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

Conclusions 179
Ecological factors determining forest dynamics in the European Alps
The analysis of the sensitivity of FORECE to structural simplifications (chapter 2) made it
possible to derive a hypothesis on the most important factors determining the successional
dynamics. According to this hypothesis, forest succession in the European Alps can be
portrayed realistically using the following factors: Tree growth is governed by the availability
of light, nitrogen, water, and sufficient summer warmth. Major factors influencing
sapling establishment are winter minimum temperature, browsing, and again light availability.
Tree mortality can be portrayed with two simple functions related to maximum
longevity and the occurrence of stress. The point here is not that these factors would not
have been identified before (Waring & Schlesinger 1985, Kimmins 1987, Lyr et al.
1992); rather, it is notable how few ecological factors are sufficient to synthesize a realistic
picture of successional processes.
The systematic simulation studies performed in chapter 5 suggest that the simplification
of a complex model like FORECE does not have to hamper its realism. On the contrary,
the simplification and the improvements introduced when developing FORCLIM have increased
its capability to simulate realistic forest dynamics especially along climate gradients.
Moreover, it was possible to simplify some of the remaining equations, such as the
formulation of maximum tree growth. Some of these simplified equations even turned out
to be biologically more sound. Finally, the parameter space of the model could be reduced
drastically, from more than 1300 parameters in FORECE to 540 in FORCLIM.
Applicability of FORCLIM to study the impact of climatic change on mountainous forests
The theoretical analysis conducted in chapter 2 and the simulation experiments in section
5.3 showed that the FORECE model has not been built and is not apt for impact studies of
climatic change. Since most forest gap models share many common features, the same
may be surmised for many of these models (Shugart 1984). On the other hand, the validation
experiments performed with FORCLIM revealed that this model yields plausible results
when it is applied along climate gradients in central Europe (section 5.3) and under
the climatic conditions of eastern North America, for which it has not been developed
(section 5.4). Moreover, the simulated species composition appears to be reasonably
robust to changes of the species parameters (section 5.1). Thus it may be conjectured that
FORCLIM yields realistic results also when applied to study the impact of climatic change
on forest ecosystems in these areas.