While a single plant is capable of fixing inorganic carbon dioxide (CO2) from the air, the entire ecosystem surrounding the plant, including water, other organisms and soil conditions, influences how efficiently the ecosystem exchanges CO2.
Understanding how photosynthesis, energy allocation and productivity differs in plants in various ecosystems is challenging. In the forest ecosystem, taller trees grow over decades to absorb all of the available solar radiation in the canopy, depriving seedlings of the sunlight required to increase their biomass. Likewise, marsh grasses adapt to changes in marsh elevation that affect the amount of flooding the grasses experience and the energy the plant allocates to leaf or root growth.
In order to better understand how the dynamics of ecosystems can change plant productivity, scientists from the University of South Carolina and Christopher Newport University studied the photosynthesis and respiration, or energy expenditure, of a single species of marsh grass, Spartina alterniflora, that grows in tall or short forms depending on the elevation of the marsh and the proximity of plants to tidal creek water.
The team published their study on December 11, 2024 in the journal Ocean-Land-Atmosphere Research.
"In saltmarshes of eastern North America, there is typically a spatial productivity gradient of the dominant grass, Spartina alterniflora. The grass is tallest and most productive near the edges of tidal creeks while the interior marsh areas are covered by a short, less productive form of Spartina. Our objective was to quantify the exchange of CO2 gas between the atmosphere and the grass canopy and soil. We did this to better understand how marsh sites of high and low productivity compare," said James T. Morris, professor at the University of South Carolina and first author of the research paper.
Importantly, the researchers ensured that all of the photosynthetic parameters, or factors that influence the efficiency of photosynthesis in a plant, in the study were equal, including the amount of available sunlight, temperature and the species of plant. The scientists carefully measured grass growth, photosynthesis and respiration using sealed environmental chambers that allowed the team to measure the activity of a specific area of the marsh ecosystem. Specifically, measurements were collected from tall forms of the marsh grass located closer to the creek at lower elevations and from short forms of the plant located further from the creek at a higher elevation.
By regularly measuring grass biomass, CO2 gas uptake for photosynthesis and CO2 release through respiration over the course of a year, the team was able to compare carbon fixation between the tall and short forms of grass. In some cases, the short- and tall-form grasses showed similar characteristics, such as demonstrating highest levels of canopy respiration, or energy expenditure above ground, in early March when the standing biomass of both grass forms is lower. Gross photosynthesis, or the total amount of CO2 consumed for photosynthesis, for both plant forms also plateaus in mid-summer.
More importantly, the short and tall forms differed in their productivity over the course of a single year. Soil respiration, or a measure of the amount of CO2 released and energy consumed by plant roots, was higher in short-canopy grasses compared to tall. Interestingly, the leaf weight-specific rate of photosynthesis at a common canopy biomass was similar in both short and tall grasses, but the study found that the short canopy plants grew less than the tall plants.
Because canopy growth of short-canopy grasses slowed earlier during the growing season than tall-canopy grasses, the team also found that tall-canopy grasses capture more atmospheric CO2 over the course of the year than short-canopy grasses, which grew further from the creek at higher elevations.
"We found that the photosynthetic parameters of the grasses were equivalent and the differences in their productivity were determined by differences in the partitioning of growth between leaves and roots. The less productive short form of grass invests more energy in growth of roots. A second major finding was that in a single growing season the biomass of the most productive form of grass expands to intercept all of the available solar energy much like a mature forest," said Morris.
Based on their results and those of other researchers, the team hypothesizes that the variability in net carbon sequestration between different salt marshes is due to changes in differences in relative marsh elevation, climate and marsh age.
The next step for the research team is to resolve a discrepancy in the amount of measured carbon the grasses were investing toward the growth of the canopy and roots, respectively, which should be roughly equal. "We discovered that a major part of the [plant] carbon budget is missing [in our measurements]. We were unable to balance total plant growth with total photosynthesis. The next step will be to identify the source of the missing carbon," said Morris.
Gary J. Whiting from the Department of Organismal and Environmental Biology at Christopher Newport University in Newport News, VA also contributed to this research.
This work was supported by National Science Foundation (NSF) awards 2203324 and 1654853.
