Elevated CO 2 stimulates marsh elevation gain, counterbalancing sea-level rise (original) (raw)
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Elevated CO 2 stimulates marsh elevation gain, counterbalancing sea-level rise
Proceedings of the National Academy of Sciences of the United States of America, 2009
Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient ؉ 340 ppm) accelerated soil elevation gain by 3.9 mm yr ؊1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas. coastal wetlands ͉ nitrogen pollution ͉ tidal marsh loss ͉ root productivity ͉ salinity
Tidal marsh plant responses to elevated CO2, nitrogen fertilization, and sea level rise
Global Change Biology, 2013
Elevated CO2 and nitrogen (N) addition directly affect plant productivity and the mechanisms that allow tidal marshes to maintain a constant elevation relative to sea level, but it remains unknown how these global change drivers modify marsh plant response to sea level rise. Here we manipulated factorial combinations of CO2 concentration (two levels), N availability (two levels) and relative sea level (six levels) using in situ mesocosms containing a tidal marsh community composed of a sedge, Schoenoplectus americanus, and a grass, Spartina patens. Our objective is to determine, if elevated CO2 and N alter the growth and persistence of these plants in coastal ecosystems facing rising sea levels. After two growing seasons, we found that N addition enhanced plant growth particularly at sea levels where plants were most stressed by flooding (114% stimulation in the + 10 cm treatment), and N effects were generally larger in combination with elevated CO2 (288% stimulation). N fertilizati...
Hydrobiologia, 2012
Higher atmospheric concentrations of CO 2 can offset the negative effects of flooding or salinity on plant species, but previous studies have focused on mature, rather than regenerating vegetation. This study examined how interacting environments of CO 2 , water regime, and salinity affect seed germination and seedling biomass of floating freshwater marshes in the Mississippi River Delta, which are dominated by C 3 grasses, sedges, and forbs. Germination density and seedling growth of the dominant species depended on multifactor interactions of CO 2 (385 and 720 ll l -1 ) with flooding (drained, ?8-cm depth, ?8-cm depthgradual) and salinity (0, 6% seawater) levels. Of the three factors tested, salinity was the most important determinant of seedling response patterns. Species richness (total = 19) was insensitive to CO 2 . Our findings suggest that for freshwater marsh communities, seedling response to CO 2 is species-specific and secondary to salinity and flooding effects. Elevated CO 2 did not ameliorate flooding or salinity stress. Consequently, climate-related changes in sea level or human-caused alterations in hydrology may override atmospheric CO 2 concentrations in driving shifts in this plant community. The results of this study suggest caution in making extrapolations from species-specific responses to community-level predictions without detailed attention to the nuances of multifactor responses.
Smithsonian Contributions to the Marine Sciences, 2009
Coastal marshes must accumulate soil to keep up with rising sea levels. It is unknown how the response of these ecosystems to global change will infl uence their ability to continue to keep up with sea-level rise. Here, we describe an in situ experimental chamber approach for manipulating key environmental variables, such as atmospheric CO 2 and soil N availability, in a brackish marsh. We outfi tted each chamber with surface elevation tables (SETs) to closely monitor soil elevation change, a sensitive indicator of marsh vulnerability to sea-level rise. Further, the design facilitates measurements of ecosystem exchange of CO 2 , plant productivity, porewater chemistry, and other environmental parameters.
Responses of Coastal Wetlands to Rising Sea-Level Revisited: The Importance of Organic Production
Research Square (Research Square), 2023
A network of 15 Surface Elevation Tables (SET) at North Inlet estuary, SC, have been monitored on annual or monthly time scales beginning from 1990 to 1996. The initial elevations spanned a range from suboptimal to superoptimal relative to the vertical growth range of the dominant vegetation, Spartina alterniflora. Of 98 time series, 20 have elevation gains equal to or exceeding the local rate of sea-level rise (SLR, 0.34 cm/yr). The elevation gain in North Inlet is dominated by organic production and, we hypothesize, is proportional to net ecosystem production. The rate of elevation change was 0.47 cm/yr in plots experimentally fertilized for 10 years with N&P compared to nearby control plots that have gained 0.1 cm/yr in 26 yr. The excess gains and losses of elevation in fertilized plots are accounted for by changes in belowground biomass and turnover. This is supported by bioassay experiments in marsh organs where in 3 years the belowground biomass of fertilized S. alterniflora plants increased by 1,772 g m-2 yr-1 , which is equivalent to 2.1 cm/yr. Root biomass was greater in the fertilized treatment than in controls, but in both treatments, roots plateaued at about 973 g m-2 and 613 g m-2 , respectively. Growth of belowground biomass was dominated by rhizomes, which continued to grow at a rate of 1,227 g m-2 yr-1 in the fertilized treatment after 3 years. Wetlands like North Inlet could be classified as autonomous because they depend on in situ organic production to maintain elevation. Autonomous wetlands are more vulnerable to SLR because their elevation gains are limited by net ecosystem production whereas minerotrophic wetlands are limited ultimately only by the mineral sediment supply.
