Christopher N. Janousek

Despite growing interest in the contributions of tidal wetlands to natural climate solutions, data remain scarce on how land use affects their carbon accumulation rates (CAR). Additionally, environmental factors driving the large observed variability in CAR among sites are poorly understood. To address these knowledge gaps, we measured short‐term (∼1 year) and long‐term (∼100 years) CAR using feldspar marker horizons, ²¹⁰ Pb profiles, and surface elevation tables from 39 sites in seven estuaries in Oregon and Washington, USA. Sites varied in wetland type (tidal marsh, tidal swamp, nontidal pasture), land‐use history (reference, restored, and disturbed), and salinity (fresh to polyhaline). Across both timescales, CAR was lowest in disturbed pastures and highest in restored marshes, reflecting patterns in sediment accretion rates. Short‐term CAR exceeded long‐term CAR by approximately 50% on average and was only weakly correlated with long‐term CAR, with differences among hydrogeomorphic setting. Short‐term CAR was well predicted by summer inundation frequency and relative sediment load, together explaining 60% of variance, whereas long‐term CAR was poorly predicted by contemporary environmental conditions. These findings highlight the importance of aligning predictor and response timescales when modeling blue carbon dynamics, and reinforce the climate mitigation potential of tidal wetland restoration.

Tidal wetland restoration is an important tool to enhance estuarine ecosystem functions. Despite substantial investments, ecological evaluation of the long-term outcomes of wetland restoration is uncommon, and such studies are seldom conducted across multiple sites, hindering understanding of restoration efficacy and design. We assessed vegetation similarity between 16 legacy tidal marsh restoration projects aged 22 to 62 yr and paired reference wetlands along the US Pacific coast as well as the influence of key environmental drivers on vegetation community structure. Across this large geographic region, restored sites generally had relatively high total plant cover dominated by native species. However, many restored sites had lower plant species richness than reference marshes at both the assemblage and site scales, and had less complex canopies. Moreover, there were commonly differences in species composition between restored and reference sites, often driven by differences in the abundance of dominant or subdominant native species. Across all marshes, elevation and salinity gradients strongly influenced community structure more than restoration status. Restored-reference differences were also driven more by site elevation than site age. Overall, our results suggest that restoration can effectively recover tidal marsh native plant communities, but that differences in species composition and diversity are likely to persist in marshes not restored to optimal elevations even after decades of development. In future projects, careful planning for site elevation and active intervention such as plantings for floristic diversity may improve biodiversity recovery. Our multisite approach can serve as a model for regional-scale monitoring to identify strategies to enhance future restoration success.

Coastal wetlands, including seagrass meadows, emergent marshes, mangroves, and temperate tidal swamps, can efficiently sequester and store large quantities of sediment organic carbon (SOC). However, SOC stocks may vary by ecosystem type and along environmental or climate gradients at different scales. Quantifying such variability is needed to improve blue carbon accounting, conservation effectiveness, and restoration planning. We analyzed SOC stocks in 1,284 sediment cores along >6,500 km of the Pacific coast of North America that included large environmental gradients and multiple ecosystem types. Tidal wetlands with woody vegetation (mangroves and swamps) had the highest mean stocks to 1 m depth (357 and 355 Mg ha⁻¹, respectively), 45% higher than marshes (245 Mg ha⁻¹), and more than 500% higher than seagrass (68 Mg ha⁻¹). Unvegetated tideflats, though not often considered a blue carbon ecosystem, had noteworthy stocks (148 Mg ha⁻¹). Stocks increased with tidal elevation and with fine (<63 μm) sediment content in several ecosystems. Stocks also varied by dominant plant species within individual ecosystem types. At larger scales, marsh stocks were lowest in the Sonoran Desert region of Mexico, and swamp stocks differed among climate zones; otherwise stocks showed little correlation with ecoregion or latitude. More variability in SOC occurred among ecosystem types, and at smaller spatial scales (such as individual estuaries), than across regional climate gradients. These patterns can inform coastal conservation and restoration priorities across scales where preserving stored carbon and enhancing sequestration helps avert greenhouse gas emissions and maintains other vital ecosystem services.

