Improving Projections of Tidal Marsh Persistence under Climate Change with Remote Sensing and Site-Specific Data

Tidal marshes are dynamic ecosystems that are threatened by climate change and sea-level rise. To characterize baseline condition and historic climate sensitivities, and improve projections into the future, new methods are required that integrate data from the field and remote sensing platforms. Marsh elevation response models can be calibrated with site-specific data to determine the vulnerability of a marsh to sea-level rise and help guide management decisions. Elevation models are sensitive to initial elevation, the rate of accretion, and aboveground biomass. The overarching goal of this dissertation was to develop techniques to improve these important model inputs and evaluate the range of spatial and temporal variation. Light detection and ranging (lidar) is an invaluable tool for collecting elevation data, however dense vegetation prevents the accurate measurement of the tidal marsh surface. In Chapter 2, I describe the development of a technique to calibrate lidar digital elevation models with survey elevation data using readily available multispectral aerial imagery from the National Agricultural Inventory Program (NAIP). Using survey elevation data across 17 Pacific Coast tidal marshes, I demonstrated the utility of the Lidar Elevation Adjustment with NDVI (LEAN) technique to account for the positive bias in lidar due to vegetation. LEAN reduced error from an average of 23.1 cm to 7.2 cm root mean squared error and removed the positive bias caused by vegetation. This increase in accuracy will facilitate more accurate assessments of current and future vulnerability to sea-level rise. The phenology of aboveground biomass in tidal marsh plants in relation to climate variation has not been explored in the Pacific Northwest (PNW). In Chapter 3 I explain how I leveraged the Landsat archive and cloud computing capabilities to assess how Tasseled Cap Greenness (TCG, a proxy for aboveground biomass) in three PNW tidal marshes has responded to recent variation in climate to characterize sensitivity to climate change. Through analysis of over 3700 Landsat images obtained from 1984-2015, I found increased annual precipitation resulted in a higher peak TCG, while warmer May temperatures resulted in an earlier day of peak TCG. These results also demonstrate how time-series analysis of remote sensing data can be used to examine the sensitivity of tidal marsh plants to climate variability and directional change. The range of variation in tidal marsh accretion rates has not been characterized across the PNW. For Chapter 4, I collected and analyzed twenty-two soil cores from eight estuaries to estimate historic accretion rates with radioisotope dating techniques and evaluated the amount and source of variation across estuaries. I found that tidal marshes across the PNW had accretion rates greater than the current rate of sea-level rise, ranging from 2.3 – 7.3 mm yr⁻¹. Using a watershed-scale analysis, I found that long-term average annual fluvial discharge was the top predictor of tidal marsh accretion rates. Additionally, I found that calibrating the Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) with accretion rate data from nearby estuaries can result in uncertainties of up to 41% (50 cm) after 100 years. Finally, in Chapter 5, I demonstrate that a range of 62 cm of error is possible in WARMER models after a 100 year simulation when both uncorrected lidar and non-local accretion rates are used, fundamentally changing the interpretation of the results. Altogether, this dissertation illustrates the importance of collecting site-specific wetland vegetation and elevation data and demonstrates how lidar and multispectral remote sensing data can be leveraged to improve our understanding of how climate variability and change impacts coastal ecosystems

