Elevated CO₂ stimulates marsh elevation gain, counterbalancing sea-level rise

Results and Discussion

Elevated CO₂ stimulated above-ground productivity in the marsh by 30% in 2006 (Table 1; two-way ANOVA, CO₂ effect: P < 0.05) but not in 2007 (P > 0.70) during a severe drought. These responses were consistent with a 20-year record of elevated CO₂ treatment in a previous CO₂ study on the same marsh (13). Strikingly, elevated CO₂ accelerated the rate of soil elevation gain over 2 years of treatment. Least-squares linear trends revealed a slight loss of elevation in ambient CO₂ (−0.9 mm yr⁻¹) compared with an elevation gain (3.0 mm yr⁻¹) in the elevated CO₂ treatment (Fig. 1A; two-way ANOVA comparing linear trends, n = 5, CO₂ effect: P < 0.05). Nitrogen addition tended to affect elevation negatively, although the overall nitrogen effect was not significant. Deposition of mineral sediment on the marsh surface is negligible at this site. Root zone expansion by accumulation of plant material is essential to maintaining a constant surface elevation relative to rising sea level. Indeed, changes in marsh surface elevation mirrored changes in root zone thickness (Fig. 1B). Elevated CO₂ caused an increase in thickness of 4.9 mm yr⁻¹ compared with 0.7 mm yr⁻¹ in the ambient CO₂ treatment, an effect that could increase marsh tolerance for SLR.

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Fig. 1.

Relative surface elevation (A) and root zone thickness (B) over 2 growing seasons in a brackish marsh (Chesapeake Bay) exposed to ambient (Amb) or elevated (Elev) atmospheric [CO₂] in open-top chambers and to factorial soil nitrogen (+N) enrichment. The slopes of the linear trends in elevation are inset in each panel (mean ± 1 SE). Elevated CO₂ increased elevation (two-way ANOVA, CO₂ effect: P < 0.05), an effect that mirrored CO₂ stimulation of root zone thickness (P < 0.05). Nitrogen addition had a nonsignificant tendency to reduce total elevation gain (P = 0.18) and root zone expansion (P = 0.13), despite stimulating above-ground plant productivity.

Increases in soil elevation were driven by a stimulation of subsurface plant productivity. Changes in soil elevation over each growing season closely followed the pattern of treatment effects on fine root productivity (Fig. 2; r² = 0.86, P < 0.05). By the end of the second year, above-ground litter also may have entered the soil and contributed to a CO₂ effect on elevation. Rhizome and subsurface stem productivity were likely also important but could not be quantified nondestructively in the field. Although the presence or absence of an elevated CO₂ effect on above-ground production each year depended on climatic conditions, elevated CO₂ strongly stimulated fine root productivity both years, by 48% in 2006 (two-way ANOVA, n = 5, P < 0.05) and by 26% in 2007 (P < 0.05). This finding concurs with previous work at this site and many other CO₂ studies that report an inordinately large stimulation of root growth (18).

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Fig. 2.

Relation of subsurface biovolume production to vertical change in the field. Values represent treatment means (±1 SE) of relative elevation change and fine root productivity over each field season, 2006 (circles) and 2007 (squares). Amb, ambient; Elev, elevated; N, nitrogen.

The timing of the below-ground response further supports the important role of plant growth in driving soil elevation gain. The CO₂ effects on root zone elevation arose during the first growing season (Fig. 1), concurrent with the production of live plant biomass. CO₂ effects on elevation during the growing season were not reversed by plant senescence and decomposition. Because anaerobic conditions in saturated soils substantially slow soil organic matter decay (11), the effects of productivity on elevation during the growing season persist through the nongrowing season. Consequently, elevated CO₂ forced the soil surface upward by stimulating the accumulation of both live and dead organic matter.

