Salt marshes are a key feature of the coastline in Nova Scotia. Where conditions are suitable, these wetlands exist at the threshold between land and sea. The grasses that form the main vegetation cover in salt marshes are specialized on areas where they are periodically flooded by the tide, but at elevations high enough that they are not permanently underwater. They are only found in wetlands exposed to salt water; these plants are specialists. Botanists call these salt specialists “halophytes”, from the Greek words meaning “salt plant”. 

Historical practices of “reclaiming” land from the ocean using dykes or other tidal barriers stop the influx of saline water. Consequently, the halophytes disappear quickly, replaced by other kinds of plants that cannot tolerate salt. Happily, the reverse process happens just as quickly: when we “re-reclaim” salt marshes by removing tidal barriers and restoring tidal flow, the salt plants come back, usually within the first year or two[1].

This affinity for salt also has what I call “forensic” uses: it can indicate that salt water is getting in to places where it shouldn’t. In the Minas Basin, some dykes protect valuable farmland, but the dykeland infrastructure is threatened by sea-level rise and the increase in violent storms. These can result in seawater overtopping the dyke, but also damage aboiteaux (the one-way gates that allow freshwater out but prevent the tidal water from coming into the dykeland). Higher ocean water levels can also lead to water seeping through the dyke soil to the other side (“piping”). The presence of halophytes on the land side of dyke is almost always associated with one or more of the above phenomena, a great indicator that something is going wrong with the dykeland infrastructure. It also implies that in these dykeland systems halophyte seeds or rhizomes are almost ubiquitous in the local environment, blowing in on the wind or floating in the water, just waiting for salty conditions to return.

Extensive salt marsh vegetation landward of the dyke in a Minas Basin agricultural dykeland: salt water is getting in via a damaged aboiteau. ©J. Lundholm.

Saline environments are generally considered stressful for plants. When salt contacts plant tissues there can be direct damage, and salty soils make it harder for plants to extract water with their roots. All plants can exclude sodium and chloride ions from entering their roots, but halophytes can exclude these at much higher concentrations than regular plants. Ecologists have shown that if you transplant a halophyte into a salt-free environment, it may thrive, sometimes even growing faster and larger than in their typical salt-rich habitats.

If the halophytes prefer to grow without salt, why are they found almost exclusively in salty environments? Ecological experiments shed light on this. Researchers have measured halophyte performance when transplanted into non-saline places, comparing survival in patches where their neighbours (potential competitors) have been removed with those where the halophyte has typical freshwater species as neighbours. Usually, the halophytes show poor survival and growth when the freshwater neighbours are present in salt-free areas. They don’t usually occur in freshwater habitats because the freshwater species outcompete the halophytes in salt-free places. This implies that our salt plants don’t actually like salt; rather, they are excluded from more benign freshwater environments. Their strategy is to tolerate a stressful environment that other plants can’t. This phenomenon is thought to explain the zonation of dominant plant species in tidal wetlands: the most salt tolerant (and flooding tolerant) species form the “low marsh”, in the lowest elevations of the salt marsh, while brackish[2] and freshwater species are found at the higher elevations. Generally speaking, as you go from the salty ocean to the freshwater upland, the lower elevation species are the stress-tolerant ones, thriving in salty environments, while the species living at higher elevations are the competitive ones.

We’ve seen this in our restoration projects as well: upon opening a site to tidal flow following the removal of restrictions such as dykes, we often see smooth cordgrass, our main low marsh species, establishing all over the site, in high and low elevations alike. Over time, the dominant competitors in the higher elevation marsh zones eventually establish and outcompete the smooth cordgrass from those areas. Are our salt marsh plants just making the most out of a bad situation or are there cases where the halophytes are actually halophiles (“salt lovers”)?

Toxic substances can often be beneficial to an organism in small doses. Many of our medicines work like that: positive effects at low doses but toxic above some threshold. And salt in our diet works the same way: too little sodium in a human diet is fatal (we need 110 to 550 mg per day, according to Wikipedia), as we use this ion in many key metabolic processes, but too much salt also leads to negative consequences.[3] This phenomenon is called “hormesis”. I was curious as to whether any of the halophytes I work with in our salt marshes show this effect, so I did a deep dive into the published research on halophytes.

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Inland halophyte Red Glasswort (Salicornia rubra), next to a salt lake in Saskatchewan. ©J. Lundholm

What I found was that for the most part, halophytes exhibit the ecological pattern I mentioned previously: they tolerate salt but don’t really benefit from it. However, there were some exceptions where hormesis was clearly happening. Some of our salt marsh grasses and sedges performed slightly better when transplanted into saline environments, compared with freshwater habitats (even when neighbours were removed). For our low marsh dominant Smooth Cordgrass, the authors speculated that salt in the sediment might help the plant cope with other toxins that their roots are exposed to.

Other species had better germination rates with salt exposure than without. This phenomenon might represent a germination cue. Seeds often stay dormant until they sense the right conditions for germination, often warm temperatures and ample moisture. But for halophyte seeds, exposure to salty water may be how the plant knows when conditions are right for germination. I imagine the salt telling the seed: “You’re in a salt marsh — this is a good competitor-free space for you — go for it!” This phenomenon has been observed in species that are adapted to establish following wildfires (which eliminate potential competitors and increase the availability of some soil nutrients). Seeds of some of these species require exposure to some of the chemicals in smoke to germinate. After a fire, the chemicals are left as residue in the soil and carried to the seeds via rainwater.

My research group found another benefit of salt for a halophyte, in this case, not a salt marsh plant but one that grows on rocky barrens at coastal sites where the plants are frequently overwashed by ocean water. This is Roseroot (Rhodiola rosea), and we were growing it inland on a green roof in the absence of any salt exposure. The ones we transplanted to the green roof were plagued by aphids, while the plants growing in their natural setting on the barrens had very few. It was the same species of specialist aphid feeding on both groups of plants, and we speculated that the salt exposure of the barrens plants probably kept the soft-bodied pests to a minimum. Spraying the rooftop plants with a salt solution did cut down the aphid populations but eventually caused damage to the plants too. We figured it was difficult to artificially replicate the occasional overwash of seawater the plants would receive in their natural setting. Perhaps this is a classic case of hormesis: the right amount of salt keeps pests in check but too much salt kills the plants.[4]

Roseroot (Rhodiola rosea) on a coastal barren. ©J. Lundholm

 

In summary, our salt marsh plants are not just tolerating a habitat considered stressful by most plant species, but they may actually do better with a little bit of salt. The relationship between halophytes and their environment is complicated and requires the lens of ecology to understand it. Different stages of the plant life cycle, the presence of neighbouring plants and pests can all influence whether salt exposure benefits a halophytic plant. And in all cases, how much salt is present seems to matter: the dose makes the medicine.

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[1] For more information on tidal wetland restoration in the Maritimes, see TransCoastal Adaptations and CB Wetlands and Environmental Specialists

[2] Brackish wetlands have some salt from seawater but are less salty than true salt marshes due to mixing with freshwater sources. The plants that live in brackish conditions show some salt tolerance compared with species that typically inhabit fully freshwater wetlands.

[3] Even water works like this: drinking too much water can cause severe ionic imbalance and even prove fatal…

[4] This work is part of Hughstin Grimshaw-Surette’s honours thesis: https://library2.smu.ca/bitstream/handle/01/26606/Grimshaw-Surette_Hughstin_Honours_2016.pdf?sequence=1