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

142 Chapter 5
this phenomenon: (1) Most simply, the parameters denoting the drought tolerance of the
tree species may have been estimated erroneously; (2) the soil water balance submodel is
not capable of tracking soil moisture content under warm-dry conditions, especially
where soils are sandy; (3) the feedbacks between vegetation properties (e.g. LAI) and
soil water balance (e.g. evapotranspiration) that have been neglected in the model
formulation may become important under these conditions; (4) neither the “dry days”
approach (FORECE) nor the evapotranspiration deficit approach (FORCLIM) are
appropriate indices for expressing drought stress as experienced by trees; (5) the indices
are appropriate, but the relationship between the index and the annual growth increment is
wrong. Further research is required to address these issues.
Transition to insubrian and mediterranean forests: None of the three models is
capable of simulating the transition from beech forests to insubrian and mediterranean
oak-chestnut forests (Fig. 5.9): When the precipitation sum is above 1000 mm/yr, the
models do not simulate any drought. However, the large precipitation sum in these areas
(e.g. Ticino, Switzerland) does not mean that there is no drought: Often there are extreme
precipitation events where a large fraction of the monthly precipitation falls within a few
days. The monthly averages used in all three models do not capture the properties of such
distributions, and this may allow beech to dominate although it should be outcompeted
due to summer drought. Moreover, the moderate standard deviations of precipitation used
in the present analysis are not characteristic of these areas (cf. the omission of Locarno
for the derivation of climatic input data); this may also prevent the occurrence of dry
months with concomitant drought. Finally, it should also be taken into account that under
mediterranean conditions other species become abundant that were not included in the
species pool for European conditions, such as Quercus ilex. Also for this reason, the
present model approaches the limits of applicability in this area.
5.3.3 Conclusion
From the analysis of the steady-state species compositions of FORECE, FORCLIM-E/P
and FORCLIM-E/P/S in a space spanned by the annual mean temperature and the annual
precipitation sum, we may conclude that the two FORCLIM model variants produce
steady state species compositions that conform well to field-based empirical expectations
in large parts of this (T,P) space (e.g. Ellenberg & Klötzli 1972, Ellenberg 1986;
Fig. 5.1). On the other hand, the FORECE model contains several unrealistic thresholds
(Fig. 5.4, 5.5, 5.6).