Close Encounters

Rising sea levels influence a battle to beat the shade in plants

Increasing salinity could free some shade-averse plants from the shadows of their neighbours.

As sea levels rise, so land close to the shore will become more saline. Salinity stress will affect species in different ways, and this will have further effects. Jespersen and colleagues have examined how salinity stress will affect competition for light between two competing wetland grasses.

Left, Phragmites australis and right, Spartina anglica. Images: Canva.

Spartina anglica is a C4 grass introduced to Europe for land reclamation and is now invasive. While it can tolerate salt, it is short. In contrast, the C3 species Phragmites australis can grow twice to four times as tall, but it isn’t so salt tolerant. What P. australis do, though, is shade other plants, like S. anglica. Jespersen and colleagues sought to discover how salinity will affect these grasses and if there would be further effects on competition between the two kinds of grass.

The team grew the plants at five different salinity levels. They also varied access to daylight, 100% for one set of experiments and 50% for another. They then grew the plants for over a hundred days, with full access to nutrients, to see how the plants developed.

For P. australis, the results were dramatic. A plant that grew over eighty grams dry weight in salt-free conditions was reduced to fewer than twenty grams dry weight at the highest salinity. The number of shoots it grew per pot more than halved, as did its height. For S. anglica, in contrast, there was little change.

Both species struggled in the simulated shade.

“Our results, when related to expected increases in soil salinity, show that the competitive outcome between S. anglica and P. australis in the transition zone, and in other species with similar contrasts, could change in the future. This is because P. australis is able to suppress vegetation by attenuating light fluxes in its understory (Hirtreiter and Potts, 2012) whereas in saline conditions lower biomass, height, and shoot density reduce the ability to compete for light (Gaudet and Keddy, 1988, Craine and Dybzinski, 2013), while higher nutrient requirements likely add to this (Berendse, 1985). Interactions between high salinity and other factors that affect transpiration, such as high light fluxes, are likely to favour species C4 species and other salt tolerant species with low transpirational demands,” write Jespersen and colleagues.

As S. anglica is a C4 plant, it can benefit from higher salinity damaging competition. That, in turn, will give it freedom from shade and allow it to thrive. So rising sea levels could help the grass invade more coastland habitats.