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

48 Chapter 3
photosynthesis (gALGF, Fig. 3.2). Gap models are geometrically explicit in the vertical
dimension, but most of them use a very simple approach to model crown geometry: All
the leaves are assumed to be concentrated at the top of the stem (Botkin et al. 1972a,b).
This assumption is not as unrealistic as it may appear; for example, Schulze et al. (1977)
found that in a Picea excelsa forest more than 70% of the annual CO 2 uptake was
attributable to the needles exposed to direct sunlight at the top of the crown. Leemans and
Prentice (1989) argued that sun angles in the boreal zone often are so low that most direct
sunlight incides from the side and not from above, making an explicit consideration of
true crown geometry necessary. For the present study, which deals with forests at temperate
latitudes where sun angles are much higher, the simple crown geometry of conventional
forest gap models (Shugart 1984) seems appropriate.
The following climate dependent constraints are used in FORCLIM: The direct effects of
temperature are modelled as the annual sum of degree-days (uDD). Woodward (1988)
showed that this index correlates well with the distribution of plant species (gDDGF), although
it may lack a physiological basis (Bonan & Sirois 1992). The water content of the
rooting zone is used to model the effect of drought stress (uDrStr) on growth (gSMGF,
Cramer & Prentice 1988), assuming that it is indicative of the water availability for plants.
In their classic fertilizer trials, Mitchell & Chandler (1939) found that tree growth increases
in a well predictable manner with increasing soil nitrogen concentrations (uAvN). Aber
et al. (1979, 1982) and Pastor & Post (1985) incorporated these findings in forest gap
models, and the same approach is used in FORCLIM (gSNGF).
The possible direct effects of atmospheric CO 2 on tree growth (“CO 2 fertilization”) are
still hotly debated in the literature (e.g. Eamus & Jarvis 1989, Overdieck & Forstreuter
1991, Körner 1993). While the short-term effects of enhanced CO 2 concentrations on
photosynthesis and water-use efficiency of tree seedlings and saplings seem to be well established
(e.g. Strain & Cure 1985), the long-term effects on older trees and whole ecosystems
remain undetermined and can not be extrapolated simply from the findings at
smaller scales (Eamus & Jarvis 1989). These authors also noted that at the ecosystem
scale “recourse must be made … to modelling” (p. 8). Simulation studies dealing with
this problem typically found that the response at the ecosystem scale is much smaller than
the increase in the growth rate of the single trees (e.g. Shugart & Emanuel 1985) or even
that there is no response at the ecosystem scale at all (e.g. Luxmoore et al. 1990). Based
on these studies and in view of the large uncertainties concerning this issue, the hypothesized
direct effects of atmospheric CO 2 on tree growth are neglected in FORCLIM.