USGS Multimedia Gallery: POTENTIAL EFFECTS OF ELEVATED CO2 AND CLIMATE CHANGE ON COASTAL WETLANDS

Click on the link below and watch the video of the USGS-United States Geological Survey.
Please notice: I have enhanced important aspects in bold.
USGS Multimedia Gallery: Potential Effects of Elevated CO2 and Climate Change on Coastal Wetlands

Narrator: Humans are directly altering the Earth’s environment through changes in terrestrial land cover as well as indirectly through activities that affect climate and biogeochemical cycles.
One of the ways humans are affecting the environment is through increases in atmospheric carbon dioxide through the burning of fossil fuels.
How do scientists know that CO2 has changed?
Scientists collect ice cores from the Antarctic and other ice fields and measure CO2 concentrations in trapped air bubbles. By analyzing CO2 in air bubbles at different depths, scientists can reconstruct changes that occurred in the Earth’s atmosphere over thousands of years. For 400,000 years, CO2 concentrations fluctuated, but never exceeded 300 parts per million. However, since the advent of the industrial age in the 1800s, concentrations have risen rapidly to reach the current concentration of 390 parts per million.
Independent, high-precision measurements made at a number of monitoring stations around the world show that CO2 has continued to rise in modern times at increasingly rapid rates and will likely continue to increase well into the future.
Scientists are interested in both direct and indirect effects of higher CO2 concentrations on the Earth’s ecosystems. In this video, we’ll take a closer look at what this change might mean for coastal wetlands and some of the difficulties inherent in making predictions about what those effects might be.
How might changes in CO2 concentrations affect coastal wetlands?
CO2, being a greenhouse gas, will contribute to global warming and changes in climate, which will affect plant communities. Rising global temperature also contributes to sea-level rise, which will alter flooding and salinity regimes affecting wetland plant growth and distribution. CO2 also has a direct fertilization effect on plants.
Plants respond to higher CO2 with increases in net photosynthesis and water use efficiency, leading to greater productivity. Some plants are more responsive to increases in CO2—these are called C3 species. C4 species are less responsive to CO2 because they are already quite efficient at capturing CO2.
Because of species differences in sensitivity to CO2, the composition of a wetland is one factor that will determine whether increases in CO2 will have an effect or not.
One simple prediction is that C3 species will increase in abundance relative to C4 species in a mixed community as CO2 increases.
What types of wetlands might respond to rising CO2?
Sea-surface temperature varies with latitude and affects plant communities along shorelines. Low-lying coastal areas are dominated by different types of intertidal wetlands—typically salt marshes at temperate latitudes and mangroves at tropical latitudes.
Where such vegetation types overlap are key locations to study effects of climate change and elevated CO2, particularly where the overlapping species are C3 versus C4 types. Just such a community occurs in the southern part of the US in the coastal fringe of Louisiana. Here, the black mangrove, (Avicennia germinans) and the salt marsh grass (Spartina alterniflora) co-occur.
How will C3 mangrove and C4 marsh species respond to higher CO2?
We can hypothesize that the two species compete for space and other resources, and that CO2 will have an indirect effect through change in climate and freeze frequency, which will mainly influence mangroves that are more sensitive to cold.
At this latitude, periodic freezes kill the sensitive mangrove shoots, which may later recover, but the winter temperatures reduce growth and keep the mangroves short and shrub-like.
We can hypothesize, however, that if the climate warms, the higher CO2 will have a direct fertilization effect on mangroves. We might further predict that increases in nutrient inputs to these wetlands would favor mangroves by enhancing their capacity to respond to CO2. This might shift the competitive advantage in favor of mangroves.
We can test these predictions in controlled experiments. This is the Wetland Elevated CO2 Experimental Facility at the USGS National Wetlands Research Center. Using sophisticated control systems, the plants inside these greenhouses can be subjected to different concentrations of carbon dioxide. Scientists use an elevated concentration of 720 ppm CO2, which is about double the ambient concentration. Whether and when we might reach that higher concentration is unknown and dependent on a number of factors. However, it’s still important to know what might happen if CO2 continues to increase.
How will a doubling of CO2 affect the salt marsh-mangrove community?
Scientists collect segments of marsh, which are established in containers called mesocosms, and transport them to the CO2 facility.
