ABSTRACT
An individual-based tree population model is
presented to simulate the forest dynamics on former heathlands in
Northern Germany. This is the JABOWA-II forest gap model, which
is based on and tested with the results of long-term ecological
research in forests of Eastern North America.
In this model, the growth of individual trees on a forest plot of 100m
2 is calculated. The calculations are based on the growth
response
of trees to environmental factors: light, temperature, soil moisture
and
nutrients. Every tree species is characterized by a set of
response
functions, representing the physiological amplitude or the fundamental
niche
of the species. The model uses stochastic functions to calculate
random
events such as reproduction and death of trees. The projections
reflect
the outcome of inter- and intraspecific competition between trees over
time
and therefore calculate the realized niche of the species. The
model
can be viewed as a mathematical expression of Ellenberg's ecograms of
tree
species, with the addition of a time axis for the process of forest
succession.
Forest species compositions under a variety of
environmental
conditions projected by the model are in good agreement with
descriptions
in the ecological literature. The fact that the same ecological
model
makes realistic projections for both forests in eastern North America
and
Central Europe speaks for the generality of the underlying processes of
forest
dynamics, upon which the model is based. A series of projections
for
a specific case study - the forest succession on former heathlands in
Northern
Germany - was conducted. The results confirm the importance of
the
proximity and amount of seed sources, especially during early stages of
heathland
succession.