Other perturbations, particularly those that may accompany SLR, could modify the influence of elevated CO₂ on marsh elevation gain. To understand better how the CO₂ response under current sea-level conditions might differ with accelerating SLR, we examined the effect of CO₂ in combination with flooding and salinity in a mesocosm study. This experimental approach allowed isolation of root-zone controls on soil elevation dynamics and manipulation of factors known to influence root growth. The objective of our treatments was to vary the depth and duration of flooding in combination with a range of salinities anticipated under different SLR scenarios (17).

As found in the field study, CO₂ had a significant effect on marsh elevations in mesocosms (17). However, the patterns of soil expansion, driven by below-ground production, varied in a complex manner because of interactions of CO₂, flooding, and salinity (Fig. 3). Under intermittent flooding, CO₂ enhanced elevation at lower salinities but had no effect or a negative effect at higher salinities (Fig. 3). Under constant flooding, however, the pattern of CO₂ response was different, with expansion greatest both above and below the current salinity range for this marsh type (Fig. 3). In particular, CO₂ treatment increased elevation in flooded mesocosms at high salinities (i.e., conditions that simulated future SLR). Although the precise CO₂ response in nature will depend on many other factors in addition to salinity and flooding, our findings indicate that future CO₂ effects on elevation in Kirkpatrick Marsh will likely be modified by conditions that accompany SLR.

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Fig. 3.

Elevated CO₂ effects on marsh elevation in greenhouse mesocosms subjected to different salinity and flooding treatments. Values are the difference in elevation (mm) between mesocosms under elevated CO₂ (720 ppm) and ambient CO₂ (380 ppm). The y axis values represent mean differences in CO₂ response (± 1 SE) under intermittent (circles) and constant (squares) flooding in relation to mean interstitial salinity (± 1 SE) measured biweekly in mesocosms (x axis). Second-order polynomial curves were fitted to the data; t tests of quadratic coefficients indicated a significant change over salinity for both flooding treatments (P < 0.05). Patterns of response for the 2 flooding treatments were significantly different (F = 3.03, P = 0.10; analysis of covariance).

Agricultural and municipal runoff increases nitrogen (N) loading in estuaries, particularly those that drain densely populated watersheds. In the field study, nitrogen addition, simulating coastal eutrophication, tended to negate elevation gains caused by elevated CO₂ alone (Fig. 1A, Inset). Although nitrogen addition stimulated above-ground biomass, it decreased root productivity (Fig. 2 and Table 1). N addition commonly decreases carbon allocation to root productivity (19). Furthermore, N addition has been shown to stimulate decomposition in other nitrogen-limited peat-based wetlands (20). Nitrogen pollution may contribute to marsh loss (5), although the mechanisms have not been described. In our study, N addition tended to reduce elevation gain, partially by reducing root productivity, but also may have stimulated organic matter decomposition.

Plant community composition will modify ecosystem responses to elevated CO₂. Plants with the C₃ photosynthetic pathway typically exhibit a stronger CO₂ response than C₄ plants (13). The CO₂ effect may be particularly important for C₃-dominated marshes experiencing SLR, which includes many brackish marshes and mangrove swamps. In this marsh community, the responsive C₃ species is more flood-tolerant and less salt-tolerant than the C₄ species (21, 22). The mesocosm experiment also provided insight into C₃ vs. C₄ contributions to soil expansion (17). Separation of roots by species was not possible, but species contributions to soil volume in the mesocosms could be assessed by measuring the final volume of shoot bases (i.e., the below-ground portion of the shoot that serves as a storage organ in graminoids). Vertical change in soil elevation was positively correlated with subsurface shoot volume of the C₃ but not the C₄ species (17). Consequently, the potential for CO₂ stimulation under an accelerating SLR scenario may depend on the inherent tolerance of component species to salinity, flooding, and other factors as well as their potential to respond to CO₂ (17).

