
grasslands (Jenkinson & Rayner 1977, Parton et al. 1987, Verberne et al. 1990) (cf. the
review in Ågren et al. 1991). However, most of these models do not treat explicitly the
effects of climatic parameters and substrate chemistry on decomposition rates. The LINK-
AGES model by Pastor & Post (1985) fulfils both requirements. Moreover, this model
was used successfully in many subsequent studies (Pastor & Post 1986, 1988, Shugart
& Urban 1989, Martin 1992, Pastor & Naiman 1992). Therefore, the FORCLIM submodel
for soil organic matter turnover was derived from LINKAGES (Fig. 3.3).