Projects

Salt marsh upland migration: Past, present and future

End Date: 1/31/2022

Objectives

The central question of our proposed research is: Are there any management strategies that can be implemented to promote salt marshes’ upslope migration? The goal is to enable more effective land management (including land acquisition and prescribed fire) that could help facilitate salt marsh upland migration to mitigate extensive losses of these important ecosystems under sea level rise and climate change. Particularly, we have four objectives:

  1. To quantify the historical and current salt marsh landward migration rates.
  2. To develop an ecohydro-vegetation-fire coupled model to predict hotspots where salt marsh vegetation could migrate upslope.
  3. To predict salt marsh upland migration under a variety of SLR, climate change, and prescribed fire scenarios, and compare these future migration rates to the historical and current rates.
  4. To predict area losses at the water boundaries under a variety of SLR scenarios using the model developed at the PI’s lab, and compare them to the area gains at the upland boundaries from Objective 3 to derive net gains/losses of salt marshes.

We will test the following research hypotheses closely related to the four objectives (note the null hypotheses are opposite to these research hypotheses):

H1: Salt marsh upland migration rates increase over time with accelerating SLR.

H2: Prescribed fire at marsh-pine savannas ecotones with 2 year frequency facilitates salt marsh landward migration under SLR and climate change.

H3: Extreme events of climate will affect spatial patterns of salt marsh migration.

H4: Losses of salt marshes at water boundaries may be compensated by upland migration, with prescribed fire as a management strategy.

This proposed project fits in the research priority “Model the rate of transition from salt marsh to uplands and determine the management implications of marsh migration” under the focus area of "healthy coastal ecosystems" in the Request for Proposal.

Methodology

We will implement remote sensing image analysis, field sampling experiments, and simulation models to conduct this proposed research. Our study area will cover the gradients from salt marshes to wet pine savannas with a focus on salt marsh-pine savanna ecotones at the Grand Bay National Estuarine Research Reserve (NERR) in southeastern Mississippi.

The proposed integrated model is the main core of the proposed project. It will synthesize the interactions between vegetation, hydrology, SLR, climate change and fire events along the gradients from salt marsh to upland ecosystems. It includes vegetation dynamics with competition accounted for (vegetation module), a ecohydrological model to predict evapotranspiration (evapotranspiration module) and soil salinity (soil salinity module), feedbacks between vegetation and fire, feedbacks between salinity and fire, and feedbacks between vegetation and salinity. The feedbacks between fire and vegetation/salinity can inform decision making in prescribed fire planning, and identifying the hotspots for landward migration can facilitate decision making on land acquisition.

We will compare the current (1990s-2010s) and predicted future (2100) landward migration rates to the historical migration rate (1950s-'70s and 1970s-'90s) to test H1.

The comparison of salt marsh migration patterns in the future scenarios with and without prescribed fires with different frequencies will help test H2.

The comparison of salt marsh migration in the future climate scenarios with extreme events (particularly, droughts and flooding) and future’s average climate scenarios will help test H3.

We will compare the area losses of salt marshes to the potential area gains at the upland boundaries through upland migration. This comparison will help test H4.

Rationale

Salt marshes provide a wide array of ecosystem services but they have been experiencing extensive loss under sea-level rise (SLR). Maintaining salt marshes entails massive restoration efforts. One other similarly important but largely understudied mechanism to mitigate the loss is salt marshes’ upland migration, provided suitable upland space (“accommodation space”) exists. However, salt marshes are constrained by their ability to migrate due to interspecific competition with upland vegetation including forests. The management strategies that increase suitable upland types and reduce interspecific competition have the potential to promote landward migration, such as land acquisition to avoid urban development and prescribed fire (controlled or management burning) that could open up space to facilitate migration.

As SLR, climate change and land management interact with each other to affect spatial patterns of salt marshes’ upland migration over time, there requires a comprehensive framework to synthesize these interactions and feedbacks to help predict upland migration and evaluate effectiveness of management strategies. Models show an effective tool for this purpose. Models have been developed to simulate salt marsh landward migration. However, these models predict change of vegetation community zones based elevation change. Such elevation envelope model, like climate envelope model, is useful for predicting potential niches, but not realized niches as they assume the adaptive capacity of the species and species interactions such as competition remain the same under changing environment. Currently there is a lack of integrative modeling framework to predict landward migration of salt marshes by integrating multiple drivers including management strategies (SLR, climate change and prescribed fire) and accounting for vegetation competition. This proposal will address this key knowledge gap by developing a coupled ecohydro-vegetation-fire model for predicting spatial and dynamic patterns of landward migration of salt marshes under a variety of SLR, climate change, and prescribed fire events with different frequencies.