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

140 Chapter 5
below 900 mm/year; (5) the transition from beech forests to insubrian and mediterranean
forests dominated by oak (Quercus spp.) and chestnut (Castanea sativa).
Location of the timberlines: All three models agree that the dry timberline extends
from the point (13 °C, 600 mm/year) down to about (5 °C, 400 mm/year) (Fig. 5.9).
Unfortunately, Rehder (1965) and Ellenberg (1986) did not include the location of this
timberline in their hypothesis. However, the location of the alpine timberline can be compared
to the simulation results: For P l < 1600 mm/year, the FORECE model produces a
timberline that contradicts the expectations formulated by Rehder and Ellenberg, whereas
both FORCLIM models conform to their hypothesis. The fact that the alpine timberline
simulated by FORCLIM extends ≈0.5 °C too far into the alpine zone is partly due to its
definition: 20 t/ha of aboveground biomass simply may not be a forest any more.
Temperature gradient under moist conditions: All three models simulate the
transition from the subalpine coniferous to mixed deciduous forests at lower elevations;
however, FORECE and FORCLIM diverge to a large extent concerning the distribution of
single species:
The strong dominance of Pinus cembra along the alpine timberline in the FORECE model
(Fig. 5.9) has been discussed already (Fig. 5.5), as well as the abrupt decline of Fagus
silvatica in the middle of the upper montane beech-silver fir zone (Fig. 5.6). FORECE
simulates oak exclusively in the warm-dry zones approaching the dry timberline, although
the species should be generally present when the annual mean temperature is
higher than about 8 °C (cf. Fig. 5.7). Thus, while the general pattern simulated by
FORECE may be correct, the model contains several thresholds that are unrealistic and occur
in the wrong places.
The FORCLIM-E/P model produces a plausible gradient of species composition from the
alpine timberline up to about 10 °C: A small belt of P. cembra close to the timberline is
followed by the subalpine spruce zone, by the montane spruce-silver fir-beech zone, and
ends with the beech forests typical of the Swiss Plateau, corresponding to a large extent
to the hypothesis by Rehder (1965) and Ellenberg (1986). A characteristic difference to
the FORECE model is that the gradients are smoother, which is reflected in more subtle
differences between the zones; for example, the FORCLIM-E/P model simulates a small
spruce-silver fir zone between the subalpine spruce zone and the montane beech-silver
fir-spruce zone, a level of detail that is not present in the Rehder-Ellenberg scheme, but
which appears to be realistic.