Rising tides bring rising salinity to coastal plants, but will they cope?

Rising sea levels mean increased salinity for plants on the shore, but how well do coastal plants cope with salt? New research says they're surprisingly bad at it.

Coastal plants are on the front line of climate change. Not only do they have the rising temperatures and associated disruption that other plants face. They also have the threat of rising sea levels. A new study by Tiffany Lum and Kasey Barton looks at how two plants from Hawai’i, Jacquemontia sandwicensis (Convolvulaceae) and Sida fallax (Malvaceae), cope with increased salinity.

A seedling on the coast
Jacquemontia sandwicensis seedling at Kaʻena Point State Park, Hawaiʻi. Credit: Tiffany Lum.

Dr Kasey Barton said that for Hawai’i, these are two obvious plants to test for salinity tolerance. “We selected these species because they are two of the most common and widespread dune plants in Hawaii. Because of that, they form the foundation of the coastal dune ecosystems, providing habitat for nesting seabirds, native bees, and other native plants.”

Understanding how plants respond to increased salinity is important in areas where inundation from the sea will become more common. While plants may occupy the same shore, they will not respond to salt in the same way. This might seem obvious. What Lum and Barton added to their experiment was the observation that the same plant will also react differently, depending on its life stage.

The research aimed to look at the two plants at four ontogenetic stages. Seeds, seedlings, juvenile plants and mature plants. Dr Barton explained that the plants are all different at each of the four stages. “Ontogeny is a genetically regulated developmental pathway. As such, there are genetic thresholds, although, in practice, it’s not always easy to identify these thresholds by looking at plants. Technically, seedlings are plants that still depend on seed reserves from the maternal plants. Identifying when seedlings become juvenile plants can be tricky. Identifying the transition from juvenile to mature ontogenetic stages is more straightforward because it coincides with the onset of flowering. All flowering plants are mature.”

Lum and Barton put plants at the different ontogenetic stages into the greenhouse and watered some of them with high-salinity water. They could then watch and see how the plants reacted. Unluckily for them, Dr Barton found something else was watching one of the experiments. “Unfortunately, we got white flies in the greenhouse during the summer, which infested the S. fallax and led us to harvest the plants early. We typically don’t have many pest problems in the greenhouse, so this was a surprise, and we were unable to get it under control.”

Dr Barton also found part of the experiment had a tendency to wilt under the glass, but not the plant part. “Our greenhouses are very hot because they don’t have good cooling systems, although coastal plants are adapted to very sunny and hot conditions, so high temperature wasn’t a problem for the plants, just for us working in the greenhouses.”

When the results came in, Lum and Barton found that the coastal plants reacted to salt in an unexpected way. Dr Barton explained: “Our most important finding is also one that really surprised us, and that is that these two species are actually not very tolerant to high salinity. We expected that because they grow in dune habitats where there is common salt spray and storm surges, that they would be adapted to tolerate salinity. However, they performed really poorly under high salinity, and so probably depend on frequent rain to remove salinity from the sandy soil.”

The could be bad news for conservation efforts, said Dr Barton. “The resilience of our coastal beaches depends on persistence of native coastal plants. Coastal dunes and beaches are important areas for recreation and cultural activities, and with sea level rise, the dune plants are experiencing higher levels of salinity. If plants aren’t able to tolerate this increased salinity, then the plants will die, and the beaches will erode away.”

“These species form the foundation of the coastal dune ecosystems, providing habitat for nesting seabirds, native bees, and other native plants. They also occur with other more rare coastal plants that depend on these coastal dune communities. We would lose entire ecosystems and beach stability if these two species disappear due to climate change.”

The aim now is to learn more about plant tolerance for salinity. Dt Barton said the experiment showed that it’s not a simple task to model responses. “Another surprising discovery from this research was that the two species, which are similar in form, lifespan, and seed size, responded very differently to the salinity treatments. This suggests that it will be difficult to generalize from our study to all the other species that grow in the coastal dune habitats.”

“Moving forward, we need to run similar experiments on more species to better understand why species differ, what ontogenetic stages are most vulnerable to salinity, and how we can manage this threat. We’d like to do some field experiments in addition to the greenhouse experiments so we can test how salinity affects plants under the added stress of high light, high temperature, and wind. That will really give us a better idea of how these plants will respond to sea level rise in the future.”

Further reading

Lum, T. D., & Barton, K. E. (2019). Ontogenetic variation in salinity tolerance and ecophysiology of coastal dune plants. Annals of Botany. https://doi.org/10.1093/aob/mcz097