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

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3 . The forest model FORCLIM
3.1 Structure of FORCLIM
Conventional forest gap models (e.g. Botkin et al. 1972a,b, Shugart 1984, Kienast 1987)
are formulated as one large model. While this approach is useful for small models, the
complexity of forest gap models makes it difficult to keep an overview. An alternative
concept is to formulate several independent submodels and to assemble them in a modular
fashion to form a complete forest gap model. This approach bears several advantages:
The structure of the ecosystem model becomes clearer, the couplings between submodels
are explicit, and it is easy to exchange a submodel without affecting the others. Consequently,
the FORCLIM 1 model is divided into three submodels (cf. Fig. 3.1):
• Environment: This submodel provides time-dependent abiotic variables. It
generates weather data (W) and uses these data to calculate bioclimatic output
variables (B). The environment submodel does not depend on any of the other
submodels and acts as an input model.
• Plants: The plant submodel calculates establishment (E), growth (G), and
mortality (M) of trees on a forest patch. It requires bioclimatic variables and
nitrogen availability as input and calculates litter production as an output.
• Soil: The soil submodel tracks the decay of plant litter (L) and humus (H) in the
soil as a function of bioclimatic variables. It calculates the amount of nitrogen
available for plant growth.
The dynamics of soil organic matter are considered explicitly in the FORCLIM model for
two main reasons: First, such a submodel calculates the availability of soil resources as a
function of weather variables, thus avoiding earlier approaches that implicitly contained
climatic information (Botkin et al. 1972a,b, Kienast 1987; cf. Fischlin et al. 1994).
Second, the explicit consideration of soil carbon dynamics makes it possible to assess the
1 FORCLIM is an acronym for "FORests in a changing CLIMate"