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

The forest model FORCLIM 69
Stress-induced mortality
Analogous to the age-related mortality, the increased mortality rate induced by environmental
stress (gP m2,c ) is based on the assumption that only a small fraction of trees will
survive a given number of years when they are subject to such stress (Shugart 1984,
Pastor & Post 1985, Kienast 1987, Solomon & Bartlein 1993):
gP m2,c =
kSlowGrP SGr c > kSGrYrs
0 else
(3.31)
where SGr c is the the number of consecutive years the cohort's diameter has increased
less than 10% of the maximum diameter increment (kMinRelInc) or less than 0.3 mm
(kMinAbsInc). Hence, SGr c provides a memory for past environmental stress; therefore
it is a state variable (Eq. 3.32):
SGr c (t+1) =
SGr c (t) + 1 ƒ(e) c < kMinRelInc ∨ ∆D c
∆t
0 else
< kMinAbsInc
(3.32)
where t denotes the discrete time in years.
Disturbance-related mortality
As noted in section 3.1, the disturbance-related mortality is formulated in a simple manner,
i.e. the probability that the trees on the patch are killed by a disturbance is regulated
by the model parameter kDistP (Eq. 3.33):
gP m3 = kDistP (3.33)
Overall mortality probability
Eq. 3.34 describes the calculation of the overall mortality probability for each tree and
each year (gP m ). The trees are subject to the disturbance-related mortality first (gP m3 ); if
they survive, they may die from the age-related mortality (gP m1,s ) and, finally, from the
stress-induced mortality rate (gP m2,c ). In the simulation model, the overall mortality
probability (gP m ) is determined for each tree using Monte Carlo techniques.
gP m = gP m3 + (1 – gP m3 )·(gP m1,s + [1 – gP m1,s ]·gP m2,c ) (3.34)