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

Model applications 155
The simulated winter temperature during the period from 1525 to 1979 was always above
-5 °C; thus its ecological significance in the FORCLIM model is negligible (chapter 3).
Similarly, the simulated drought stress is mostly below 10%, with a few exceptions
where stronger drought occurred, the strongest being almost 30% in the “mediterranean”
year 1540 (cf. Pfister 1988). Hence, the variable that could have the largest effect on simulated
forest dynamics is the annual sum of degree-days (Fig. 6.1). Most of the periods
outlined by Pfister (1988) as mid-term climatic variations are evident from Fig. 6.1,
such as the warm period from 1530-1564, the maximum of the Little Ice Age from 1688-
1701, the rapid warming from 1702-1730, and the cool phase from 1812-1860. These
variations had strong effects e.g. on agricultural yield (Pfister 1988) – did they also have
effects on the characteristics of near-natural forests, such as species composition and total
aboveground biomass?
The simulated forest dynamics from 1525-1979 (Fig. 6.3) do not show any relationship
to the climatic variations visible in Fig. 6.1. The variability of the simulated aboveground
biomass is due to the stochastic formulation of tree establishment and mortality in
FORCLIM (cf. Tab. 3.6), not to the changing abiotic environment. Thus, we may conclude
that the forest simulated by FORCLIM-E/P at the CLIMINDEX site is well buffered
against climatic variations of the duration and magnitude that occurred during the last 450
years, corroborating the findings by Davis & Botkin (1985). From an evolutionary point
of view, these results are plausible as well: Trees typically have lifespans of several centuries;
given the fact that climatic variations like the ones reconstructed by Pfister (1988)
occur on the timescale of decades, trees must be capable of surviving such anomalies,
otherwise they could not grow to adult size and would not be able to reproduce.
Hence, the inertia to climatic variations of the simulated species composition probably is
characteristic of real forests of the Swiss Plateau as well. However, these findings can
not be generalized to other areas. For example, under conditions of strong environmental
stress, such as close to the alpine or the dry timberlines, it is conceivable that climatic
variations on the timescale of decades might lead to breakdown phenomena – at least in
forest models. However, further studies would be required to address this issue.
Finally, it should also be noted that this experiment could not have been performed using
the FORECE model by Kienast (1987). In an earlier study, we have shown that the “dry
days” approach incorporated in FORECE and other forest gap models leads to unrealistic
forest breakdown events when the same realization of weather is used to drive successional
dynamics on all the patches that are simulated (Fischlin et al. 1994).