| dc.description.abstract | Salt marshes are highly valuable habitats and offer many important ecosystem services. They protect
the coast from erosion and serve as pollution filters and nursery ground for fish. Furthermore,
they are capable of storing a very high amount of carbon in a relatively short time. Their protection
and restoration is therefore crucial for the mitigation of global warming. Salt marshes are shaped
by a variety of biogeomorphic feedbacks which depend on many different biotic and abiotic aspects.
In order to clarify the conditions needed for such biogeomorphic interactions and the effect of different
vegetation colonisation patterns on it, four scaled landscape experiments were conducted in
the tidal flume "The Metronome". Two experiments were unvegetated control experiments and
two included vegetation. In one of these, the seeds were spread by the flowing water and in the
other, circular patches were sown and expanded manually at random locations.
Vegetation focused the flow more effectively and led to a split-up into several channels further
from the inlet, resulting in a longer and straighter main channel and an overall further landward
expansion of the system. Despite these clear morphological differences, quantitative measures of
the system, like eroded volume or drainage density, were very similar in all experiments, showing
that these are mainly shaped by the hydrodynamic boundary conditions and that the experiments
are very comparable.
The vegetation-induced morphological differences described above were more pronounced in the
experiment with hydrochorous seed spreading than in the experiment with patchy seeding. This
is a result of the patch locations being pre-determined randomly. This disables the biogeomorphic
feedback between low flow velocities and vegetation establishment, causing further reduction of
flow velocities. The completeness of this positive feedback loop is crucial for the emergence of a
self-organised landscape. Even though the patches were very dense and covered a larger area of
the system than the vegetation in the experiment with hydrochorous seed spreading, they were not
able to foster the evolution of a self-organised landscape in a similar way.
Only locally, the patches sometimes had strong biogeomorphic effects. Their density enables them
to show such effects, like the stabilisation of features or the initiation of channels around them, very
clearly, but only if they are in the locally favourable position for that. Since this was not the case
everywhere, the overall morphology in the experiment with patchy seeding was less self-organised
and more similar to the unvegetated control experiments even though the biogeomorphic feedbacks
were more pronounced in some locations. The feedback loops of biogeomorphic feedbacks thus
need to be complete to enable the emergence of a fully self-organised landscape. | |
