Root Structure and Function Gradients in Salt Marshes

• Salt marshes are unique ecosystems characterized by micro-heterogeneity in elevation, soil, tidal inundation, salinity, and oxygen availability.
• In these extreme conditions, plants are adapted to each zone, so root traits also vary.
• Root biomass, density, tissue type, and size of roots are adapted for each of the three prominent zones in salt marshes.

Salt marshes are ecosystems that provide several valuable ecosystem services, such as protecting coasts from storms, stabilizing shorelines, providing forage, breeding grounds, and habitats for wildlife, and acting as carbon sinks. The underground root systems, which are adapted to the extreme environments in these ecosystems, are a crucial component of the system. This article will cover some of the root traits and their functions that benefit the plants and the ecosystem.

Salt Marshes Zonation

Salt marshes occur in areas between high and low tide in sheltered coasts and along estuaries, where there is little wave action but regular sedimentation. Extreme and adverse conditions characterize the ecosystems, which experience daily tidal inundation, salinity, mechanical disturbances, and waterlogging. Soils are anoxic a few mm below the surface but are also rich in organic matter due to high plant productivity and sedimentation from microphytobenthic production.

The salt marshes have distinct zonation in the dense halophytic vegetation, adapted to live in saline environments, and are composed of grasses, herbs, and shrubs. The vegetation that occurs in patches starts at sea level and changes along a gradual elevation. Three distinct zones are recognized in salt marshes. These are the low marshes, high marshes, and upper border, as shown in Figure 1. Below the low marshes are subtidal and tidal mudflats that are submerged in low and mid-tide and have constant water, salinity, and temperatures. Above the salt marsh zones and beyond the upper border are the uplands that are never flooded.

Low marshes: Low marshes are flooded by high tides daily for several hours. Cordgrass (Spartina alterniflora) grows in this zone, as it can withstand flooding. The shallow waters are warmed by the sun, increasing their temperature. When water dries, salt crusts are formed, increasing salinity. There are large amounts of organic sediments in the water and mud, and the oxygen levels are low.

Mid to high marshes: At slightly higher elevations, the soil is sandier and is covered by high tide only for an hour or two. High water evaporation leaves higher salt concentrations in this zone, which is also called the salt pans. Cordgrass cannot survive here, and only salt-resistant species like glasswort, saltwort, and salt grass grow here.

Marsh or upper borders: These are only rarely covered with seawater, which can occur during high spring tides for a few hours in a year. The salinity in the soil is low, as fresh rainwater washes away any deposits from seawater. Soil oxygen levels are also normal. Phragmites or common reeds are common here.

At lower elevations, environmental severity controls vegetation zonation, while interspecific competition drives the upper limits of salt marshes.

Figure 1: “Profile of a Long Island Sound Salt Marsh Ecosystem (Illustration by Lucy Reading-Ikkanda/LISS.” (Image credits: https://lispartnership.org/wp-content/uploads/2015/08/ SLAMMdid-you-know-fact-sheet2-V05.pdf)

Factors Causing Salt Marsh Zonation

The distribution of plant species and types is not random in salt marshes, but is correlated with ecohydrological zones, due to the gradient in biogeochemical processes and abiotic factors. The zones are a result of the combination of various factors like tidal flooding, oxygen availability, salinity, and biotic interactions.

Tidal flooding: The duration and frequency of floods are crucial. For example, black needle rush (Juncus roemerianus) grows at higher elevations with less flooding. Flooding causes waterlogging, which can lower or increase the intake of nutrients.

Oxygen availability: Oxygen depletion occurs due to the flooding and waterlogged conditions, soil conductivity, topography, and position of channels of flooding. In the absence of oxygen, toxic sulfides are formed, so plant exposure to toxins in soils increases. Root respiration is also limited, which can affect plant growth. Germination and seedling growth are also negatively impacted in anoxic soils.

Salinity: The frequency and duration of flooding influence salt concentrations in the soil. Soil salinity increases with elevation until the mean high sea level and then decreases as flooding reduces. Most angiosperms cannot survive in salinity, and only plants with special adaptations can survive in this zone.

Just as vegetation changes in response to small-scale heterogeneity along the zones, so do root traits, which must adjust to varying abiotic conditions. The variability in root traits, even within a species across zones, remains poorly understood. Some important root traits and their functions in the salt marsh zones that are well-established are discussed below.

