Sara Knox

Brackish wetland plant and microbial communities are a diverse mix of freshwater- and saltwater-adapted species in competition with each other. This has led researchers to predict that carbon cycling in brackish wetlands may be more resilient to changes in salinity than in fresh- or saltwater systems. Rush Ranch, a brackish tidal wetland near Suisun Bay, California, experienced drought-induced salinization in the 2015 and 2016 growing seasons followed by a freshwater flushing event in 2017. During the drought, salinity rose from the baseline of 4.5 ppt to an average of 10.3 ppt, peaking at 12.5 ppt. During these summers, gross primary productivity (GPP) decreased by 30%. Stepwise linear regression revealed that salinity was a major driver of GPP at this brackish wetland. We trained a random forest model to predict GPP based on environmental data from low salinity years. Naive to the salinization event, the model over-predicted GPP during high salinity years. These results provide ecosystem-scale evidence that increased salinity can decrease GPP at brackish tidal wetlands. This relationship is a starting point for incorporating the effect of changes in salinity on GPP in wetland carbon models, which could improve wetland carbon forecasting and management for climate resilience.

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Peatland rewetting, which is a management effort to restore water levels in previously drained peatlands, is important for re-establishing the role of peatlands as carbon (C) sinks. Since rewetted peatlands often have a highly variable response to year-to-year changes in climatic conditions and to functional changes, long term studies of C fluxes are needed in these ecosystems. Here, we evaluated the impact of climate variability and functional change on the interannual variability of CO₂ and CH₄ fluxes at Burns Bog, a rewetted temperate bog in Canada, based on five years of continuous eddy-covariance measurements. We found that the study site alternated between being an annual-scale net CO₂ sink and source, ranging from -32.6 ± 21.5 (± 95% CI) to 11.9 ± 15.1 g CO₂-C mˉ² yearˉ¹, respectively, while consistently being a CH₄ source, ranging from 11.6 ± 0.7 to 18.0 ± 1.6 g CH₄-C mˉ² yearˉ¹. On a five-year average, CH₄ emissions (13.7 ± 2.5 g CH₄-C mˉ² yearˉ¹; ± SD across years) completely offset the CO₂ sink (-12.3 ± 20.4 g CO₂-C mˉ² yearˉ¹; ± SD across years) on a carbon equivalent basis, resulting in the site losing an average of 1.3 ± 23.9 g C mˉ² yearˉ¹ (± SD across years). This finding indicates that excluding CH₄ fluxes from the net ecosystem C budget results in a significant overestimation of the net C uptake at this peatland site. The bog was the greatest annual CO₂ source in the year with a dry and warm summer, emphasizing the importance of temperature and water table depth at the bog. Regardless of the GHG metrics (i.e., global warming potential or sustained global warming potential) used in calculating the annual CO₂-eq GHG balance, the bog consistently had a positive radiative balance during each year of the study period. Despite mainly acting as a GHG source, the rewetted bog will likely have cooling effect on climate over long timescales compared to drained bogs that are large CO₂ sources.

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Peatland disturbance through drainage threatens to liberate large amounts of C stocks by increasing emissions of carbon dioxide (CO₂) from the soil. Restoration through re-wetting, on the other hand, could play an important role in climate change mitigation or adaptation by reducing CO₂ emissions and increasing the ability of peatland to sequester atmospheric CO₂. However, this can come at the cost of increased CH₄ emissions, an extremely potent greenhouse gas, and therefore rewetting can lead to a biogeochemical compromise between CO₂ uptake and storage, and CH₄ release. Currently, there is large uncertainty surrounding the extent of this compromise in ecosystems at different stages of recovery and with differing environmental conditions, making it difficult to predict how well these ecosystems are able to regain their function as CO₂ sinks following restoration. To assess the effect of re-wetting, I analysed eddy-covariance flux measurements alongside environmental variables from sites that have undergone different restoration techniques and consequently have different environmental conditions, mainly water table height (WTH). By the end of the one-year study period, the site with a higher water table, i.e., the wetter site, was a CO₂ sink, and the drier site was a CO₂ source. CH₄ emissions were higher at the wetter site annually and in the growing season, and whilst both sites had a positive radiative balance when calculated using sustained global warming potentials, the wetter site had a lower radiative balance on both a 20- and 100- year time horizon than the drier site, implying the importance of CO₂ sink status for climate benefits. These results emphasize the role that WTH and soil temperature have on promoting or inhibiting CO₂ and CH₄ emissions, and therefore can be used to inform management decisions and predict future trends in peatland ecosystems undergoing restoration.

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