PROGRAM | Plant & Soil Sciences

By: Alma Delia Vazquez Lule Chair: Rodrigo Vargas

ABSTRACT

Salt marshes are one of the most productive ecosystems in the world with the capacity to store large amounts of carbon per unit area, referred as Blue Carbon. This carbon can remain in the ecosystem, be emitted to the atmosphere as carbon dioxide (CO₂) or methane (CH₄), or laterally exported by the tidal exchange. The influence of vegetation on the CO₂ and CH₄ exchange between salt marsh ecosystems and the atmosphere is uncertain, because of high unresolved temporal and spatial variability in these ecosystems. Data based knowledge about the spatial and temporal variability of salt marshes is needed for a better understanding of the role of these ecosystems into the global carbon cycle. In this PhD study, I aim to understand the influence of the salt marsh vegetation on the exchange of carbon between a temperate salt marsh and the atmosphere. I describe and quantify the influence of different plant phenological phases on the CO₂ and CH₄ exchange, as well as their influence on the Gross Primary Productivity (GPP) at the ecosystem and canopy scale (i.e., canopy photosynthesis by each dominant salt marsh species; F_A). For that, I use proximal remote sensing (PRS; PhenoCam, hyperspectral reflectance data and spectral vegetation indices) to measure the temporal and spatial variability of carbon exchange. This study was performed on the mid-Atlantic Coast of the United States, in the State of Delaware. The study site is a temperate tidal salt marsh dominated by grasses (i.e., Spartina alterniflora and S. cynosuroides). My main results show that contrasting biophysical factors influence Net Ecosystem Exchange (NEE) of CO₂ and CH₄ exchange across the diel cycle and plant phenological phases (i.e., Greenup, Maturity, Senescence, Dormancy). I find that plant phenological phases have a substantial influence on the exchange of carbon, being Senescence and Dormancy the phases with higher CH₄ and CO₂ emissions to the atmosphere (Chapter 2). My results show that plant phenological phases also have an influence on the daily GPP variability, and that PRS is able to model and predict this variability across the annual cycle and during the beginning of the growing season, but challenges remain for the rest of the plant phenological phases, as a response to changes in the salt marsh vegetation and exposition of soils. I find that vegetation indices used to explain changes in the chlorophyll/carotenoid ratio were more useful to model GPP variability, in contrast to some indices used to explain changes on the greenness condition of the vegetation. My results also show that the use of hyperspectral data from the visible and infrared regions (VIS-IR) coupled with the partial least square regression (PLSR) approach, is more useful to model and predict daily GPP than specific areas of the electromagnetic region such as the Sun Induced Fluorescence (SIF), red edge (RedEdge) and infrared (IR) (Chapter 3). I find that the spatial heterogeneity in salt marshes influences the relationship between canopy photosynthesis (F_A) and leaf nutrients for the most dominant species of vegetation. Nitrogen leaf content (N) has an influence on the F_A of S. cynosuroides but not on the F_A of S. alterniflora, as a response of the availability of vegetation to uptake this nutrient from soils under lower redox conditions. Leaf nutrients such as phosphorus (P), potassium (K) and sodium (Na) are related with F_A for the most dominant salt marsh species in this ecosystem. My results show the promising application of hyperspectral PRS and PLSR approach for linking information of leaf nutrients with F_A in canopy salt marshes (Chapter 4). My PhD results are useful to better understand and monitor the carbon cycle in temperate salt marshes, to reduce the uncertainty on the carbon exchanged within the atmosphere and to improve estimations and models of blue carbon in coastal wetlands.

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