Response of Salt Marsh and Mangrove Wetlands to Changes in Atmospheric CO 2, Climate, and Sea Level
2012
Coastal salt marsh and mangrove ecosystems are particularly vulnerable to changes in atmospheric CO 2 concentrations and associated climate and climateinduced changes. We provide a review of the literature detailing theoretical predictions and observed responses of coastal wetlands to a range of climate change stressors, including CO 2 , temperature, rainfall, and sea-level rise. This review incorporates a discussion of key processes controlling responses in different settings and thresholds of resilience derived from experimental and observational studies. We speci fi cally consider the potential and observed effects on salt marsh and mangrove vegetation of changes in (1) elevated [CO 2 ] on physiology, growth, and distribution;
Plant species determine tidal wetland methane response to sea level rise
Nature Communications, 2020
Blue carbon (C) ecosystems are among the most effective C sinks of the biosphere, but methane (CH4) emissions can offset their climate cooling effect. Drivers of CH4 emissions from blue C ecosystems and effects of global change are poorly understood. Here we test for the effects of sea level rise (SLR) and its interactions with elevated atmospheric CO2, eutrophication, and plant community composition on CH4 emissions from an estuarine tidal wetland. Changes in CH4 emissions with SLR are primarily mediated by shifts in plant community composition and associated plant traits that determine both the direction and magnitude of SLR effects on CH4 emissions. We furthermore show strong stimulation of CH4 emissions by elevated atmospheric CO2, whereas effects of eutrophication are not significant. Overall, our findings demonstrate a high sensitivity of CH4 emissions to global change with important implications for modeling greenhouse-gas dynamics of blue C ecosystems.
Global Change Biology, 2005
Increased atmospheric CO 2 concentration (Ca) produces a short-term stimulation of photosynthesis and plant growth across terrestrial ecosystems. However, the long-term response remains uncertain and is thought to depend on environmental constraints. In the longest experiment on natural ecosystem response to elevated Ca, we measured the shoot-density, biomass and net CO 2 exchange (NEE) responses to elevated Ca from 1987 to 2003 in a Scirpus olneyi wetland sedge community of the Chesapeake Bay, MD, USA. Measurements were conducted in five replicated open-top chambers per CO 2 treatment (ambient and elevated). In addition, unchambered control plots were monitored for shoot density. Responses of daytime NEE, Scirpus plant biomass and shoot density to elevated Ca were positive for any single year of the 17-year period of study. Daytime NEE stimulation by elevated Ca rapidly dropped from 80% at the onset of the experiment to a long-term stimulation average of about 35%. Shoot-density stimulation by elevated Ca increased linearly with duration of exposure (r 2 5 0.89), exceeding 120% after 17 years. Although of lesser magnitude, the shoot biomass response to elevated Ca was similar to that of the shoot density. Daytime NEE response to elevated Ca was not explained by the duration of exposure, but negatively correlated with salinity of the marsh, indicating that this elevated-Ca response was decreased by water-related stress. By contrast, circumstantial evidence suggested that salinity stress increased the stimulation of shoot density by elevated Ca, which highlights the complexity of the interaction between water-related stresses and plant community responses to elevated Ca. Notwithstanding the effects of salinity stress, we believe that the most important finding of the present research is that a species response to elevated Ca can continually increase when this species is under stress and declining in its natural environment. This is particularly important because climate changes associated with elevated Ca are likely to increase environmental stresses on numerous species and modify their present distribution. Our results point to an increased resilience to change under elevated Ca when plants are exposed to adverse environmental conditions.
