Where do halophytes grow? Influence of elevation, flooding, and salinity in a non-tidal saltmarsh

Imagine living in a packet of Salt’n’Vinegar crisps. Bear with me. The packet gets filled up with water, dried out, then filled up with water again. Doesn’t sound like much fun, does it? Well, this salinity and periodic flooding is the sort of environment that halophytic plants in coastal saltmarshes have adapted to grow and thrive in. 

These saltmarshes can either be tidal (experiencing inundation with seawater pretty much every day) or non-tidal (disconnected from the sea, with less frequent flooding and longer dry periods). In tidal saltmarshes, the distribution of halophytes is strongly influenced by elevation, with the frequency and duration of flooding combining to limit oxygen supply to sediments. But where do halophytes grow in non-tidal saltmarshes, and why?

A recent study by Vélez-Martín and colleagues aimed to figure this out, by examining whether elevation, flooding period, or salinity best explained the distribution of halophytes in the non-tidal saltmarshes of Doñana National Park, in south-west Spain. Over 7 consecutive years, Vélez-Martín and colleagues recorded vegetation in permanent quadrats on the Doñana saltmarshes, and determined sediment salinity, surface elevation, and the duration and extent of surface flooding. 

So what did they find? First, the Doñana saltmarshes supported a much wider range of species than their tidal counterparts, with a mosaic of halophytic plants in the Amaranthaceae family (AKA chenopods) including numerous annual species and a few perennial shrubs. They wrote that “the consequences of elevation for halophyte distribution in seasonally flooded saltmarshes are fundamentally different from those in tidal marshes”, and that these differences “could explain greater species diversity in non-tidal marshes and the absence of key saltmarsh species prominent in tidal marshes”.

The researchers also noted clear differences in the drivers of distribution for perennial versus annual halophytes. Specifically, they found that flooding period best explained the abundance of annual species, while elevation was more important in explaining the abundance of perennial species, and that the independent effects of salinity varied according to individual species’ salt tolerance. Finally, the researchers also touched on the possible effects of climate change for the saltmarshes of Doñana National Park, noting that “the vegetation of non-tidal marshes will be particularly susceptible to the more extreme annual cycles of temperature and rainfall predicted for Mediterranean climates”. 

So, in the non-tidal saltmarshes of Doñana National Park, where do halophytes grow? For perennials, it depends on elevation. For annuals, it depends on flooding period. For individual species, it depends on salinity. And, for the future, it will depend on mitigating the impacts of climate change for these saltmarsh habitats.