Introduction
Coastal wetland ecosystems are some of the most efficient ecosystems for
carbon sequestration (Mcleod et al., 2011). At the interface of land and
ocean, these ecosystems are characterized by mixing of water, nutrients,
and sediment from watersheds and nearshore marine waters. Productive
plant communities occupy the continuum from upstream to estuary mouths,
despite the dynamic gradients in physicochemical conditions. Dense
vegetation slows water velocity, allowing sediment accretion that
further buries organic matter produced or trapped in coastal wetlands.
In addition, soils are often anoxic, slowing the decomposition of buried
organic matter and allowing soil organic matter to accumulate. Land
surface models (LSMs) are useful for examining carbon cycling at
ecosystem scales and are critical for estimating carbon cycling in
global scale Earth system model simulations. However, despite the
importance of coastal wetlands in carbon storage, and their
vulnerability to sea level rise, LSMs have limited representation of
coastal ecosystems (O’Meara et al., 2021; Ward et al., 2020).
Plant functional types (PFTs) developed for terrestrial vegetation are
inadequate for coastal wetlands because the controls on vegetation
productivity differ between these ecosystems (LaFond-Hudson and Sulman,
2023). Wetland plants are sometimes exposed to soil moisture, pH, and
nutrient concentrations rarely experienced by terrestrial plants, so
wetland vegetation traits sometimes stray from the predictable patterns
of resource tradeoffs observed in terrestrial plants (Moor et al. 2017).
Models sometimes lack critical for processes in wetlands if the
processes are less influential in terrestrial ecosystems. For example,
in the Energy Exascale Earth System Model (E3SM)’s land surface model
(ELM), vegetation productivity is limited by low soil moisture, but not
by saturated soils (Oleson et al. 2013), so the model likely
overestimates productivity in coastal wetlands. The model also does not
represent the impact of salt on soil water potential and photosynthesis.
These missing processes lead to uncertainty in global carbon cycle
projections; improving these processes will open opportunities to
explore carbon cycle questions within coastal wetland ecosystems using
LSMs.
Inundation decreases oxygen availability to roots under saturated soil
conditions and by covering stems and leaves under flooded conditions
(Colmer and Vosenek 2009). Plant adaptations to flooding can include
aerenchymous tissue that transports oxygen to the root zone, increased
vertical growth under flooded conditions, and floating tissues for
plants adapted to permanently flooded conditions. Despite the abundant
water in coastal wetlands, salinity affects plants by lowering the soil
water potential outside of roots, creating challenges for water uptake.
Plants may respond by limiting water uptake, or by taking up salt along
with water and accumulating salt in tissues (Munns and Tester, 2008).
Plants that limit water uptake must conserve water; this strategy
requires lowering stomatal conductance and carbon uptake. Plants that
accumulate salt have adaptations to alleviate ion toxicity within
tissues, such as storage in vacuoles and production of osmolytes to
maintain osmotic potential of cells. Although these adaptations are
better for chronic salt exposure, these adaptations are energetically
costly.
The distribution of plant communities is tightly coupled to average
salinity concentrations and hydroperiod, but the physicochemical
conditions of coastal wetlands are dynamic. Additionally, sea level
rise, droughts, and more frequent, intense storms are likely to further
vary water levels and salinity in coastal wetlands. Land surface models
will be more capable of exploring carbon cycling in coastal wetlands if
vegetation dynamics can be accurately simulated at the salinity and
hydroperiod for which vegetation is well adapted, as well as the
dynamics when vegetation is exposed to conditions for which it is poorly
adapted. For example, much of the research on salt marshes in the past
two decades concerns the fragility of salt marshes in the face of
accelerating sea level rise. Some evidence suggests they are resilient
to moderate increases in the duration of inundation via enhanced
vegetation productivity that contributes to elevation gain (Kirwan et
al., 2010; Morris et al., 2002). However, in some locations, marshes are
eroding because local sea level rise rates are outpacing marsh elevation
gain (Wasson et al., 2019). At the same time, saltwater intrusion into
ecosystems that are not adapted to saline conditions can cause
widespread mortality of vegetation and consequent loss of carbon stocks.
Incorporating vegetation responses to salinity and inundation into LSMs
is important for simulating the impacts of seasonal patterns and
long-term trends in water levels and salinity on vegetation productivity
in coastal ecosystems. In this paper, we describe a new capability of
the DOE’s Energy Exascale Earth system model (E3SM) land model (ELM) to
represent vegetation responses to salinity and inundation. We show how
the vegetation responses to salinity and inundation improve model
accuracy in a salt marsh ecosystem. Several scenarios are then presented
to demonstrate how a salt marsh might respond to systemic changes in
salinity and water level, as might occur in departures from average
annual precipitation.