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

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2 . Analysis of existing forest gap
models
This chapter starts with an analysis of the model formalism of forest gap models (section
2.1). Section 2.2 presents simulation studies with the FORECE model, which are used to
exemplify the type of basic simulation results produced by forest gap models and to
analyse some of their statistical properties. These considerations provide the basis for an
ecological analysis in section 2.3: First, the sensitivity of the FORECE model to structural
simplifications is investigated in order to derive a minimum set of ecological factors that
are necessary to model forest dynamics in the European Alps (section 2.3.1). Second, the
set of climate-dependent factors remaining after the simplification procedure is analysed
for its sensitivity to alternate formulations (section 2.3.2).
Throughout the thesis, the nomenclature of the European tree species follows Hess et al.
(1980). Their scientific and common names are listed in Appendix I.
2.1 Model formalism
Zeigler (1976) distinguished the following categories of model formalisms: Differential
Equation System Specifications (DESS), Discrete EVent System Specifications (DEVS),
and Sequential Machines (SM, i.e. discrete time models). It is often difficult to specify to
which of these three formalisms complex models belong, because the mathematical equations
are not published in detail, the various submodels may be based on different formalisms,
or the formalism used for the mathematical model does not correspond to the
one used in the simulation model, i.e. in the computer code. For example, the tree growth
equation of forest gap models most often is described as a differential equation (DESS,
Botkin et al. 1972a,b, Shugart 1984), while the descriptions of tree establishment and
tree death suggest that these processes are simulated using a discrete event model
(DEVS). However, forest gap models typically are implemented as discrete-time models
(SM) with an annual time step. Hence, there arises the question to which formalism these
models conform.