Abstract:
Tropical forests are well known for their exceptional species richness ? high diversity of
plant species constitute the basis for an equivalently rich fauna. An astonishing variety
of plant life strategies has evolved, manifesting itself also in different compositions of life
history traits in trees. This thesis investigates the role of tree life history traits (growth,
mortality and recruitment) on different processes structuring species-rich forests. Our
study system is a montane rainforest located in the Tropical Andes hotspot of biodiversity
in southern Ecuador. Here, we find a mosaic of steep ridges and deeply incised
valleys, covered with predominantly broadleaf forest. Forest structure and species composition
differ considerably depending on altitude and topographic position. The forest
cover is frequently interrupted by scars of landslides, which constitute an important type
of natural disturbance in this ecosystem.
We utilize ecological models as tools to gain deeper insights into key processes driving
the maintenance of tree species richness and affecting forest recovery after landslides.
The first part of this thesis concerns the question of species coexistence. We develop
a theoretical model to analyze how different trade-offs between life history traits (tree
growth, seed dispersal, tree mortality) affect tree species coexistence. We find that the
considered trade-offs alone are not sufficient to explain long-term species coexistence.
Additional ?stabilizing? mechanisms seem to be indispensable to facilitate coexistence in
species-rich forests. Such mechanisms could result from biotic interactions, that alter
the relation between inter- and intra-specific competition depending on (local) species
abundances (e.g. density-dependent mortality). Other possible coexistence mechanisms
likely to be relevant to our particular study system are driven by external, abiotic factors
like a complex topography resulting in locally differing habitat types (each supporting a
different set of species), or the character of a prevailing disturbance regime (e.g. shallow
landslides).
In the second part of the thesis, we investigate the growth dynamics of the ridge forest
in our study system. To this end, we utilize the process-based forest growth model
FORMIND. We show that after calibration, the model successfully reproduces forest
dynamics on different levels of complexity (e.g. basal area and stem size distribution).
We then use this forest model to investigate the influence of landslide disturbances on
forest dynamics both on the local scale of a single landslide and on the landscape scale.
On landslide sites, changes in environmental conditions might lead to changes in different
tree life history traits. We analyze scenarios with changes in different traits (tree recruitment,
tree growth, tree mortality) and find that while tree biomass can recover within the
first hundred years after a landslide, the time until forest structure and species composition
is restored is considerably longer (approximately 200 years). Changes in different
traits result in differing spatial distributions of tree biomass: reduced tree growth leads
to a more homogeneous distribution of biomass, whereas reduced recruitment and increased
mortality yield a more heterogeneous biomass distribution (?patchy? vegetation).
On the landscape level, overall forest biomass is substantially reduced by landslides (8 -
I
14%), compared to only 2 -3% of the area marked by visible traces of landslides. Thus
this particular type of disturbance considerably influences the total forest carbon balance.
In a complementary investigation we study abiotic and biotic factors that potentially
trigger landslide occurrence in our study system. For this, we develop an extension of
a standard physically-based model of slope stability. We find that due to the predominantly
shallow tree roots, some of the observed landslides might be triggered by the
vegetation itself.
This thesis demonstrates that ecological models are useful tools to gain deeper insights
into important processes shaping forest communities. They can be applied for theoretical
questions such as the question of species coexistence, as well as for more applied,
management related questions like predicting forest recovery after disturbances.