RESPONSES OF COASTAL WETLANDS TO RISING SEA LEVEL
Salt marsh ecosystems are maintained by the dominant macrophytes that regulate the elevation of their habitat within a narrow portion of the intertidal zone by accumulating organic matter and trapping inorganic sediment. The long-term stability of these ecosystems is explained by interactions among sea level, land elevation, primary production, and sediment accretion that regulate the elevation of the sediment surface toward an equilibrium with mean sea level. We show here in a salt marsh that this equilibrium is adjusted upward by increased production of the salt marsh macrophyte Spartina alterniflora and downward by an increasing rate of relative sea-level rise (RSLR). Adjustments in marsh surface elevation are slow in comparison to interannual anomalies and long-period cycles of sea level, and this lag in sediment elevation results in significant variation in annual primary productivity. We describe a theoretical model that predicts that the system will be stable against changes in relative mean sea level when surface elevation is greater than what is optimal for primary production. When surface elevation is less than optimal, the system will be unstable. The model predicts that there is an optimal rate of RSLR at which the equilibrium elevation and depth of tidal flooding will be optimal for plant growth. However, the optimal rate of RSLR also represents an upper limit because at higher rates of RSLR the plant community cannot sustain an elevation that is within its range of tolerance. For estuaries with high sediment loading, such as those on the southeast coast of the United States, the limiting rate of RSLR was predicted to be at most 1.2 cm/yr, which is 3.5 times greater than the current, long-term rate of RSLR.
Wetland carbon storage controlled by millennial-scale variation in relative sea-level rise
Nature, 2019
Coastal wetlands (mangrove, tidal marsh and seagrass) sustain the highest rates of carbon sequestration per unit area of all natural systems 1,2 , primarily because of their comparatively high productivity and preservation of organic carbon within sedimentary substrates 3. Climate change and associated relative sea-level rise (RSLR) have been proposed to increase the rate of organic-carbon burial in coastal wetlands in the first half of the twenty-first century 4 , but these carbon-climate feedback effects have been modelled to diminish over time as wetlands are increasingly submerged and carbon stores become compromised by erosion 4,5. Here we show that tidal marshes on coastlines that experienced rapid RSLR over the past few millennia (in the late Holocene, from about 4,200 years ago to the present) have on average 1.7 to 3.7 times higher soil carbon concentrations within 20 centimetres of the surface than those subject to a long period of sea-level stability. This disparity increases with depth, with soil carbon concentrations reduced by a factor of 4.9 to 9.1 at depths of 50 to 100 centimetres. We analyse the response of a wetland exposed to recent rapid RSLR following subsidence associated with pillar collapse in an underlying mine and demonstrate that the gain in carbon accumulation and elevation is proportional to the accommodation space (that is, the space available for mineral and organic material accumulation) created by RSLR. Our results suggest that coastal wetlands characteristic of tectonically stable coastlines have lower carbon storage owing to a lack of accommodation space and that carbon sequestration increases according to the vertical and lateral accommodation space 6 created by RSLR. Such wetlands will provide long-term mitigating feedback effects that are relevant to global climate-carbon modelling. Broad biogeographic drivers, such as vegetation, climate, topography or water chemistry, are often emphasized as important global-scale controls on organic matter accumulation, decomposition and carbon stocks within tidal wetlands 7. However, relative sea-level trends over the Holocene varied across the globe, principally on the basis of distance from maximal ice-sheet extent during the last glacial period, and have a profound influence on the contemporary character of coastal wetlands 8,9. In Europe and North America, where studies of coastal wetland sea-level rise (SLR) impacts are concentrated, sea levels have been rising over the past few millennia at a decelerating rate up to the present (Fig. 1a, b). Tidal marshes in these locations, particularly when sediment supply is low-moderate, are often characterized by deep sediments that are highly organic 4,10,11 , in contrast to coastal wetlands in locations where the sea level has been stable for the past few millenia 12 , in spite of similarities in floristics 13. We review published data, contribute new observations on soil carbon concentrations (%C) in tidal-marsh sediments and compare %C values over the active root zone (0-20 cm) and sub-surface depths (20-50 cm, 50-100 cm and >1 m) for 345 locations that vary in rates of SLR over the late Holocene (Supplementary Information). We find that variation in RSLR over past millennia is a primary control on carbon storage. Overall %C varies consistently between RSLR zones (areas spatially