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

180 Chapter 8
Several problems remain when attempting to use FORCLIM to study the impact of climatic
change on near-natural forests: First, such applications basically deal with extrapolations
in time and beyond current ecological conditions. The fact that FORCLIM – although
developed for European conditions – provides plausible descriptions of forest dynamics
also in eastern North America may provide a clue that such extrapolations may be legitimate.
However, it would be highly desirable to conduct validation experiments under
conditions of climatic change, e.g. in the early Holocene (cf. Solomon et al. 1980, 1981,
Solomon & Tharp 1985, Solomon & Bartlein 1993). Although few suitable palaeoecological
records are available and it is difficult to derive independent climatic data to
drive FORCLIM, such experiments would be important to increase our confidence that the
model is appropriate for studying some possible impacts of climatic change on forests.
Second, some factors that are important in mountainous terrain are not considered in
FORCLIM, such as soil erosion and landslides, which may occur after forest dieback phenomena
and may render large areas inappropriate for forest growth. Moreover, air pollution
in conjunction with climatic change may lead to unexpected synergistic effects, such
as an increased sensitivity of forests to climatic change (e.g. Schulze et al. 1989), and
herbivores could also modify the response of forests to climatic change (e.g. Fajer et al.
1989). Finally, in FORCLIM it is assumed that seeds of every species are always available.
In reality, migration of trees is slow (e.g. Fenner 1985, Roberts 1989, Leck et al.
1989), and the growth of new species at a given site often would start later than predicted
by FORCLIM because of delayed immigration. Thus, the changes of community composition
projected by FORCLIM often are too fast and may overestimate the recovery rates especially
after forest dieback phenomena.
In spite of these restrictions, which have to be taken into account especially when interpreting
the results obtained from FORCLIM, it is concluded that this model yields realistic
results when applied along climate gradients and thus can be considered to be appropriate
for assessing the impact of climatic change on the species composition of near-natural
forests in large parts of central Europe and eastern North America.
Implications for impact assessments of climatic change
The study of forest dynamics for the last 500 years at a site representative of the Swiss
Plateau, using reconstructed monthly temperature and precipitation data to drive the
model, showed that these historical climate variations have no impact on the simulated