Processes and parameters underlying the failure of salt marsh vegetation in different sediments
Berg, M.C. van den (2021)
Salt marshes are considered valuable habitats and provide a wide range of ecosystem
services, including contributing to coastal protection, stabilising coastlines, carbon
storage and providing habitat and marine nursery grounds. Therefore, many
projects have attempted to create and restore salt marshes but are often hindered
by a lack of thorough understanding of initial vegetation establishment. To determine
if vegetation can establish on an intertidal flat, Balke et al. (2011) developed
the Windows of Opportunity (WoO) framework. The framework consists of three
successive periods or windows,in which certain hydrodynamic conditions can not
be exceeded. In the first window, vegetation requires a short disturbance-free period
to develop roots (WoO1). This is followed by a period with calm hydrodynamic
conditions (WoO2) in which the vegetation’s roots can gain more strength and a
period in which the high-energy events do not exceed the vegetation limits (WoO3).
This thesis aims to determine the conditions under which juvenile pioneer salt marsh
vegetation fails and how this knowledge can be applied for the restoration and creation
of salt marshes.
An experiment was used to study the above and below ground development of juvenile
pioneer salt marsh vegetation in different sediments. The plants were subsequently
tested in a wave flume, using irregular waves to examine failure. For
this experiment the pioneer salt marsh species “Salicornia procumbens ” was selected;
this species is native to the Dutch coast and often one of the first plants to
establish on bare intertidal flats. Four batches of seedlings of different ages were
cultivated and tested in defaunated cohesive sediment (mud) and non-cohesive sediment
(sand). During the flume experiment the wave height and flow velocity were
measured at several locations in the wave flume.
The development of Salicornia seedlings aboveground was comparable in sediment
of the cohesive and non-cohesive type, although, in cohesive sediment, the plants
became more complex in a shorter period. Belowground, the bio morphology of the
Salicornia seedlings was significantly different. In sand, a complex root system developed
with numerous long thin roots, while in cohesive sediment, the roots were thick and short and the root system relatively simple. This difference was most likely
related the increase in soil strength as a result of the consolidation in cohesive sediment
and other sediment properties like nutrient availability. Another consequence
of this consolidation and increase in soil strength was that the erosion resistance
increased rapidly in recently deposited cohesive sediment.
The irregular waves in the flume stressed the seedlings due to the to-and-fro motion
of the plants and erosion as a result of the oscillating flow velocities produced by the
waves. Seedlings in cohesive sediment received on average more wave energy over
time because of the larger frontal surface area of these seedlings. Moreover, distinct
failure mechanisms were observed between the sediment types. In non-cohesive
sediment, erosion was the dominant process causing failure, while in cohesive sediment,
the to-and-fro motion of the plants that pried out and broke the roots was the
dominant process causing failure. Furthermore, the seedlings growing in cohesive
sediment could withstand a more extended period of wave loading and more wave
energy before failure, compared to seedlings of similar age in sand.
In practice, sediment with higher clay content may result in a higher survival rate of
Salicornia seedlings on the intertidal flats, especially near the regions with harsher
hydrodynamic conditions. Salicornia stands enable perennial salt marsh plants to
establish on the intertidal flats, for example, by trapping vegetative tillers of these
plants. These species are essential for further increasing biodiversity and plant succession
on a recently established salt marsh as well as stabilising the soil. This, subsequently,
will benefit the ecosystem services like wave attenuation, carbon storage
and provides more habitat and marine nursing grounds.
Van_den_Berg_MA_ET.pdf