There is substantial interest in restoring tidal wetlands because of their high rates of long‐term soil carbon sequestration and other valued ecosystem services. However, these wetlands are sometimes net sources of greenhouse gases (GHG) that may offset their climate cooling potential. GHG fluxes vary widely within and across tidal wetlands, so it is essential to better understand how key environmental drivers, and importantly, land management, affect GHG dynamics. We measured methane (CH ₄ ) and nitrous oxide (N ₂ O) fluxes at 26 reference and restored tidal wetland sites and eight nontidal pastures (mostly diked former tidal wetlands) in five estuaries in the Pacific Northwest (PNW), USA. We measured fluxes 7–8 times over one year to assess the effects of environmental drivers, wetland type, and land management on CH ₄ and N ₂ O fluxes. Linear relationships between CH ₄ fluxes and environmental drivers were poor, but a machine‐learning approach with boosted regression trees provided strong predictability for fluxes based upon wetland surface elevation, water‐table level, and salinity. Less important variables were groundwater pH, wetland type, and temperature. Under oligohaline conditions, CH ₄ fluxes were variable and sometimes very high, but fluxes at salinities above 2 ppt were relatively low on an annual basis. Fluxes of CH ₄ were higher in restored tidal marshes and wet pastures than in reference tidal marshes, tidal swamps, and dry pastures. The N ₂ O model had lower predictive power than the CH ₄ model, with wetland type as the most important factor, although N ₂ O fluxes across all wetland types were low (median of zero). Our results indicate that estuarine hydrologic gradients are a key driver of CH ₄ fluxes and that wetland land use impacts on CH ₄ fluxes are largely mediated by their varying environmental conditions. In the PNW, estuarine wetlands that have low salinity, lower elevation, and have high water tables are more likely to have increased CH ₄ emissions that may offset their carbon sequestration benefits until they gain enough elevation through accretion. This study also provides a transferrable modeling approach to predict the consequences of coastal wetland management on GHG fluxes using monitoring data from a limited set of key environmental drivers.

Wetland restoration is an increasingly popular nature‐based method for flood risk mitigation in coastal communities. In this study, we present a novel method using hydrodynamic modeling and harmonic analysis to quantify wetlands' ability to reduce future nuisance flooding. The method leverages a hydrodynamic model calibrated to present day data and was run for a range of future sea‐level rise (SLR) and wetland restoration scenarios to quantify changes to tidal harmonic amplitudes and phases. The harmonic constituents are used to generate water surface elevations over a time period of interest (e.g., one year) and compared to critical exceedance thresholds such as levee elevations. Then, changes to nuisance flooding are calculated by counting the number of hours critical thresholds are exceeded under different SLR and wetland restoration scenarios. We applied the method to Coos Bay, Oregon, USA as a test case. We found restoration reduces the number of hours nuisance flooding occurs in downtown Coos Bay from 15 hr (present day conditions) to 0 hr (fully restored condition) under median SLR (82 cm by 2100). Restoration had spatially variable impacts on reducing peak flood elevations with minimal impacts near the estuary mouth and greatest impact 32 km inland. The effectiveness of restoration was heavily dependent on future SLR. Restoration was maximally effective in 2050 under all SLR scenarios, less effective in 2100 under median SLR, and not effective under high SLR. Modeling results suggest increased tidal prism and accommodation space are driving restoration‐associated reductions in tidal amplitudes.

