The Salt Marsh Accretion Response to Temperature eXperiment (SMARTX) is a whole-ecosystem warming experiment in Smithsonian's Global Change Research Wetland (GCReW) focused on quantifying the impacts of multiple ecosystem stressors (warming and elevated CO2) on carbon cycling in tidal wetlands. The interdisciplinary team includes researchers from SERC, Virginia Institute of Marine Science, Oak Ridge National Laboratory, USGS, and Georgia Southern University. SMARTX has been supported by the US Department of Energy Environmental System Science program since 2016.
The SMARTX heating system was designed by Roy Rich and is maintained by SERC's Technology in Ecology Lab. Each transect is set up along a heating gradient running from ambient (foreground) to +5.1 °C above ambient (background). Soils are heated down to 1.5 m, using vertical "pins" made from resistance cable. The aboveground temperature is elevated using infrared heaters. Elevated CO2 (750-800 ppm) is achieved through open-top chambers at the ambient and +5.1 °C ends of the transects.
Though brackish wetlands are not typically assumed to be large sources of methane, we have found that CH4 emissions (measured with manual chambers) are substantially higher from the warmed plots, suggesting large shifts in anaerobic metabolism. This discovery has lead to further work looking at the effect of warming and elevated CO2 on anaerobic carbon mineralization and redox potential and how plant traits affect CH4 dynamics.
Aboveground and belowground plant responses
In coastal wetlands, plant biomass can be a key component of whether or not the ecosystem will survive rising sea levels, epescially highly organic sites like GCReW. In the first two years of SMARTX, we discovered that plants respond asynchronously to warming, with a large increase in belowground biomass occurring at +1.7 °C, an effect that we attribute to changes in N cycling under the different warming scenarios. Satya Kent, SMARTX's technician, is currently working on a 5-year dataset to determine how warming shifts phenological trends in Schoenoplectus americanus.
In 2021, we installed minirhizotron tubes in the ambient and +5.1 °C plots, to start tracking the effects of warming and elevated CO2 on root phenology, longevity, and turnover rates.
Soil redox potential
One way that plants influence ecosystem responses to global change is by regulating the soil oxidation-reduction environment through supplying both electron donors and electron acceptors. To study treatment effects on this plant response, we installed a network of probes that have been continuously measuring redox potential in the +5.1 °C plots (ambient and elevated CO2) since March 2020. We've found that in the rooting zone, redox potential is consistently higher in elevated CO2 plots, which we attribute to higher rates of oxygen transport through the sedges. In January 2022, we added an additional network of probes in the ambient plots, to look at temperature effects.
Elevation gain is a key component of coastal wetland survival; when the marsh can no longer keep pace with sea-level rise, flooding increases and eventually kills the vegetation, exposing the carbon-rich soils to erosion. Collaborations from the Virginia Institute of Marine Science track elevation gain and loss in the SMARTX plots using soil elevation tables (SETs).
Kerrie Sendall and students from Rider University focus on plant physiological responses to warming and elevated CO2, by measuring stomatal conductance, chlorophyll fluorescence, and other stress-related physiological traits of species and relating them to plant growth and performance.
From field data to models
SMARTX has two integrated modeling efforts, that operate at different skills. Matt Kirwan's Coastal Geomorphology and Ecology Lab at the Virginia Institute of Marine Science is working on a point-based model designed to capture ecogeomorphic feedbacks that influence carbon cycling in tidal marshes. Teri O'Meara, at Oak Ridge National Lab, leads the global-scale modelling component, updating a terrestrial component of DOE's Energy Exascale Earth System Model (E3SM) to represent tidal wetlands.