Under our current changing climate, plants and animals must respond to rising sea levels, altered precipitation patterns, and increasing air and water temperatures and concentrations of greenhouse gases, including atmospheric carbon dioxide (CO2). While some species may adapt to changing conditions, these factors have the potential to drive latitudinal and elevational shifts in species distributions.
Increasing CO2 concentration may affect plant distributions through its effects on photosynthesis; rising CO2 acts to reduce stomatal conductance and transpiration in plants, thereby improving water-use efficiency. Studies have shown that elevated CO2 generally stimulates the growth of plants that utilize the C3 photosynthetic pathway, whereas it has less of an effect on plants utilizing the C4 pathway.
Because plants may react differently to CO2 enhancement, interspecific interactions between plants can be altered. The C3 mangrove species Avicennia germinans (black mangrove) and the C4 marsh grass Spartina alterniflora (smooth cordgrass) are two common wetland species on the subtropical Gulf of Mexico (Gulf) coast of the United States that are responding to changing climatic conditions.
Scientists have documented landward shifts of mangrove forests over the past several decades along the Gulf and in other regions of the world (e.g., Australia, China, Mexico, New Zealand). In many settings, mangrove forests have encroached into marshes and mudflats, replacing those habitat types. Similar inland migration of brackish and salt marshes can be prevented in some areas by existing human infrastructure, including roads, housing, and commercial development; this process has been described as “coastal squeeze.”
As a consequence, marsh habitat loss has occurred, and the type and quantity of ecosystem goods and services provided have been affected. Mangrove expansion in the Gulf has been attributed primarily to sea-level rise, reduced freshwater input, and increased intervals between winter freeze events, all factors that favor the expansion of cold-intolerant and salt-tolerant mangrove species. Drought is an additional factor that affects the resiliency of Gulf coastal wetlands, and it has been implicated as a contributing factor in severe vegetation dieback events that have occurred in brackish and salt marshes.
Because it is likely that the effects of increased CO2 concentration and drought will become more prominent under future climate projections, we were interested in how the stressful conditions imposed by these factors impact early growth interactions between A. germinans and S. alterniflora. We conducted a greenhouse study that simulated elevated atmospheric CO2 concentrations of about 700 ppm, double the current ambient conditions, as well as water regimes of drought and flooding. Propagules of A. germinans and seeds of S. alterniflora were placed in pots individually (i.e., monoculture plantings) and in mixed-species plantings, and the plants were exposed to the experimental conditions for 10 months.
Neither species responded to elevated CO2 during the study; this lack of response was perhaps related to the short duration of the experiment. Significant responses to the water regime treatment, however, were identified. The biomass of A. germinans pneumatophores (aboveground root structures) was lower under drought conditions, perhaps an indication of increased belowground oxygen availability, and the belowground biomass of both species was lower under flooded conditions. The experiment also provided interesting evidence of competitive interactions between the species during their early growth stages. Stem biomass results indicated that the two species competed for aboveground resources (e.g., light), and belowground resource competition was indicated by reduced A. germinans pneumatophore biomass when grown in monoculture compared to mixed-species plantings.
We believe the study provides insight into how A. germinans and S. alterniflora may respond following major disturbance events that lead to vegetation dieback, events that may increase in frequency based on climate models that predict regional warming trends and decreasing annual rainfall over the coming decades. Following a marsh dieback event, species that colonize the bare sediment exposed will likely vary depending on propagule or seed availability as well as environmental conditions.
This research is described in the article entitled, Early growth interactions between a mangrove and an herbaceous salt marsh species are not affected by elevated CO2 or drought, recently published in the journal Estuarine, Coastal and Shelf Science.
This work was conducted by Rebecca Howard and Camille Stagg from the U.S. Geological Survey Wetland and Aquatic Research Center and Herry Utomo of Louisiana State University Agricultural Center, Rice Research Station.