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

104 Chapter 4
such as Davos agrees with descriptions of other near-natural forests of the area (e.g.
Melico-Piceetum Ellenberg & Klötzli 1972; cf. also Ellenberg 1986).
Similar to the site Bever, the stronger occurrence of summer drought causes shifts of the
species composition at the site Bern (Fig. 4.9): Spruce (P. excelsa) loses importance. On
the other hand, the biomass of A. alba increases considerably, and that of Quercus robur
increases slightly. Abies clearly is overrepresented now, which is due to its large maximum
height and the formulation of asymmetric competition, as discussed already by
Kienast & Kuhn (1989b). The rest of the community remains virtually unchanged.
At the site Sion, radical changes are observed when the abiotic environment is stochastic
(Fig. 4.9): Total biomass decreases to about one third, Tilia and Taxus disappear, and
Castanea is reduced to very low biomass. The forest simulated by FORCLIM-E/P is an
oak-pine forest close to the arid timberline, which corresponds better to phytosociological
expectations (Ellenberg & Klötzli 1972, Burnand 1976). It should be noted that small
changes of the drought tolerance parameters (kDrT) of P. silvestris, Q. robur, and Q.
pubescens can lead to strong changes in the relative proportions of these species at the
site Sion; the simulated species composition therefore should be interpreted with caution.
The simulation time required to simulate 1200 years of successional dynamics on one
patch with FORCLIM-E/P is 40 seconds (Macintosh Quadra 700); for the same run on the
same machine, the FORTRAN version of the FORECE model requires 226 seconds. In
other words, simulation time with FORCLIM-E/P is less than one fifth of FORECE
(19.4%). Most of the increased efficiency is attributable to the model simplifications
(section 2.3) and not to the different programming and simulation environment. Hence
large-scale simulation experiments can be performed more efficiently with FORCLIM than
with FORECE (cf. chapter 5).
4.3.3 FORCLIM-E/P/S
In this model version, neither the weather nor the availability of nitrogen are kept constant,
and it is also possible to evaluate the amount of belowground organic matter.
Fig. 4.10 & 4.11 show the results obtained from FORCLIM-E/P/S at the four sites. A
common pattern is visible at all sites: the accumulation of biomass is slower in the E/P/S
than in the E/P model because nitrogen availability limits tree growth markedly as long as
soil organic matter is accumulating (cf. section 4.2). Moreover, even if the average nitro-