Species interactions change the response of saltmarsh plants to flooding

Saltmarshes are on the front line of climate change. As sea-levels rise, inundation becomes more common. This rise will impact the plants we rely on for coastal protection, but how? A new study by Ryan Edge and colleagues has examined saltmarsh plants and found that their resilience can be affected by their neighbouring species. Co-author Dr Hannah Mossman explained how they came to look at multiple species. “We know from previous work that positive and negative interactions between plant species are important for shaping saltmarsh vegetation, so we were interested in investigating whether these interactions influenced how plants respond to flooding. This is important to test as we know that plant species composition varies both within and between saltmarshes, so if species composition affected responses to flooding, then we can’t expect every saltmarsh to respond to flooding in the same way.”

While there’s a lot of coastline that isn’t saltmarsh, Dr Mossman said that this was an excellent place to start looking at how species combine or compete with each other. “Saltmarshes are a great model system to investigate interactions because the number of species is low and so studying even a small number of species is useful as it represents real levels of diversity in communities.” So while the paper is on salt marshes, it has relevance for anyone looking at how species interactions affect ecological responses to environmental change.

The three plants studied sea aster, sea plantain, and sea arrowgrass, are all common in the middle elevations of saltmarshes. But Dr Mossman said that they weren’t interchangeable. “There are subtle differences in where each species can occur, with sea arrowgrass, for example, being able to grow in more waterlogged conditions than the other species. We, therefore, expected that these differences in occurrence in the field would be reflected in different sensitives to flooding [*spoiler alert* this was true to an extent, but responses were complex!]. Sea arrowgrass and sea plantain are also uncommon on restored saltmarshes, which we work on a lot and so we are always keen to know more about these species to help restoration efforts.

The current experiment lasted five months. Dr Mossman explained that while the length might look odd, not being quite half a year, there was a good reason for it. “Five months was selected to give plants time to grow but not get the point of senescence (although perennial, the study species die back to an extent after the end of the growing season). The study wasn’t planned to be a precursor of a long-term experiment, but of course, such an experiment over multiple growing seasons would be interesting to see if there are any multi-generational effects of flooding!”

The surprising result from the paper, for me, was that plant mortality was so low. I’m used to planting things in good conditions and seeing them die. The authors, who know what they’re doing, found other results much more surprising, as Dr Mossman said. “We were actually reassured by the low mortality as we had hoped to design a flooding treatment that changed conditions with a species’ niche rather than making conditions unsuitable for them. The most surprising result was the extent to which species interactions influenced responses to flooding – we thought that they might strengthen or dampen responses to flooding, but didn’t expect that the direction of flooding effects would change between species composition treatments.”

But how difficult was it to create an artificial saltmarsh in Manchester, and flood it, reliably, for five months? Dr Mossman said that the plants did most of the hard work. “Saltmarsh plants are quite easy to work with as they are inherently tough as they live in pretty inhospitable environments, but it was important that we didn’t stress them out more than was designed in our experimental treatment. It helped that I’ve always been green-fingered thanks to my mum working in a garden centre (and growing the best vegetables)! Although it was a lot of work to keep the trays topped up in hot weather, which was why we avoided the peak of summer.”

Despite being centred around saltmarsh, the paper has a message for planting in response to climate change. At the simplest level, it’s about the need for ecosystems instead of monocultures, but the conclusions are a lot more subtle than that. Dr Mossman said, “The complexity of flooding responses we observed, with species responding differently depending on which other species they are growing with, gives lots of potential for saltmarshes as a whole to be resilient to flooding. The next step is to get back out into the field to test this in real saltmarshes and to run some experiments in a new bit of kit we’ve developed that allows custom control of tidal cycles and the control of different nutrient treatments – TINA, the tidal inundation – nutrient addition machine.”

The results of this and future experiments should have relevance to people around the world working on how ecosystems can help buffer the effects of rising sea-levels.