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

132 Chapter 5
where ∆T m is the average deviation of the monthly mean temperature from the annual
mean temperature (Fig. 5.2), and the equation is scaled so that at the average precipitation
sum of all sites (P l
* = 1180 mm/yr) their average temperature amplitude is reached
(A l
* = ∆T Jul – ∆T Jan = 18.4 °C, Fig. 5.2). The monthly precipitation sum and the
standard deviations of temperature and precipitation are calculated from the data presented
in Fig. 5.2.
Simulation studies were conducted with the FORECE model and two FORCLIM variants,
FORCLIM-E/P and -E/P/S. The annual mean temperature was varied from -2 to 13 °C,
and one simulation experiment was performed every 0.5 °C. The annual precipitation
sum was varied from 400 to 2000 mm/yr, with one experiment every 100 mm/yr, yielding
a total of 31·17 = 527 simulation experiments. For each of these points in the (T,P)
space, the equilibrium species composition was estimated using n = 200 points and an
interval (∆t) of 150 years, discarding the first 1000 years (cf. section 4.4).
The field capacity parameter (kFC) was assumed to be 30 cm throughout the (T,P) space
and in all models. For the model variant FORCLIM-E/P, available nitrogen was assumed
to be 100 kg/ha throughout the (T,P) space. For both FORCLIM variants the cross-correlation
coefficients were assumed to be -0.6 in summer (April–Sept.), and 0 in winter
(Oct.–March). For FORECE, the additional site parameters were taken from Tab. 2.2
(Bern), and the 200 samples with ∆t = 150 years were extracted from the default output
file. The simulation studies with the FORCLIM models were executed on Macintosh computers;
because simulation studies with FORECE require much more computing time, they
had to be performed on a Sun SS630 workstation and still took more than three weeks.
Data processing for creating three-dimensional plots was done with the software Microsoft
EXCEL V4.0, and unsmoothed contour plots were drawn using MATHEMATICA
V2.03 on an Apple Macintosh computer model Quadra 700.
5.3.2 Results & discussion
First, let us examine the pattern of total aboveground biomass simuated by the three
models (Fig. 5.3): The most striking feature is that FORECE simulates the highest biomass
close to the alpine treeline, whereas both FORCLIM variants predict a steady increase
with increasing temperature and precipitation. It may be argued that the assumption
in FORECE that the SOILQ parameter is constant throughout the (T,P) space is unreal-