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

122 Chapter 5
fore was assumed to be twice as high as for the kDDMin parameter, i.e. ±40%. The uncertainty
associated with the winter temperature parameter (kWiT) is also large and was
assumed to be ±2 °C. Since drought gradients in the landscape are steep, the drought tolerance
parameter (kDrT) is difficult to determine. Its uncertainty was assumed to be ±0.1.
Please note that an absolute uncertainty was used since small kDrT values are not more
precise than large ones, rather the reverse is true. Tab. 5.1 gives an overview of the
plausibility range of each species parameter. Two tables with the minima and maxima for
all parameters may be found in Appendix V.
5.1.2 Simulation experiments
The site Airolo (Appendix III) was chosen for the sensitivity analysis because it is located
in the transition zone between subalpine coniferous and mixed deciduous forests, where
the simulated community may be especially sensitive to parameter changes. Each species
parameter was set to the lower and the upper end of its plausibility interval, and the
steady state species composition of the FORCLIM-E/P/S model was estimated for each parameter
change, thus resulting in 2·420 = 840 samples. The steady states were estimated
using n = 200 points and ∆t = 150 years (cf. section 4.4). Two types of analyses
were performed:
1) To quantify the robustness of the simulated species composition to changes of
species parameters, the simulated steady states were compared to a conjectured
standard steady state calculated with the default parameter set given in
Tab. 3.11 (n = 20'000 points, ∆t = 150 years). For these comparisons, the
percentage similarity coefficient (PS, Eq. 2.3) was used. A sample steady state
is significantly different from the standard steady state if PS < 0.871
(α = 5%, determined from an investigation for FORCLIM-E/P/S at the site
Airolo similar to the ones performed in section 4.4).
2) To quantify the response of particular species to changes of their parameters, it
was tested whether the average biomass (µ) of the corresponding species is
significantly different from the standard biomass (µ * ) of that species. Since the
sampling distribution of the mean tends to normality with increasing sample
size (and here, n = 200; Zar 1984), the range µ * ±1.96·SE includes µ at a confidence
level (α) of 5%, where SE is the standard error of the mean. For the
species with significant differences, the percentage change was calculated.