A summary of water-quality and salt marsh monitoring, Humboldt Bay, California

This report summarizes data-collection activities associated with the U.S. Geological Survey Humboldt Bay Water-Quality and Salt Marsh Monitoring Project. This work was undertaken to gain a comprehensive understanding ofwater-quality conditions, salt marsh accretion processes, marsh-edge erosion, and soil-carbon storage in Humboldt Bay, California. Multiparameter sondes recorded water temperature, specific conductance, and turbidity at a 15-minute timestep at two U.S. Geological Survey water-quality stations: Mad River Slough near Arcata, California (U.S. Geological Survey station 405219124085601) and (2) Hookton Slough near Loleta, California (U.S. Geological Survey station 404038124131801). At each station, discrete water samples were collected to develop surrogate regression models that were used to compute a continuous time seriesof suspended-sediment concentration from continuously measured turbidity. Data loggers recorded water depth at a 6-minute timestep in the primary tidal channels (Mad River Slough and Hookton Slough) in two adjacent marshes (Mad River marsh and Hookton marsh). The marsh monitoring network included five study marshes. Three marshes (Mad River, Manila, and Jacoby) are in the northern embayment of Humboldt Bay and two marshes (White and Hookton) are in the southern embayment. Surface deposition and elevation change were measured using deep rod surface elevation tables and feldspar marker horizons. Sediment characteristics and soil-carbon storage were measured using a total of 10 shallow cores, distributed across 5 study marshes, collected using an Eijkelkamp peat sampler. Rates of marsh edge erosion (2010–19) were quantified in four marshes (Mad River, Manila, Jacoby, and White) by estimating changes in the areal extent of the vegetated marsh plain using repeat aerial imagery and light detection and ranging (LiDAR)-derived elevation data. During the monitoring period (2016–19), the mean suspended-sediment concentration computed for Hookton Slough (50±20 milligrams per liter [mg/L]) was higher than Mad River Slough (18±7 mg/L). Uncertainty in mean suspended-sediment concentration values is reported using a 90-percent confidence interval. Across the five study marshes, elevation change (+1.8±0.6 millimeters per year[mm/yr]) and surface deposition (+2.5±0.5 mm/yr) were lower than published values of local sea-level rise (4.9±0.8 mm/yr), and mean carbon density was 0.029±0.005 grams of carbon per cubic centimeter. From 2010 to 2019, marsh edge erosion and soil carbon loss were greatest in low-elevation marshes with the marsh edge characterized by a gentle transition from mudflat to vegetated marsh (herein, ramped edge morphology) and larger wind-wave exposure. Jacoby Creek marsh experienced the greatest edge erosion. In total, marsh edge erosion was responsible for 62.3 metric tons of estuarine soil carbon storage loss across four study marshes. Salt marshes are an important component of coastal carbon, which is frequently referred to as “blue carbon.” The monitoring data presented in this report provide fundamental information needed to manage blue carbon stocks, assess marsh vulnerability, inform sea-level rise adaptation planning, and build coastal resiliency to climate change

Potential effects of sea-level rise on plant productivity: species-specific responses in northeast Pacific tidal marshes

Coastal wetland plants are adapted to varying degrees of inundation. However, functional relationships between inundation and productivity are poorly characterized for most species. Determining species-specific tolerances to inundation is necessary to evaluate sea-level rise (SLR) effects on future marsh plant community composition, quantify organic matter inputs to marsh accretion, and inform predictive modeling of tidal wetland persistence. In 2 macrotidal estuaries in the northeast Pacific we grew 5 common species in experimental mesocosms across a gradient of tidal elevations to assess effects on growth. We also tested whether species abundance distributions along elevation gradients in adjacent marshes matched productivity profiles in the mesocosms. We found parabolic relationships between inundation and total plant biomass and shoot counts in Spartina foliosa and Bolboschoenus maritimus in California, USA, and in Carex lyngbyei in Oregon, USA, with maximum total plant biomass occurring at 38, 28, and 15% time submerged, respectively. However, biomass of Salicornia pacifica and Juncus balticus declined monotonically with increasing inundation. Inundation effects on the ratio of belowground to aboveground biomass varied inconsistently among species. In comparisons of field distributions with mesocosm results, B. maritimus, C. lyngbyei and J. balticus were abundant in marshes at or above elevations corresponding with their maximum productivity; however, S. foliosa and S. pacifica were frequently abundant at lower elevations corresponding with sub-optimal productivity. Our findings show species-level differences in how marsh plant growth may respond to future SLR and highlight the sensitivity of high marsh species such as S. pacifica and J. balticus to increases in flooding.Keywords: Root-to-shoot ratio, Zonation, Plant biomass, Tidal wetlands, Marsh organ

Stress gradients structure spatial variability in coastal tidal marsh plant composition and diversity in a major Pacific coast estuary

Understanding the drivers of variability in plant diversity from local to landscape spatial scales is a challenge in ecological systems. Environmental gradients exist at several spatial scales and can be nested hierarchically, influencing patterns of plant diversity in complex ways. As plant community dynamics influence ecosystem function, understanding the drivers of plant community variability across space is paramount for predicting potential shifts in ecosystem function from global change. Determining the scales at which stress gradients influence vegetation composition is crucial to inform management and restoration of tidal marshes for specific functions. Here, we analyzed vegetation community composition in 51 tidal marshes from the San Francisco Bay Estuary, California, USA. We used model-based compositional analysis and rank abundance curves to quantify environmental (elevation/tidal frame position, distance to channel, and channel salinity) and species trait (species form, wetland indicator status, and native status) influences on plant community variability at the marsh site and estuary scales. While environmental impacts on plant diversity varied by species and their relationships to each other, overall impacts increased in strength from marsh to estuary scales. Relative species abundance was important in structuring these tidal marsh communities even with the limited species pools dominated by a few species. Rank abundance curves revealed different community structures by region with higher species evenness at plots higher in the tidal frame and adjacent to freshwater channels. By identifying interactions (species–species, species–environment, and environment–trait) at multiple scales (local, landscape), we begin to understand how variability measurements could be interpreted for conservation and land management decisions

Accuracy and precision of tidal wetland soil carbon mapping in the conterminous United States

© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 9478, doi:10.1038/s41598-018-26948-7.Tidal wetlands produce long-term soil organic carbon (C) stocks. Thus for carbon accounting purposes, we need accurate and precise information on the magnitude and spatial distribution of those stocks. We assembled and analyzed an unprecedented soil core dataset, and tested three strategies for mapping carbon stocks: applying the average value from the synthesis to mapped tidal wetlands, applying models fit using empirical data and applied using soil, vegetation and salinity maps, and relying on independently generated soil carbon maps. Soil carbon stocks were far lower on average and varied less spatially and with depth than stocks calculated from available soils maps. Further, variation in carbon density was not well-predicted based on climate, salinity, vegetation, or soil classes. Instead, the assembled dataset showed that carbon density across the conterminous united states (CONUS) was normally distributed, with a predictable range of observations. We identified the simplest strategy, applying mean carbon density (27.0 kg C m−3), as the best performing strategy, and conservatively estimated that the top meter of CONUS tidal wetland soil contains 0.72 petagrams C. This strategy could provide standardization in CONUS tidal carbon accounting until such a time as modeling and mapping advancements can quantitatively improve accuracy and precision.Synthesis efforts were funded by NASA Carbon Monitoring System (CMS; NNH14AY67I), USGS LandCarbon and the Smithsonian Institution. J.R.H. was additionally supported by the NSF-funded Coastal Carbon Research Coordination Network while completing this manuscript (DEB-1655622). J.M.S. coring efforts were funded by NSF (EAR-1204079). B.P.H. coring efforts were funded by Earth Observatory (Publication Number 197)

Taxonomy based on science is necessary for global conservation

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Wetlands in Intermittently Closed Estuaries Can Build Elevations to Keep Pace With Sea-Level Rise

Sea-level rise is a threat to coastal ecosystems, which have important conservation and economic value. While marsh response to sea-level rise has been well characterized for perennially open estuaries, bar-built intermittently-closed estuaries and their sea-level rise response are seldom addressed in the literature – despite being common globally. We seek to advance the conceptual understanding of sea-level rise response of marshes by incorporating the unique nature of intermittently-closed estuaries in a marsh model. We hypothesize that intermittently-closed-estuary marshes may be more resilient to sea-level rise than open-estuary marshes due to greater initial elevation capital and higher accretion rates due to closure events. Using California, USA as a case study, spatial analysis shows that marshes in intermittently-closed-estuaries had significantly greater elevations (x̄ = 1.93 m ± 0.2 standard error, n = 14) than marshes in permanently open estuaries (x̄ = 0.94 m ± 0.1 standard error, n = 8; P = 0.003). We then used a process-based model to determine marsh elevation change under 840 simulated responses to sea-level rise to 2100. Our modeling shows that regular annual mouth closure can promote accretion rates and increase marsh elevations fast enough to match even high rates of sea-level rise, as fluvial sediment pulses can be captured in the estuary. Modeled suspended sediment concentration had the strongest effect on accretion, followed by probability of annual mouth closure. Intermittently closed estuaries are critical environments where marshes may be sustained under high rates of sea-level rise, thus reducing the anticipated global loss of these important ecosystems. Our results begin to fill an important gap in the knowledge about marsh accretion and identify research needs to inform coastal management

Future Marsh Evolution Due To Tidal Changes Induced by Human Adaptation to Sea Level Rise

Abstract With sea level rise threatening coastal development, decision‐makers are beginning to act by modifying shorelines. Previous research has shown that hardening or softening shorelines may change the tidal range under future sea level rise. Tidal range can also be changed by natural factors. Coastal marshes, which humans increasingly depend on for shoreline protection, are ecologically sensitive to tidal range. Therefore, it is critical to examine how changes in tidal range could influence marsh processes. A marsh accretion model was used to investigate the ecological response of a San Francisco Bay, California, USA marsh to multiple tidal range scenarios and sea level rise from 2010 to 2100. The scenarios include a baseline scenario with no shoreline modifications in the estuary, a shoreline hardening scenario that amplifies the tidal range, and 14 tidal range scenarios as a sensitivity analysis that span tidal amplification and reduction of the baseline scenario. The modeling results expose key tradeoffs to consider when planning for sea level rise. Compared to the baseline, the hardening scenario shows minor differences. However, further tidal amplification prolongs marsh survival but decreases Sarcocornia pacifica cover, an important species for certain threatened wildlife and an effective attenuator of wave energy. Conversely, tidal reduction precipitates marsh drowning but shows gains in Sarcocornia pacifica cover. These mixed impacts of tidal amplification and reduction shown by the model indicate potential tradeoffs in relation to marsh survival, habitat characteristics, and shoreline protection. This study suggests the need for a cross‐sectoral, regional approach to sea level rise adaptation