6. Controls on Marsh Plant Productivity
GCE scientist Steven Pennings measures short-form Spartina alterniflora plants in a permanent plot in the mid-marsh at a monitoring station on Sapelo Island GA.
Salt marshes are highly productive. Their vigorous growth takes up nutrients from the water and stores large amounts of nutrients and carbon in the soil. The plant growth supports a productive food web both in marshes and in adjacent habitats where plant biomass is exported. In addition, the structure provided by vigorous plant growth provides habitat for many species and helps protect coastlines from erosion and storms. But what determines how productive the marshes are?
GCE scientists have determined that biomass of the most common salt marsh plant, Spartina alterniflora, can vary up to three-fold from one year to the next. They used two techniques. First, starting in 2000, the GCE team installed 160 permanent plots at ten marsh sites on the Georgia coast. Each fall, at the time of year when plant biomass is greatest and plants are flowering, GCE researchers measured the height of each plant in these plots and noted whether it was flowering. This information was converted to standing biomass using established relationships between height, flowering and biomass. Second, GCE scientists examined images from the Landsat 5 satellite that were acquired between 1984 and 2011. This allowed the group to examine much larger spatial scales and to look back in time to before the GCE-LTER program began.
Both approaches found that productivity of Spartina alterniflora was linked to processes that reduce salinity in the porewater around the plant roots. These processes include high discharge of freshwater from rivers to the coast, heavy local precipitation into the marshes, and high sea levels that prevent evaporative concentration of salts. In addition, productivity was reduced in the hottest years during which plants were heat-stressed. Because drought has been more common in recent years compared to the 1980's, Spartina productivity has declined by about one third over the past three decades.
River discharge and other aspects of climate in the GCE domain are functions of three climate signals: the Bermuda High, the El Nino/Southern Oscillation, and the Atlantic Multidecadal Oscillation. The complex interplay between major weather patterns makes it difficult to predict future trends of river discharge and precipitation for the Georgia Coast; however, the Bermuda High is the climate signal most likely to affect river discharge during the growing season, which is one of the strongest drivers of plant productivity. Climate models agree that sea levels and temperatures will continue to rise, but because high sea levels and temperatures have opposite effects on plants, it is difficult to predict future patterns of productivity for Georgia marshes at this time.
The plot-level sampling also allowed GCE scientists to measure disturbance in the marshes. Disturbance by wrack (floating vegetation debris) was more common along creekbanks than in the mid-marsh; disturbance by snails was more common in the mid-marsh. Both disturbances varied several-fold among years, and both were more important at barrier island marshes compared with mainland marshes. Although disturbance sharply reduces standing biomass in affected areas, it is not common enough to overwhelm the effects of climatic conditions on standing biomass at the landscape level. Because the plot-level sampling occurs only once a year, however, it may underestimate the importance of disturbance. Ongoing GCE research is examining how marshes respond to disturbances that occur at different times of the year.
Standing biomass and productivity of Spartina alterniflora plants is also affected by herbivory. A range of consumers, including deer, crabs, snails, planthoppers, grasshoppers and stem-boring insects, eat Spartina and affect its biomass to different degrees. A major challenge to better understanding primary production in salt marshes is to integrate the "top-down" effects of consumers with the "bottom-up" effects of abiotic conditions.
This work is ongoing (see project page). GCE scientists continue to sample the permanent plots each fall, and the remote sensing group is working on expanding their results to the entire Georgia Coast and is also continuing to acquire new imagery each year. Future analyses will expand to include other marsh plant species, more complicated climate indices, and spatially-explicit models of processes and feedbacks that determine above- and below-ground plant production. Collectively, these approaches should help to answer a key question: Are the large, multi-decadal declines that we documented in Spartina alterniflora biomass cyclical in nature, or are they long-term, directional changes related to global warming?
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This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959 and OCE-1237140. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.