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 123<br />
5.1.3 Results & discussion<br />
The detailed results <strong>of</strong> all <strong>the</strong> simulation experiments are listed <strong>in</strong> Appendix V; <strong>the</strong>y are<br />
summarized here, and <strong>the</strong>ir statistical properties are presented and discussed as well.<br />
ROBUSTNESS OF THE SIMULATED SPECIES COMPOSITION<br />
The percentage similarity coefficients between <strong>the</strong> sample steady states and <strong>the</strong> standard<br />
steady state are generally high (PS > 0.72 except for kDrt with Fagus silvatica;<br />
Tab. 5.2). The overall species composition appears to be little sensitive to changes <strong>of</strong><br />
s<strong>in</strong>gle parameters with<strong>in</strong> <strong>the</strong> plausibility range. This is a dist<strong>in</strong>ct difference to <strong>the</strong> FORECE<br />
model: For example, with an ITENO <strong>in</strong>dicator parameter <strong>of</strong> F. silvatica <strong>of</strong> 5 (Kienast<br />
1987), this species atta<strong>in</strong>s almost 40% <strong>of</strong> <strong>the</strong> total biomass simulated by FORECE at<br />
Airolo. However, when ITENO is <strong>in</strong>creased by just one class (to 6), F. silvatica disappears<br />
completely (cf. <strong>the</strong> simulations with ITENO = 6 <strong>in</strong> Kienast 1991). Similar phenomena<br />
can be observed <strong>in</strong> FORECE for o<strong>the</strong>r species at Airolo (e.g. Larix decidua) as<br />
well as at o<strong>the</strong>r sites, such as with <strong>the</strong> IMST parameter <strong>of</strong> Quercus spp. at Sion.<br />
Tab. 5.2 suggests that <strong>the</strong> FORCLIM-E/P/S model is sensitive ma<strong>in</strong>ly to <strong>the</strong> parameters<br />
<strong>of</strong> <strong>the</strong> most abundant species. Thus, <strong>the</strong>ir relative proportions are subject to considerable<br />
uncerta<strong>in</strong>ty: note for example that <strong>the</strong> lowest PS coefficients occur with F. silvatica, <strong>the</strong><br />
most abundant species. <strong>On</strong> <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> set <strong>of</strong> dom<strong>in</strong>at<strong>in</strong>g species produced with<br />
<strong>the</strong> default parameter set seems to be ra<strong>the</strong>r robust to errors <strong>of</strong> parameter estimation, i.e.<br />
<strong>the</strong>re are no species that turn up or disappear completely and alter <strong>the</strong> species composition<br />
qualitatively when <strong>the</strong>ir species parameters are changed with<strong>in</strong> <strong>the</strong> plausibility range.<br />
WHICH SPECIES PARAMETERS ARE MOST SENSITIVE?<br />
Accord<strong>in</strong>g to Tab. 5.3 and consider<strong>in</strong>g <strong>the</strong> lower end <strong>of</strong> <strong>the</strong> plausibility <strong>in</strong>terval <strong>of</strong><br />
species parameters, <strong>the</strong> model appears to be most sensitive to <strong>the</strong> nitrogen tolerance parameter<br />
(kNTol), followed by <strong>the</strong> species type (sType) and <strong>the</strong> growth scal<strong>in</strong>g constant<br />
(kG). For <strong>the</strong> upper end <strong>of</strong> <strong>the</strong> plausibility <strong>in</strong>terval, <strong>the</strong> rank<strong>in</strong>g is kNTol > kL a (shad<strong>in</strong>g<br />
tolerance <strong>of</strong> adult trees) > kG and kDrT (drought tolerance). The effects <strong>of</strong> <strong>the</strong> uncerta<strong>in</strong>ty<br />
<strong>in</strong>herent <strong>in</strong> kNTol on <strong>the</strong> simulated species composition suggest that <strong>the</strong>re is a strong coupl<strong>in</strong>g<br />
between FORCLIM-S and FORCLIM-P, as hypo<strong>the</strong>sized earlier (cf. section 4.3.4).