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

The forest model FORCLIM 47
TREE GROWTH
The mechanisms underlying tree growth, such as photosynthesis and the allocation of the
various forms of carbon to tree organs, which lead to processes like shoot elongation,
leaf development, and root growth, are not modelled explicitly in FORCLIM. It is not that
these mechanisms are not important in tree growth. Rather, they operate on small temporal
scales and equilibrate fast, so that they are not evident at the scale of the annual growth
rate of a whole tree (Shugart 1984). Whittaker & Marks (1975) argued that the enlargement
of a system requires a redesign of its proportions. Similarly, the dimensions of a
tree as they enlarge change in ways that maintain their functional balance. This allows to
calculate the dimensions of many tree organs from the dimension of other organs that are
simpler to measure. Such “allometric” relationships are widely used in forest science and
forestry (e.g. Burger 1945-1953, Mitscherlich 1970, King 1991, Woods et al. 1991,
Smith et al. 1991, Wang et al. 1991).
A tree dimension that is easily measured is the diameter at breast height (D). According to
the approach used in most forest gap models, in FORCLIM it is assumed that tree growth
can be expressed adequately as an increase in diameter at breast height, which thus is one
of the two state variables characterizing each tree cohort (Fig. 3.2). Allometric relationships
are used to calculate other tree measures from the diameter, such as height (Huang
et al. 1992), leaf weight and leaf area (Burger 1945-53), stemwood biomass (Woods et
al. 1991) and the production of twig and root litter (Pastor & Post 1985).
The following internal constraints on diameter growth are considered in FORCLIM: Under
optimal conditions, i.e. full sunlight, abundant nutrients, optimum temperature and adequate
soil moisture supply, gross photosynthesis is proportional to the photosynthetic
surface of the tree, i.e. its leaf area, and respiration is proportional to tree volume (Moore
1989). The latter assumption neglects that the fraction of nonliving tissue of a tree increases
with its age, but it is assumed that this still constitutes a reasonable approximation
(Shugart 1984).
Given this basic relationship for diameter growth under optimal conditions, the effects of
the abiotic and biotic environment are used to modify the optimal growth rate, which
results in the actual growth rate of every tree cohort (Fig. 3.2).
One of the most important external constraints on tree growth is shading, which determines
the amount of light a tree receives and thus the amount of energy available for