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

10 Chapter 1
only a subset of the parameters (Kercher & Axelrod 1984, Dale et al. 1988, Botkin &
Nisbet 1992) or species-poor forests (Leemans 1991). Thus, there arises the question
whether the essence of the original hypothesis of forest dynamics behind these models
has been cluttered by ornaments, whether all the details present in current forest gap
models are necessary for producing realistic successional characteristics, and whether
simpler models could provide equally valid descriptions of forest ecosystems. Moreover,
such models would be easier to interpret ecologically and would allow for a more detailed
analysis of their behaviour.
Although forest gap models originally were not built to study the effects of a changing
climate on forest ecosystems (Botkin et al. 1972a,b) and in spite of their ill-known behaviour,
they have been applied extensively to study the possible impacts of future climatic
change on forests. The direct fertilizing effects of CO 2 were investigated by Botkin
et al. (1973) and Shugart & Emanuel (1985); authors concentrating on the effects of
changing temperatures and/or precipitation were Solomon et al. (1981, 1984), Solomon
(1986), Solomon & West (1987), Pastor & Post (1988), Dale & Franklin (1989),
Kellomäki & Kolström (1992), Solomon & Bartlein (1993), Kräuchi & Kienast (1993),
and Urban et al. (1993). A few studies dealt with the simultaneous effects of CO 2 fertilization
and climatic change (Luxmoore et al. 1990, Kienast 1991, Post et al. 1992,
Prentice et al. 1991, 1993, Bowes & Sedjo 1993), while others investigated the effects of
a changed disturbance regime (Overpeck et al. 1990, O'Brien et al. 1992). While these
applications are heuristically useful, extensive tests should be conducted to determine
whether forest gap models implicitly assume a constant climate. If they do so, these assumptions
should be replaced by explicitly considering the influence of climate on ecological
processes. Moreover, it would also be important to know how sensitive the
models are to different formulations of climatic influences (Bonan 1993). Like this, forest
gap models could become more reliable tools for projecting the impact of climatic change
on forest dynamics.
Since forest gap models also have been adapted for Europe, it seemed more promising to
take an existing forest gap model as a starting point for the present work than to build a
new one from scratch. In early 1990, when this study was incepted, there were two
forest gap models for European conditions: FORECE, which had been used extensively
for simulating forest succession in the European Alps (Kienast 1987, Kienast & Kuhn
1989a,b), and FORSKA, at that time a model restricted to Scandinavian boreal forests
(Leemans & Prentice 1989). Thus, FORECE was chosen as a basis for this study.