How does Spartina alterniflora react to long-term wave exposure?

A multi-national team has been investigating how Spartina alterniflora (also Sporobolus alterniflorus) reacts to long-term wave expsoure. The findings could affect planning for ecosystem-based flood defences.

Professor Dongdong Shao of Beijing Normal University explained why they chose Spartina alterniflora to study: “We chose S. alterniflora as the test species mainly because it is a perennial grass native to the east coast of North America and is spreading rapidly to estuaries and coastal salt-marshes in other parts of the world, including the Pacific coast of North America, Europe, New Zealand and China. It is, therefore, a globally relevant species. Also, it is often employed in eco-shoreline projects due to its wide range of expansion and strong salt- and waterlogging-tolerance. Testing is important as the species may be expected to be exposed to different wave regimes, as it grows both at wave exposed and more sheltered locations.”

S. alterniflora is a grass that’s invasive in many locations, and it is a problem in China, though recent research has found found some limits on its invasion. While there has been research into modelling its growth, knowing how the plant responds to various wave dynamics would help how it will react in new locations. But how do you get that information?

The experiment might seem obvious, get a bunch of plants, then stick them in a tank and make waves. This is not what happened. What the team did was move the plants, by swinging them through the water. Prof Shao explained why: “The primary technical constraint of traditional wave flumes that move the water to generate waves through wave maker lies in the inability of the wave maker to operate for several weeks without burning. Even it manages to do so, the way it generates waves will make the wave field a complete mess after continuous reflection from the downstream flume wall for several weeks. Of course, replication for physiological experiments is an added issue. As such, this study adopted a novel approach to move the plants instead of the water through a slider-crank mechanism to mimic plants living under shallow-water wave conditions for a relatively long period of time.”

By reducing the mechanical demands of a wave flume, Prof Shao believes the system opens opportunities for testing wave stress with other plants. “Further tests with other salt-marsh plants such as Phragmites australis and comparison of their responses can help inform eco-shoreline projects in terms of plant selection, suitable transplantation locations, the optimal timing for transplantation, etc. Both submerged and emergent plants can be tested, and the more challenging part would be to acclimate the plant species in the laboratory condition, particularly for sensitive species such as seagrass, and ensure their healthy growth even without adding wave stress.”

The paper should be relevant to shoreline conservation teams around the world, said Prof. Shao. “Our research presents an effective experimental facility for studying the response of salt-marsh plants to long-term wave exposure. Moreover, the results presented herein show that wave exposure leads to oxidative stress in plants and suppresses plant photosynthetic capacity and thereby, growth. In response, the wave-exposed plants exhibited activated antioxidant enzymes. Comparison between the different wave treatment groups suggested the wave effects to be generally correlated positively with wave height and negatively with wave period, i.e., waves with greater height and frequency imposed more stress on plants.”

“In addition, wave-exposed plants tended to allocate more biomass to their roots. Such allocation is favourable because it enhances root anchorage against the wave impact.”