The mesocosms are then randomly assigned to different experimental treatments designed to test the response of plants to hypothetical conditions. In this study, the mesocosms were subjected to either ambient or elevated CO2, as well as to either high or low availability of nitrogen in the soil. Three different plant assemblages were tested: Avicennia alone, Spartina alone, and a mixture of the two species. Propagules of the black mangrove were added to the mesocosms to mimic the natural dispersal process whereby they invade salt marshes. The experiment lasted a year and a half to allow sufficient time for a response.
The results of this study, which were published in the refereed journal, Global Change Biology, provided some important insights.
The graph to the left shows the final biomass of the C4 grass, Spartina, in grams per mesocosm and on the right is a photograph taken just before the end of the experiment. The data and the photograph clearly show a strong response to higher nitrogen, which caused higher stem density and taller plants compared to low nitrogen. There was no effect of CO2 as predicted.
Avicennia, however, showed a different response to the treatments. For this mangrove species, the greatest difference was between growth in monoculture and mixture. When grown alone, Avicennia grew to over 1 meter tall. However, when in mixture with Spartina, growth of Avicennia was strongly suppressed. And higher nitrogen, instead of helping Avicennia, enhanced the growth of its competitor, Spartina. CO2 stimulated Avicennia’s growth, but this occurred only when it was grown alone under higher availability of nitrogen.
So the results were contrary to the simple prediction that CO2 would enhance abundance of a C3 species in a mixed community. Mangrove seedling growth was instead strongly suppressed by Spartina, and CO2 and N enrichment could not reverse this suppression—at least not under the conditions of this experiment. Note that if we had only examined the growth of the two species separately and not in mixture, we would have come to a different, and incomplete conclusion.
This information allows us to modify our conceptual model to show that when Spartina is present and healthy, it strongly suppresses Avicennia recruitment. And higher nutrients enhance Spartina, perhaps further suppressing Avicennia.
However, if Spartina is eliminated by a disturbance, then Avicennia’s growth is enhanced by CO2, particularly where nutrient availability is high.
Both small-scale and large-scale disturbances occur in salt marshes. For example, the deposition of dead plant material, called wrack, can smother Spartina, creating bare patches where mangroves can get established. Large-scale dieback of salt marsh has also occurred in the past, which allowed suppressed mangroves to become dominant in areas once occupied by Spartina.
What other changes related to climate might influence this plant community?
With climate change, some regions will experience drier conditions and higher salinities. We can hypothesize that Avicennia is more tolerant of water stress and high salinity than Spartina, based on their natural distributions . Another piece of evidence is the large-scale dieback [= plant disease] of salt marsh, which occurred in the Mississippi River delta at the same time as a 100-year drought. A study of this sudden dieback phenomenon was reported in the journal Global Ecology and Biogeography. The results strongly indicated that dieback of Spartina was related to this extreme weather event, which left Avicennia untouched.
So we can add another pathway to the conceptual model showing how more arid conditions and higher salinity might reduce survival of Spartina. We can also hypothesize that sea-level rise, which will alter flooding regimes, will have an effect on one or both species or interact with CO2.
Scientists can determine the physiological tolerance of plants to salinity and flooding in controlled experiments in which they are subjected to these stresses. Although we have some understanding of plant stress tolerance, additional experiments are needed to determine exactly how these and other salt marsh and mangrove species will respond to environmental stresses associated with climate change and to elevated CO2. Such information is critical to the development of accurate models used to predict future changes in wetlands and to assist in planning restoration projects.
A simple question…
We began this video with a simple question—how will elevated CO2 and related changes in climate affect coastal wetlands?
Scientists begin to answer such questions by developing a basic hypothesis, based on expectations as to how C3 and C4 species might react to increased CO2. As experiments are conducted, such basic models can be improved to include previously unrecognized pathways. As we’ve seen in this video, natural ecosystems are quite complex, and a change in one factor may have a direct effect on one or more species or may act indirectly by modifying another factor. Even seemingly simple systems such as the salt marsh-mangrove community dominated by two species are quite complicated. The complexity increases as we consider additional factors and more diverse plant communities.
Complex ecological problems pose huge challenges for scientists who must tease apart all the various ways in which ecosystems may respond to climate change. Much is known, but more remains to be discovered. This video has provided a brief glimpse at some of these complexities and how scientists are figuring out what the future may hold.