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

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4 . Behaviour of FORCLIM along a
transect in the European Alps
The modular structure of FORCLIM makes it possible to examine the behaviour of each
submodel in isolation before considering combinations of the three submodels. This allows
to quantify e.g. the effects of FORCLIM-E on FORCLIM-P, and to evaluate the feedback
mechanisms between FORCLIM-P and FORCLIM-S. Thus, in a first step each submodel
will be run on its own, either for all or a selection of the sites given in Appendix
III (sections 4.1 – 4.3.1). Second, two interesting combinations of submodels will be examined:
(1) FORCLIM-E/P, a setup corresponding to many other forest gap models (section
4.3.2), and (2) the full FORCLIM-E/P/S model, a setup coming closer to a true ecosystem
model (section 4.3.3). Finally, a method for estimating efficiently the steady state
species composition of FORCLIM will be developed (section 4.4).
4.1 FORCLIM-E
The FORCLIM-E model was run for the time window 0…5000 years at all 12 sites
(Tab. 4.1). These sites correspond to an ecological gradient from cool to warm and from
ecologically wet to dry (cf. the variables uDD and uDrStr, Cleuson – Sion in Tab. 4.1).
There is an emphasis on sites around uDD = 1900 °C·d (Huttwil – Basel) because these
conditions are typical for a large part of the Swiss Plateau.
It is evident from Tab. 4.1 that these sites do not correspond to a smooth gradient of the
drought stress index. For example, there are no sites with 0.06 < uDrStr < 0.2. In fact,
drought gradients are very steep in the European Alps. In central alpine valleys
precipitation decreases from around 800 mm to 600 mm over distances as small as
30 km (e.g. from Martigny to Sion; Martigny has similar climatic parameters as Basel,
Tab. 4.1); yet this corresponds to a strong increase of drought stress. These gradients
will be explored in more detail in chapter 5.
The cross-correlation coefficients between monthly temperature means and monthly
precipitation sums affect both the actual evapotranspiration (uAET) and the drought stress