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

46 Chapter 3
TREE ESTABLISHMENT
The germination of seeds operates at small temporal and spatial scales as compared to
many other processes in forest ecosystems such as tree growth (cf. section 1.2). The
factors influencing seed bank dynamics, germination, and establishment of small plants
are very difficult to develop mechanistically in an ecosystem model (Shugart 1984), and
the establishment of trees from seeds is the result of a long chain of random events
(Botkin et al. 1972a). Mortality rates of germinating seeds, seedlings, and small saplings
are overwhelmingly high (Kimmins 1987), so that only a minute fraction of the seeds will
ever become trees. These complicated phenomena can be portrayed in a simple manner
with a few environmental filters, such as light availability and growing season temperature
(Shugart 1984).
The following environmental filters (“flags”) are used for tree establishment in the
FORCLIM model (cf. section 2.3.1): (1) Winter minimum temperature (uWiT) is used to
exclude the species that do not tolerate extremely cold winters (gWFlag, Fig. 3.2;
Ellenberg 1986, Woodward 1988). (2) Light availability at the forest floor as determined
by the canopy trees prevents establishment of light-demanding species (gLFlag; Kimmins
1987, Fig. 3.2). (3) Mammals exert a considerable influence on tree recruitment in the
European Alps (Näscher 1979, Eiberle & Nigg 1986, Liss 1988, Albrecht 1989,
Rechsteiner 1993); thus browsing is incorporated to simulate the influence of species like
red and roe deer (Cervus elaphus L., Capreolus capreolus L.), whose population dynamics
are not modelled explicitly in FORCLIM (gBFlag, Fig. 3.2). (4) To avoid establishment
of saplings that would be killed anyway because they subsequently would fail to
grow, the annual sum of degree-days (uDD) is used as an environmental filter in FOR-
CLIM (gDFlag, Fig. 3.2).
Like most forest gap models, FORCLIM is aimed at modelling forest succession under
natural conditions. Therefore forest management practices such as planting and artificial
thinning are disregarded in the present model.
Most forest gap models establish tree individuals with very similar sizes (Shugart 1984).
Since tree growth in these models is treated deterministically, the size of all the individuals
of a given species established in a given year will remain similar throughout their
lifespan. Thus in the FORCLIM model these individuals are assumed to have identical size
and are established as one tree cohort. Tree growth then may be calculated once for each
cohort instead of each tree.