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

Introduction 5
in a changing climate thus would require to formulate time-variant transition probabilities.
However, such a formulation would not be causal and does not appear trustworthy
enough for a study of climatic change. Other landscape models concentrate on primary
productivity (e.g. Kauppi & Posch 1985, 1988). A prominent model that includes primary
productivity and the hydrological cycle is FOREST-BGC (Running et al. 1987,
Running & Coughlan 1988, Running & Gower 1991). Running & Nemani (1991) used
this model for assessing the possible impact of climatic change on forest productivity and
hydrology.
A major drawback of models on the landscape scale is that none of them was designed to
predict the structure of the landscape (e.g. species composition or vegetation types) and
its productivity simultaneously. However, both features are of interest in the present
study, and landscape models therefore are of limited value.
Ecosystem models: A large effort for building ecosystem models was initiated by the
International Biological Programme (Reichle 1981). Models on this scale typically
assume either that a forest consists of a single species (Swartzman 1979, McMurtrie &
Wolf 1983) or that its composition does not change with time (Shugart et al. 1974,
Sollins et al. 1976, Dixon et al. 1978a,b, Aber et al. 1991). The temporal resolution of
these models is on the scale of hours to weeks, and the compartments ignore any differences
between individuals, species, and often even trophic levels. They take the forest as
a functional entity with superorganism-like behaviour (cf. Huston et al. 1988). This
makes it difficult to apply such models to study the transient behaviour in function of climatic
variables (Davis 1990). However, they can be quite useful to assess productivity,
assimilate allocation, transport mechanisms, and energy flow through ecosystems.
Sollins et al. (1981) noted that a major problem with models formulated on the ecosystem
scale is the lack of sufficient validation data, such as gross ecosystem respiration or the
effects of defoliating insects on net primary productivity. Moreover, the scope of these
models was to increase the understanding of forests as they are today. This justifies their
basic assumptions but renders them inappropriate for studies of climatic change.
Models using populations and functional groups: Models at this scale were used
to simulate the management of single-species stands (e.g. Kimmins et al. 1981). Other
applications included studies of the interactions between a few populations or functional
groups of organisms, most often plants (Malanson 1984, Moore & Noble 1990, Osho
1991). These models typically were built for management purposes, thus ignoring many