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. The fixation of CO₂ in any autotrophic ecosystem is ultimately limited by the availability of incident light energy. In the forest ecosystem, for instance, 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, Dr. Jim Morris and his collaborator, Dr. Gary Whiting (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. Their study titled "Components of the CO2 Exchange in a Southeastern USA Salt Marsh" reveals that the short and tall forms of grass 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. Although the leaf weight-specific rate of photosynthesis at a common canopy biomass was similar in both short and tall grasses, the short canopy plants grew less than the tall plants. 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. Overall, these results demonstrate that the differences in the productivity of the grasses are determined by differences in the partitioning of growth between leaves and roots, and not by different photosynthetic parameters. In addition, the biomass of the most productive form of grass expands to intercept all of the available solar energy in a single growing season, much like a mature forest.