Biogeomorphic behaviour

Coastal plants engineer their surrounding environment to increase changes of survival. Depending on how energetic the environment is, plants that use different survival strategies might emerge - ones that are resistant to erosion, and others that reinforce it.

Saltmarsh plants have evolved traits that help them survive the energetic environment of the coast, be that through growing large roots to help resist soil erosion, setting roots quickly from seedlings to avoid being washed away, or growing flexible leaves that resist tearing by the waves. 

The plant structure can also redirect water flow and trap sediment, which changes the elevation of the marsh (though sediment accretion or erosion). Different plant species grow best at specific elevations within the tidal frame, and deviation from these elevations risks plants being either outcompeted as conditions become more benign (high-elevation) or being lost as conditions become more exposed (low-elevation). 

Small differences in hydrological forcing (responsible for sediment transport and plant vigour) might tip the scale as to which plant communities are able to survive and ultimately engineer their environment at different points along the coast.

This research project is asking whether plant communities have evolved traits that either promote or reduce sediment accretion in order to maintain their “Golidlocks” elevations. Different plant communities may be engineering marsh morphology in order to build resilience.

what's being done?

We’re conducting an experiment to compare plant traits, soil composition, and prevailing hydrological conditions between a pioneer- and high-marsh community right next to the mudflat, to test the hypothesis that plant traits shift from reinforcing erosion (low erosion-resistance but quick seedling recovery) to resisting erosion (high erosion-resistance but slow seedling recovery) in response to small differences in the prevailing hydrological regime. Mwche Marsh on the Taf is ideally suited for this study because we already understand a lot about how the marshes have changed over time.

Along the marsh edge, we have several ‘mini buoys’ which measure inundation frequency, wave height, and current speed. The mini buoy consists of three ‘off-the-shelf’ items: an accelerometer (the same sort of thing you’d find in a smartphone) inside a UV-protected centrifuge tube, which is anchored to the mudflat by fishing line. The accelerometer in the mini buoy continuously records how the mini buoy moves. The first sign of movement indicates that the tide is in and the mini buoy has started floating (giving us the start of the inundation period). A consistent dip in the data indicates that the mini buoy is being pulled by the current in a certain direction. The angle of dip gives us current speed. If the accelerometer records that it’s swaying back and forth, this indicates that a wave is passing over the mini buoy. From this movement we can infer wave height. When the accelerometer no longer records any movement, this indicates the tide is out. The length of time since it started floating gives us the inundation frequency. 

We’ve also gathered plant and soil samples from the marsh. These will be used to determine how characteristics of the plants’ physiology differ between pioneer and high-marsh communities.

why it matters.

If we can demonstrate that saltmarsh plants play a crucial role in engineering the estuary landscape for their own benefit, we open the doors to better predicting how salt marshes will change under worsening river flooding and storm surging expected because of climate change. This in turn can help managers identify areas that are vulnerable to switching from high-marsh to pioneer-marsh environments and vice versa, which might impact on the role different marsh plant communities have on habitat provisioning.

This work is supported by the National Trust. Funding for this work has come from the Early Career Researcher Grant from the British Society for Geomorphology.