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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66 Chapter 3<br />
gL 9,c = 2.24 · 1 – e -1.136⋅(gAL gHc – 0.08) (3.23)<br />
F<strong>in</strong>ally, <strong>the</strong> light growth factor <strong>of</strong> <strong>the</strong> tree cohort (gALGF c ) is calculated by <strong>in</strong>terpolation<br />
between <strong>the</strong> above two functions, depend<strong>in</strong>g on kL a,s , a parameter denot<strong>in</strong>g <strong>the</strong> shade<br />
tolerance <strong>of</strong> adult trees (Fig. 3.7a):<br />
gALGF c = MAX gL 1,c + (kL a,s –1) · gL 9,c – gL 1,c<br />
8<br />
, 0 (3.24)<br />
Degree-day growth factor<br />
The effect <strong>of</strong> degree-days on tree growth (gDDGF s ) is modelled accord<strong>in</strong>g to <strong>the</strong> parabolic<br />
equation proposed by Botk<strong>in</strong> et al. (1972a,b) (Fig. 3.7b):<br />
gDDGF s = MAX 4 ⋅ (uDD – kDDM<strong>in</strong> s) ⋅ (kDDMax s – uDD)<br />
(kDDMax s – kDDM<strong>in</strong> s ) 2 , 0 (3.25)<br />
Soil moisture growth factor<br />
Bassett (1964) found that <strong>the</strong> basal area <strong>in</strong>crement <strong>of</strong> trees is related l<strong>in</strong>early to <strong>the</strong> amount<br />
<strong>of</strong> drought stress <strong>the</strong>y experience; thus diameter <strong>in</strong>crement can be expected to be related to<br />
drought stress (uDrStr) by a square root function. The latter relationship was <strong>in</strong>corporated<br />
<strong>in</strong> many forest gap models <strong>in</strong> order to represent <strong>the</strong> <strong>in</strong>fluence <strong>of</strong> drought on tree growth<br />
(gSMGFs, e.g. Pastor & Post 1985, Kienast 1987), tak<strong>in</strong>g <strong>in</strong>to account <strong>the</strong> maximum<br />
drought tolerance <strong>of</strong> <strong>the</strong> species (kDrT s , Prentice & Helmisaari 1991; Fig. 3.7c):<br />
gSMGF s =<br />
MAX 1 – uDrStr<br />
kDrT s<br />
, 0 (3.26)<br />
Soil nitrogen growth factor<br />
The equations by Aber et al. (1979), which are based on <strong>the</strong> fertilizer trials by Mitchell &<br />
Chandler (1939), are used to def<strong>in</strong>e <strong>the</strong> <strong>in</strong>fluence <strong>of</strong> nitrogen availability (uAvN) on tree<br />
growth rate (gSNGF s , Pastor & Post 1985):<br />
gSNGF s = MAX 1 – e kN 1,kNTols · ( uAvN - kN 2,kNTols ) , 0 (3.27)<br />
where kN 1,kNTol s and kN 2,kNTols are parameters with different values depend<strong>in</strong>g on<br />
kNTol s , <strong>the</strong> nitrogen tolerance class <strong>of</strong> <strong>the</strong> tree species (Fig. 3.7d).