On the Ecology of Mountainous Forests in a Changing Climate: A ...
On the Ecology of Mountainous Forests in a Changing Climate: A ...
On the Ecology of Mountainous Forests in a Changing Climate: A ...
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Parameter sensitivity & model validation 143<br />
The FORECE model fails to simulate <strong>the</strong> occurrence <strong>of</strong> oak (Quercus spp.) except under<br />
warm-dry conditions close to <strong>the</strong> dry timberl<strong>in</strong>e. In <strong>the</strong> FORCLIM-E/P model, oak is a<br />
codom<strong>in</strong>ant species at higher temperatures, which conforms better to <strong>the</strong> expectations by<br />
Rehder (1965) and Ellenberg (1986). In <strong>the</strong> FORCLIM-E/P/S model, <strong>the</strong> soil submodel<br />
(FORCLIM-S) produces periods with low nitrogen concentrations, and oak gets a competitive<br />
advantage, thus <strong>in</strong>creas<strong>in</strong>g its abundance (Fig. 5.7). Accord<strong>in</strong>g to <strong>the</strong> FORCLIM<br />
model, oak requires this heterogeneity <strong>of</strong> nutrient availability to be competitive; this constitutes<br />
an <strong>in</strong>terest<strong>in</strong>g hypo<strong>the</strong>sis that requires fur<strong>the</strong>r test<strong>in</strong>g.<br />
The problems encountered with all three models along drought gradients deserve to be<br />
studied <strong>in</strong> more detail: Simulation experiments performed with <strong>the</strong> FORECE, FORSKA,<br />
and FORCLIM models <strong>in</strong> <strong>the</strong> warm, dry area extend<strong>in</strong>g from Germany through Poland<br />
<strong>in</strong>to Byelorussia suggest that each model fails <strong>in</strong> different ways, also affect<strong>in</strong>g <strong>the</strong> behaviour<br />
<strong>of</strong> species such as Tilia spp. and Carp<strong>in</strong>us betulus (M. L<strong>in</strong>dner & P. Lasch,<br />
pers. comm.). They found vast differences between <strong>the</strong> models e.g. concern<strong>in</strong>g <strong>the</strong><br />
amount <strong>of</strong> simulated evapotranspiration and drought stress. Probably several <strong>of</strong> <strong>the</strong> hypo<strong>the</strong>ses<br />
listed above are <strong>in</strong>volved <strong>in</strong> caus<strong>in</strong>g <strong>the</strong> failure <strong>of</strong> forest gap models <strong>in</strong> this area.<br />
Exclud<strong>in</strong>g <strong>the</strong> areas where both FORECE and FORCLIM fail to produce plausible results,<br />
we may conclude that FORCLIM simulates more plausible species compositions and more<br />
realistic gradients, whereas FORECE conta<strong>in</strong>s many threshold effects. Especially <strong>the</strong> latter<br />
renders <strong>the</strong> application <strong>of</strong> FORECE for impact studies <strong>of</strong> climatic change questionable. <strong>On</strong><br />
<strong>the</strong> o<strong>the</strong>r hand, FORCLIM may be considered to be a valid tool for simulat<strong>in</strong>g forest dynamics<br />
for a large part <strong>of</strong> <strong>the</strong> range <strong>of</strong> temperature and precipitation explored <strong>in</strong> this experiment.<br />
5.4 Behaviour <strong>of</strong> FORCLIM <strong>in</strong> eastern North America<br />
In a well-known application <strong>of</strong> <strong>the</strong> forest gap model FORENA, Solomon (1986) studied<br />
forest dynamics at 21 locations along a latitud<strong>in</strong>al gradient <strong>in</strong> eastern North America, extend<strong>in</strong>g<br />
from <strong>the</strong> Canadian tundra to <strong>the</strong> temperate-subtropical forests <strong>of</strong> sou<strong>the</strong>rn<br />
Georgia. The application <strong>of</strong> FORCLIM to perform simulation experiments along this same<br />
gradient appears to be <strong>in</strong>terest<strong>in</strong>g, but is faced with two problems: First, <strong>the</strong> near-natural<br />
forests <strong>of</strong> Central Europe and eastern North America have no species <strong>in</strong> common; thus it<br />
is necessary to change <strong>the</strong> species pool and to derive <strong>the</strong> FORCLIM species parameters for