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.2021 Mar 25;16(3):e0248398.
doi: 10.1371/journal.pone.0248398. eCollection 2021.

Productive wetlands restored for carbon sequestration quickly become net CO2 sinks with site-level factors driving uptake variability

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Productive wetlands restored for carbon sequestration quickly become net CO2 sinks with site-level factors driving uptake variability

Alex C Valach et al. PLoS One..

Abstract

Inundated wetlands can potentially sequester substantial amounts of soil carbon (C) over the long-term because of slow decomposition and high primary productivity, particularly in climates with long growing seasons. Restoring such wetlands may provide one of several effective negative emission technologies to remove atmospheric CO2 and mitigate climate change. However, there remains considerable uncertainty whether these heterogeneous ecotones are consistent net C sinks and to what degree restoration and management methods affect C sequestration. Since wetland C dynamics are largely driven by climate, it is difficult to draw comparisons across regions. With many restored wetlands having different functional outcomes, we need to better understand the importance of site-specific conditions and how they change over time. We report on 21 site-years of C fluxes using eddy covariance measurements from five restored fresh to brackish wetlands in a Mediterranean climate. The wetlands ranged from 3 to 23 years after restoration and showed that several factors related to restoration methods and site conditions altered the magnitude of C sequestration by affecting vegetation cover and structure. Vegetation established within two years of re-flooding but followed different trajectories depending on design aspects, such as bathymetry-determined water levels, planting methods, and soil nutrients. A minimum of 55% vegetation cover was needed to become a net C sink, which most wetlands achieved once vegetation was established. Established wetlands had a high C sequestration efficiency (i.e. the ratio of net to gross ecosystem productivity) comparable to upland ecosystems but varied between years undergoing boom-bust growth cycles and C uptake strength was susceptible to disturbance events. We highlight the large C sequestration potential of productive inundated marshes, aided by restoration design and management targeted to maximise vegetation extent and minimise disturbance. These findings have important implications for wetland restoration, policy, and management practitioners.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig 1
Fig 1. Map outlining the Sacramento-San Joaquin Delta with the five restored wetland sites.
Site locations are marked (top) and enlarged (bottom) to show the wetland areas (shaded grey) and instrument tower locations (red points).
Fig 2
Fig 2. Proportion of vegetation cover for all wetland sites for each year since restoration.
The trajectories show a two-step linear fit for sites with rapid expansion in the initial establishment phase followed by a post-establishment plateau and a log-linear fit for sites with slow expansion rates. In 2013 East Pond was disturbed to seed the nearby East End wetland and is considered newly restored after this event. These data are based on aerial imagery classifications supplemented by data gathered from ground-based survey transects [44] for West and East Ponds before 2013.
Fig 3
Fig 3. Land cover proportions split by vegetation type.
This uses September 2018 as an example for all land and vegetation types (live litter, mixed vegetation, and dead litter refer to emergent macrophytes) at all the sites. Site labels are East End (EE), East Pond (EP), Mayberry (MB), Sherman Wetland (SW), and West Pond (WP). See S1 Fig in S1 File for the classified footprints.
Fig 4
Fig 4. Flux measurements against vegetation proportions.
Annual (top) and monthly (bottom) cumulative net ecosystem exchange (NEE) of CO2 (left) and gross ecosystem productivity (GEP, right) against vegetation cover during the growing season (May–September) from land cover classifications within the flux footprint for all available site-years with linear regressions and 95% confidence intervals (grey shading for linear regression and bars for data points). Negative NEE values denote carbon uptake by the ecosystem.
Fig 5
Fig 5. Monthly median aerodynamic canopy heights.
Canopy heights (m) were calculated using turbulence statistics from the eddy covariance towers for all site-years during the growing season from April to October. Eddy covariance measurements were available for Sherman in 2016 (blue square) before it was converted to a wetland and showed the canopy height of the previous pasture.
Fig 6
Fig 6. Annual sums of net ecosystem exchange (NEE) against wetland age (years since restoration).
All complete site-years are shown with bars indicating 95% confidence intervals from propagated errors of the flux measurement and gap-filling methods. The sums for the restoration year (year 0) are not shown (i.e. incomplete years). Negative NEE values denote C uptake by the ecosystem.
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References

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Grants and funding

This work was supported in the form of funding by the California Department of Water Resources through a contract from the California Department of Fish and Wildlife and the United States Department of Agriculture (NIFA grant #2011-67003-30371) awarded to DDB. Funding for the AmeriFlux core sites was provided by the U.S. Department of Energy’s Office of Science (AmeriFlux contract #7079856) and the aerial images and footprint mapping was funded by the Delta Science Program grant #R/SF-52 awarded to DDB and KSH. KK was supported by the Estonian Research Council grant No. PSG631 and by the Baltic-American Freedom Foundation Research Scholar program. KSH, ST and TLA were supported by the California Sea Grant Delta Science Fellowship (programs R/SF-70, R/SF-71 and R/SF-89 and grant no. 2271 and 5298). McIntire Stennis grant CA- B-ECO-7673-MS awarded to WLS partially supported this work. This work uses data and processing services provided by the OpenTopography Facility with support from the National Science Foundation under NSF Award Numbers 1948997, 1948994 & 1948857.

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