Patchy global data on belowground litter decomposition dynamics limit our capacity to discern the drivers of carbon preservation and storage across inland and coastal wetlands. We performed a global, multiyear study in over 180 wetlands across 28 countries and 8 macroclimates using standardized litter as measures of "recalcitrant" (rooibos tea) and "labile" (green tea) organic matter (OM) decomposition. Freshwater wetlands and tidal marshes had the highest tea mass remaining, indicating a greater potential for carbon preservation in these ecosystems. Recalcitrant OM decomposition increased with elevated temperatures throughout the decay period, e.g., increase from 10 to 20 °C corresponded to a 1.46-fold increase in the recalcitrant OM decay rate constant. The effect of elevated temperature on labile OM breakdown was ecosystem-dependent, with tidally influenced wetlands showing limited effects of temperature compared with freshwater wetlands. Based on climatic projections, by 2050 wetland decay constants will increase by 1.8% for labile and 3.1% for recalcitrant OM. Our study highlights the potential for reduction in belowground OM in coastal and inland wetlands under increased warming, but the extent and direction of this effect at a large scale is dependent on ecosystem and OM characteristics. Understanding local versus global drivers is necessary to resolve ecosystem influences on carbon preservation in wetlands.

Disturbances are a key component of ecological processes in coastal ecosystems. Investigating factors that affect tidal marsh accretion and elevation change is important, largely due to accelerating sea‐level rise and the ecological and economic value of wetlands. Sediment accumulation rates, elevation change, and flooding were examined at five marshes along a riverine‐tidal gradient in the northern San Francisco Bay‐Delta, California, USA during an Atmospheric River storm event in 2017 using Surface Elevation Tables (SETs), feldspar marker horizons (MH), and continuous water‐level sensors. Our results showed that localized marsh flooding increased during the storm event, but not evenly across sites. Marsh surface elevation increased the most at the tidal freshwater marsh site in response to the storms, with an average surface elevation gain of 45.6 ± 13.1 mm, and the least at a tidal saline marsh with an average surface elevation gain of 4.0 ± 1.2 mm. A marsh located on the large embayment did not exhibit an immediate response to the storm but had a surface elevation gain of 21.5 ± 13.7 mm 6 months after the storm. During the storm period, marsh distance to the bay was the strongest predictor of elevation change, followed by SET‐MH elevations. Conversely, during non‐storm periods, SET‐MH elevation was a relatively strong predictor of elevation change. Atmospheric Rivers appear to be a major factor affecting short‐term spatial and temporal variability in flooding and sedimentation rates in tidal marsh systems. Incorporating information about storms into monitoring could increase our understanding of how episodic storms can impact marshes.

Understanding patterns of biodiversity is a key goal of ecology and is especially pressing in the current human‐caused biodiversity crisis. In wetland ecosystems, human impacts are centered around hydrologic manipulation including the common practice of wetland diking and impoundment. Constraining how wetland management influences plant biodiversity patterns across spatial scales will provide information on how best to modify actions to preserve biodiversity and ecosystem function in managed wetlands. Here, we compare patterns of plant diversity and species presence, abundance, and community composition at several spatial scales among tidal wetlands along an estuarine salinity gradient and managed wetlands that were formerly tidal. Managed impounded wetlands had decreased alpha and gamma diversity of rare species, with less than 60% of the species richness found in tidal brackish wetlands at several spatial scales. There was little change in the overall pattern of alpha, beta, and gamma diversity for common species in impounded wetlands; however, dominant tidal brackish species, primarily perennial rhizomatous graminoids, were replaced with management target plants and non‐native annual grasses in impounded wetlands. This species replacement led to over 60% of impounded sites being classified as containing novel plant assemblages. An additional 25% of impounded sites were classified as containing tidal saline plant assemblages, suggesting potential soil salinization. Along the estuarine gradient, patchiness and codominance of common plant species drove high diversity and turnover in tidal brackish wetlands, while it remains unclear whether tidal fresh or brackish wetlands maximize rare plant diversity. With reduced species richness, altered functional dominants, and novel or saline assemblages, impounded brackish wetlands may require careful water management to balance native plant biodiversity, associated ecosystem processes, and waterfowl requirements.