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

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Parameter sensitivity & model validation 125<br />

auxiliary variable <strong>in</strong> <strong>the</strong> Kercher & Axelrod model and is calculated as a function <strong>of</strong> maximum<br />

tree age (kAm); thus <strong>the</strong> high sensitivity to kAm found <strong>in</strong> <strong>the</strong>ir study corresponds to<br />

<strong>the</strong> results given <strong>in</strong> Tab. 5.3. Unfortunately, <strong>the</strong>y did not <strong>in</strong>clude <strong>the</strong> light response parameters<br />

(kL a ) <strong>in</strong> <strong>the</strong>ir sensitivity analysis.<br />

The results from <strong>the</strong> sensitivity analysis by Dale et al. (1988) are not directly comparable<br />

with <strong>the</strong> present data because <strong>the</strong>y expressed kG as a function <strong>of</strong> maximum tree diameter<br />

(kDm), kAm, and maximum height (kHm). Their f<strong>in</strong>d<strong>in</strong>g that <strong>the</strong> sensitivity is kDm ><br />

kAm > kHm thus is difficult to compare with <strong>the</strong> results from Tab. 5.3. At <strong>the</strong> site<br />

Airolo, FORCLIM appears to be least sensitive to changes <strong>of</strong> kDm, which does not conform<br />

to <strong>the</strong> results by Dale et al. (1988). However, this f<strong>in</strong>d<strong>in</strong>g is quite important for<br />

FORCLIM because data for determ<strong>in</strong><strong>in</strong>g <strong>the</strong> kDm parameter are scarce (cf. Appendix II).<br />

In a sensitivity study <strong>of</strong> <strong>the</strong> FORSKA model, Leemans (1991) found that FORSKA is most<br />

sensitive to changes <strong>of</strong> <strong>the</strong> growth scal<strong>in</strong>g constant (correspond<strong>in</strong>g to kG), an allometric<br />

parameter for determ<strong>in</strong><strong>in</strong>g leaf area as a function <strong>of</strong> diameter at breast height (correspond<strong>in</strong>g<br />

to sType), and parameters <strong>of</strong> <strong>the</strong> light response function (kL a ). S<strong>in</strong>ce <strong>the</strong><br />

version <strong>of</strong> FORSKA used by Leemans (1991) <strong>in</strong>cluded nei<strong>the</strong>r nitrogen availability<br />

(kNTol) nor drought stress (kDrT) nor <strong>the</strong> effects <strong>of</strong> low w<strong>in</strong>ter temperatures (kWiT), it<br />

can be concluded that his results conform to <strong>the</strong> f<strong>in</strong>d<strong>in</strong>gs <strong>of</strong> <strong>the</strong> present study (Tab. 5.3).<br />

Tab. 5.3: Sensitivity <strong>of</strong> species composition at <strong>the</strong> site Airolo to changes <strong>of</strong> species parameters,<br />

summarized for each parameter. p – percentage <strong>of</strong> <strong>the</strong> number <strong>of</strong> species that show significant changes <strong>of</strong><br />

<strong>the</strong> species composition (α=5%, PS

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