Our findings bear particular importance given the threat of accelerating SLR to coastal wetlands worldwide. A recent report suggests that a 2-mm increase in the rate of SLR will threaten or eliminate a large portion of mid-Atlantic marshes (6). The threshold rate of SLR (i.e., the SLR rate at which the marsh converts to open water) depends on the rate of marsh elevation gain by accumulating sediment or endogenous organic material (7). For instance, models predict that increasing mineral sediment load should increase this threshold rate (7, 23). Nevertheless, many marshes, like Kirkpatrick Marsh, naturally receive very little exogenous sediment. Therefore, the critical SLR threshold depends almost entirely on biological mechanisms that have been more difficult to describe and predict. We suggest that elevated CO₂, by stimulating below-ground plant productivity and biogenic elevation gain, may also increase the capacity of coastal wetlands to tolerate SLR (Fig. 4).

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Fig. 4.

Conceptual comparison of SLR and atmospheric [CO₂] scenarios from the Inter-governmental Panel on Climate Change (IPCC) to hypothetical thresholds of marsh tolerance for increasing SLR. The black line approximates a simplified SLR × CO₂ relation for the past 140 years (4, 12, 29, 30). Red and orange lines represent most probable increases in atmospheric [CO₂] and SLR rates for the years 2090–2100 according to model projections for 2 moderate IPCC socioeconomic scenarios, A2 and B2 (31). The difference between the 2 lines depends on socioeconomic uncertainties. Each model output has its own error based on physical uncertainties that are not shown here. The relation between SLR and CO₂ appears approximately linear in the 100-year projections, but other relations are possible. The dotted green line represents a threshold SLR rate fixed at 4 mm yr⁻¹, a rate that would threaten many marshes (6). The dashed green line represents a hypothetical threshold rate that depends on atmospheric [CO₂] and is based on our finding of increased marsh elevation gain with elevated CO₂. The CO₂ effect (difference between the 2 green lines) portrayed here (≈2 mm yr⁻¹ at 720 ppm) is conservative compared with that measured in the field experiment (3.9 mm yr⁻¹) because of uncertainties in extrapolating from short-term CO₂ effects on elevation to actual marsh thresholds. Marshes collapse when the rate of SLR (red or orange lines) exceeds the threshold of marsh tolerance for SLR (green lines). Thresholds may have been lower in the past when [CO₂] was at preindustrial levels (≈280 ppm), because many plants likely exhibited reduced below-ground growth compared with today, and thus made smaller contributions to marsh elevation gain.

Atmospheric CO₂ has risen 40% from preindustrial levels (≈280 ppm) to the current concentration (≈388 ppm) over the past 200 years and has enhanced plant productivity globally. A CO₂ effect on coastal wetland elevation may have already mitigated a significant amount of marsh loss acting through the biological mechanisms demonstrated in our study (Fig. 4). However, the stimulatory effects of CO₂ on leaf-level photosynthesis are known to diminish with each marginal increase in CO₂ concentration (24), and a similar attenuation of the stimulatory effect of elevated CO₂ on soil elevation gain should follow. Consequently, further increases in atmospheric CO₂ beyond 720 ppm may yield diminishing benefits for plant productivity and marsh elevation, while further accelerating SLR through greenhouse warming (12). A simultaneous increase in plant stressors, such as salinity and flooding, however, may sustain or enhance the CO₂ effect, as found in the mesocosm study.

Our findings show that elevated CO₂ stimulates plant productivity, particularly below ground, thereby boosting marsh surface elevation. In addition to providing evidence for organic control of wetland topography on the landscape scale, we show how elevated CO₂ may strengthen the organic mechanisms that sustain marsh elevation relative to SLR in the future. These effects likely extend beyond biogenic marshes, because organic processes can exert control over elevation dynamics in mineral-soil wetlands as well (11). For instance, marshes in the Mississippi River delta have high rates of subsidence that were naturally countered by mineral sediment deposition (25). However, hydrological modifications by humans have diminished sediment deposition in this system. Such sediment-deficient deltas, which once would have been buffered against increasing SLR, have been rendered more vulnerable, and hence more dependent on organic contributions to sustain a constant elevation relative to SLR. To manage or restore such ecosystems to a self-sustaining state, we must explore further how global change interactions will influence key biological mechanisms that maintain soil elevations in coastal wetland ecosystems.