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

Introduction 7
Their integrative capability may be an important reason why forest gap models produce
plausible successional patterns for a wide range of forest ecosystems (e.g. Shugart
1984).
Forest gap models are fairly general tools and can be used to study a variety of phenomena,
ranging from age structure and species composition to primary productivity and nutrient
cycling of forest ecosystems (Shugart 1984). This is a distinct difference to all the
other models reviewed above, which were built to answer specific questions; for example,
productivity models are not usually capable of treating succession because the choice
of state variables implicitly assumes that forest composition is constant. Moreover, forest
gap models are an explicit quantification of a sound ecological theory (Watt 1947,
Shugart 1984) which is consistent with many field observations (Moore 1990).
Physiological models: Models of physiological processes like photosynthesis and
respiration typically work on time scales of minutes or hours; they simulate tissue development
and plant growth (e.g. Sinclair et al. 1976, Tenhunen et al. 1980, Reynolds et al.
1980, Running 1984, Eckersten 1985, Webb 1991). An application on larger time scales
and for whole ecosystems is impractical, if not impossible due to the different scales
involved. However, these models can give important guidelines about processes to be incorporated
into more aggregated models and about the choice of adequate equations for
process formulations.
Conclusion: From the above review I conclude that forest gap models offer the highest
potential for modelling forest dynamics in mountainous terrain: These models bridge several
spatial, temporal, and organizational scales, they consider many abiotic and biotic
factors explicitly, and they represent quantifications of distinct hypotheses of the factors
determining forest dynamics. Moreover, forest gap models have already been used successfully
to simulate forest dynamics in the European Alps (Kienast & Kuhn 1989a,b).
1.4 Forest gap models
Forest succession may be defined as the directional change with time of the attributes of a
single site, such as species composition and vegetation physiognomy (Finegan 1984). It
is obvious that succession can be observed on a wide variety of scales, depending on the
exact definition of the term “site”. An early approach, which has pervaded much of the
ecological literature, views succession from a holistic ecosystem perspective (Clements