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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114 Chapter 4<br />
600<br />
Airolo<br />
(E/P)<br />
Biomass (t/ha)<br />
500<br />
400<br />
300<br />
200<br />
100<br />
Ulmus scabra<br />
Quercus robur<br />
Quercus petraea<br />
Populus nigra<br />
Fagus silvatica<br />
Betula pendula<br />
Acer pseudoplatanus<br />
Acer platanoides<br />
Picea excelsa<br />
Larix decidua<br />
Abies alba<br />
0<br />
0<br />
400<br />
Year<br />
800<br />
1200<br />
Fig. 4.13: Average species composition from 200 forest patches simulated by <strong>the</strong> comb<strong>in</strong>ed<br />
FORCLIM-E/P model, assum<strong>in</strong>g a nutrient-rich soil at <strong>the</strong> site Airolo.<br />
4.4 A new method for estimat<strong>in</strong>g <strong>the</strong> equilibrium species<br />
composition<br />
In many studies us<strong>in</strong>g forest gap models, it is more important to evaluate <strong>the</strong> steady state<br />
species composition than to know <strong>the</strong> transient behaviour <strong>of</strong> <strong>the</strong> model start<strong>in</strong>g from <strong>the</strong><br />
highly unrealistic <strong>in</strong>itial condition <strong>of</strong> a bare patch. Under <strong>the</strong>se circumstances, it would be<br />
desirable to have a method for estimat<strong>in</strong>g <strong>the</strong> steady state species composition that avoids<br />
<strong>the</strong> need to simulate <strong>the</strong> transient behaviour on many patches. A way to achieve this is <strong>the</strong><br />
follow<strong>in</strong>g: Instead <strong>of</strong> simulat<strong>in</strong>g many patches (say, 200) over a comparably short time<br />
(say, 1200 years), one can simulate just one patch over a much longer time span. Discard<strong>in</strong>g<br />
<strong>the</strong> first centuries <strong>of</strong> transient behaviour, <strong>the</strong> average species composition over<br />
time will be <strong>the</strong> same as <strong>the</strong> average species composition across many patches, because<br />
<strong>the</strong> stochastic process underly<strong>in</strong>g forest gap models appears to be stationary.<br />
The species composition <strong>of</strong> two po<strong>in</strong>ts <strong>in</strong> time <strong>of</strong> one patch is autocorrelated (section<br />
2.2); hence <strong>the</strong> distance between <strong>the</strong> samples (∆t) should be chosen so that autocorrelation<br />
becomes negligible. <strong>On</strong> <strong>the</strong> o<strong>the</strong>r hand, <strong>the</strong> number <strong>of</strong> samples (n) should be sufficiently<br />
large. If <strong>the</strong> required ∆t and n fulfil <strong>the</strong> <strong>in</strong>equality ∆t·n < 200·1200, <strong>the</strong>n this estimation<br />
procedure is more efficient than <strong>the</strong> conventional method <strong>of</strong> simulat<strong>in</strong>g many<br />
patches.