Root Biomass

Root biomass is considered one of the most important underground traits for salt marshes, and is the one most reported in studies. Root biomass helps in sediment stabilization and sediment buildup and acts as a carbon sink.

• Sediment stabilization: Root biomass has several functions that help the marshes. Root biomass is crucial in ensuring sediment stability by binding soils in dynamic environments that experience regular flooding and soil shifting. It helps in preventing lateral erosion of the shoreline and acts as a buffer against wave energy.
• Sediment buildup: The root biomass makes a substantial contribution to the organic matter buildup, enough to raise the level of sediments. The contribution is in the form of living roots and rhizomes, some of which can be hundreds of years old! Even slight differences in elevation can have a significant impact on flooding frequency, water level, salinity, anoxia, and vegetation.
• Carbon sink: Plants in salt marshes invest more in underground growth to produce extensive root systems than in above-ground parts. For example, 50-90% of the biomass of Spartina alterniflora is found in roots. So, roots are crucial contributors to the salt marsh carbon sinks and nutrient cycling.
• Nutrient uptake: Roots in upper marshes, which have a nitrogen deficiency, grow large roots to explore more soil for nutrients.

Several factors can affect root biomass in salt marshes.

• Nutrients: Low marshes are nutrient-poor. So, plants grow more roots to explore for nutrients. The constant flooding and wave action make the sediment unstable, so plants grow roots to anchor themselves, which also prevents soil erosion.
• Species richness: The biodiversity of plants is another significant factor. As biodiversity increases in the high marshes, the root biomass increases. Since different species have different lengths and depths, the biomass accumulation is also greater.
• Sandy soils: In patches with more sand, plants grow more roots for better anchorage in soils, helping to stabilize sediments, as fine-grained silts are more resistant to erosion.
• Lifespan: Root longevity is higher in the stress-tolerant cordgrass than in upper marsh species like Elytrigia atherica, which can explain the high root biomass in the lower marshes.

Aerenchyma

Plants growing in the lower marshes that are most often inundated with seawater have to cope with anoxia.

Plants growing in anoxic soils also produce adventitious roots near the sediment surface to capture more oxygen from the air. For example, cordgrass in the lower marshes grows roots in the top 3 cm of the sediment and passes on the oxygen to deeper roots. The roots and rhizomes in this zone are also made of aerenchyma to enhance oxygen flow to the roots. The tissue also helps in detoxifying the roots of sulfur, iron, and manganese ions. These roots help the ecosystems when oxygen leaks into the rhizosphere, creating oxygen-rich niches in the anoxic soils, helping underground microbes, which in turn affect sediment chemistry.  These adaptations are not necessary in the upper border zones.

Root Density

Root density is vital in the lower salt marshes for trapping and stabilizing sediments. As root density increases, so does sediment stability. Intraspecific variability along the zones in root density is associated with stability. For example, cordgrass’s ability to stabilize sediments in low marshes is connected to its high root density.

Root density also increases as more species grow in an area, so increasing the number of species can improve sediment stability.

Root Type and Diameter

Plant roots, particularly in the Spartina or low marsh zone, reduced erosion and increased stability. It is not just root density but size also that matters. Spartina’s rhizomes and coarse roots were responsible for stabilizing the sediment, not fine roots, especially in sandy soils.

Fine roots (diameters <2 mm) with a short lifespan and rapid turnover are essential for nutrient uptake. These root traits are also relevant in salt marshes. The mid and upper marshes that have the most diversity of species and habitats have the highest fine root biomass compared to lower marshes dominated by cordgrass, or upper borders by Elytrigia atherica in Europe.

Due to differences in soil, where some areas of salt marshes can be sandy while other patches can be silty, differences can exist in nutrient availability. Fine roots, which are responsible for nutrient uptake, were 25% more in sandy patches than in soils with high silt content. Similarly, the delicate root surface area was greater by 50% in sandy patches than in silty soils, facilitating nutrient absorption.

So coarse roots are more in the lower marshes, and fine roots are more in the mid-marshes.

