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Exploring halophyte hydrodynamics and the role of vegetation traits on ecosystem response to disturbance at the terrestrial-aquatic interface

Active Dates 9/1/2019-8/31/2024
Program Area Terrestrial Ecosystem Science
Project Description
Mangroves grow along tropical and subtropical coastlines and intertidal zones, and are therefore very rarely limited by root-zone moisture availability. However, during the dry season, these ecosystems have been shown to behave more similarly to semi-arid ecosystems than well-watered forests. The process of salt exclusion from sea- and brackish waters during root-water uptake provides an additional energy and rate limiting step in water transport along the soil-plant-atmosphere continuum. This adaptation is responsible reductions in transpiration rates due to high tensions in the xylem system, in spite of adequate water availability. Reductions in transpiration cause increases in sensible heat flux rather than latent heat leading to an energy balance characteristic of a water-limited ecosystem. Current land-surface modeling technologies use a semi-empirical relationship to connect stomatal conductance directly to soil moisture, and are unable to replicate the behaviors of halophytes leading to errors in energy balance partitioning throughout tropical coastlines.

We propose to develop a salt-tolerant root-water uptake model for the FETCH2 advanced vegetation hydrodynamics model that will be capable of mechanistically simulating osmoregulation by halophytic plants such as mangroves. The FETCH2 model will then be integrated into the land-surface model component of the DOE’s Energy Exascale Earth System Model (E3SM) as a replacement for the current direct link between soil moisture and stomatal conductance. FETCH2 approximates flow through the xylem of plants as a flow through porous media and accounts for dynamic changes to conductance and capacitance of plant tissues caused by changes in water content. Parameter sets within FETCH2 are based on measurable hydraulic traits including stomatal sensitivity, xylem and root vulnerability to hydraulic impairment (embolism), and rooting depth, among others. However, studies have shown that these above and belowground hydraulic traits can be highly plastic and vary both spatially and temporally. Therefore, we will couple our model development with an extensive field study of mangrove hydraulic traits, their variability, and their influence on plant and ecosystem level fluxes of carbon, water, and energy.

Atmospheric demand for water vapor and soil water availability are the primary determinants of vegetation water stress for terrestrial plants. Yet in the unique case of mangroves, salinity supersedes the control of soil water availability. Our study is designed to analyze mangrove forest function across both humidity and salinity gradients which are predicted to change in response to compounding disturbances such as sea level rise, enhanced variability in precipitation and overland runoff, inundation frequency, and increased atmospheric CO2. We will combine ecosystem scale measurements of carbon, water, and energy flux with plant-level observations of sap flux, biomass water content, and leaf level gas exchange, and analysis of emergent above and below ground plant hydraulic traits in three field sites spanning a strong humidity gradient from Panama (humid), to the US Gulf Coast (subhumid), and Abu Dhabi (arid). Within each site, tidal influences, evaporation, and surface runoff create a salinity and water depth gradient from landward to ocean-ward edge. We propose to analyze trait plasticity along the axes of atmospheric response (across sites), salinity response (within sites), individual and species controls (individuals and species within and across sites), and temporal controls (across wet and dry seasons and years). This analysis of above and belowground trait plasticity will facilitate the development of a flexible parametric ‘trait-space’ for FETCH2. We hypothesize that such a trait-based plant hydrodynamics model will more accurately represent water, carbon, and energy exchange at the terrestrial-aquatic interface along tropical coastlines.
Award Recipient(s)
  • The University of Texas at Austin (PI: Matheny, Ashley)