Minirhizotrons for Salt Marshes

The increased research focus on the root traits in salt marsh plants and their role in the ecosystem can benefit from the use of advanced plant science tools, such as minirhizotrons and root imagers. Minirhizotrons’ suitability for non-destructive data collection on roots in semi-aquatic ecosystems is well-established. However, the installation of the transparent root tubes requires more attention than in terrestrial areas due to inundation and wave action. The camera of the root imager can be inserted in the tubes for a 360-degree scan of roots growing around to calculate root length, width, area, longevity, and turnover. CID BioScience Inc. produces two imagers, the CI-600 In-Situ Root Imager and CI-602 Narrow Gauge Root Imager.

Find out more about the minirhizotrons and root imagers offered by CID BioScience Inc.

Sources

 

Auckland Council. (n.d.). Coastal wetlands, saltmarshes & estuaries. Retrieved from https://www.aucklandcouncil.govt.nz/environment/plants-animals/plant-for-your-ecosystem/docscoastalplantingguides/coastal-wetlands-salt-marshes-estuaries-planting-guide.pdf

 

De Battisti, D., Fowler, M. S., Jenkins, S. R., Skov, M. W., Rossi, M., Bouma, T. J., Neyland, P. J., & Griffin, J. N. (2019). Intraspecific Root Trait Variability Along Environmental Gradients Affects Salt Marsh Resistance to Lateral Erosion. Frontiers in Ecology and Evolution, 7, 427314. https://doi.org/10.3389/fevo.2019.00150

 

Gribsholt, B., & Kristensen, E. (2002). Effects of bioturbation and plant roots on salt marsh biogeochemistry: a mesocosm study. Mar Ecol Prog Ser 241:71-87 https://doi.org/10.3354/meps241071

 

Indian River Lagoon Species Inventory. (n.d.). Salt Marshes. Retrieved from https://irlspecies.org/misc/Saltmarsh.php#:~:text=Most%20plants%20that%20grow%20in%20anoxic%20soil,oxygen%20from%20the%20atmosphere%20to%20submerged%20roots.

 

Liu, H., Xu, X., Zhou, C., Zhao, J., Li, B., & Nie, M. (2021). Geographic linkages of root traits to salt marsh productivity. Ecosystems, 24(3), 726-737.

 

Moffett, K.B., Robinson, D.A. & Gorelick, S.M. (2010). Relationship of Salt Marsh Vegetation Zonation to Spatial Patterns in Soil Moisture, Salinity, and Topography. Ecosystems 13, 1287–1302. https://doi.org/10.1007/s10021-010-9385-7

 

Moffett, K. B., Gorelick, S. M., McLaren, R. G., & Sudicky, E. A. (2012). Salt marsh ecohydrological zonation due to heterogeneous vegetation–groundwater–surface water interactions. Water Resources Research, 48(2). https://doi.org/10.1029/2011WR010874

 

(n.d.). Salt Marsh Response to Sea Level Rise. Retrieved from https://lispartnership.org/wp-content/uploads/2015/08/SLAMMdid-you-know-fact-sheet2-V05.pdf

 

Redelstein, R., Dinter, T., Hertel, D., & Leuschner, C. (2018). Effects of Inundation, Nutrient Availability and Plant Species Diversity on Fine Root Mass and Morphology Across a Saltmarsh Flooding Gradient. Frontiers in Plant Science, 9, 330150. https://doi.org/10.3389/fpls.2018.00098

 

Reyes, A. W., & Chmura, G. L. (2022). Contribution of belowground plant components to salt marsh soil volume. Estuarine, Coastal and Shelf Science, 275, 107974. https://doi.org/10.1016/j.ecss.2022.107974

 

Sarika, M., & Zikos, A. (2021). Coastal salt marshes: Structure and function of plant communities. In Handbook of halophytes: From molecules to ecosystems towards biosaline agriculture (pp. 199-237). Cham: Springer International Publishing.

 

Silvestri, S., Defina, A., & Marani, M. (2004). Tidal regime, salinity and salt marsh plant zonation. Estuarine, Coastal and Shelf Science, 62(1-2), 119-130. https://doi.org/10.1016/j.ecss.2004.08.010

 

The University of Georgia. (n.d.). Salt Marsh Ecology. Retrieved from

https://gacoast.uga.edu/about/georgia-coast/salt-marsh-ecology/