Experimental Tree Mortality Does Not Induce Marsh Transgression in a Chesapeake Bay Low-Lying Coastal Forest

David C Walters,Katya E Kovalenko,Joshua A Jones,Matthew L Kirwan,Glenn R Guntenspergen,Eliza Mcfarland,Donald R Cahoon,Alyssa Hockaday,Joel A Carr

doi:10.3389/fmars.2021.782643

Abstract

Transgression into adjacent uplands is an important global response of coastal wetlands to accelerated rates of sea level rise. “Ghost forests” mark a signature characteristic of marsh transgression on the landscape, as changes in tidal inundation and salinity cause bordering upland tree mortality, increase light availability, and the emergence of tidal marsh species due to reduced competition. To investigate these mechanisms of the marsh migration process, we conducted a field experiment to simulate a natural disturbance event (e.g., storm-induced flooding) by inducing the death of established trees (coastal loblolly pine, Pinus taeda) at the marsh-upland forest ecotone. After this simulated disturbance in 2014, we monitored changes in vegetation along an elevation gradient in control and treatment areas to determine if disturbance can lead to an ecosystem shift from forested upland to wetland vegetation. Light availability initially increased in the disturbed area, leading to an increase in biodiversity of vegetation with early successional grass and shrub species. However, over the course of this 5-year experiment, there was no increase in inundation in the disturbed areas relative to the control and pine trees recolonized becoming the dominant plant cover in the disturbed study areas. Thus, in the 5 years since the disturbance, there has been no overall shift in species composition toward more hydrophytic vegetation that would be indicative of marsh transgression with the removal of trees. These findings suggest that disturbance is necessary but not sufficient alone for transgression to occur. Unless hydrological characteristics suppress tree re-growth within a period of several years following disturbance, the regenerating trees will shade and outcompete any migrating wetland vegetation species. Our results suggest that complex interactions between disturbance, biotic resistance, and slope help determine the potential for marsh transgression.

Highlights

Transgression into adjacent uplands is an important response of coastal wetlands that are threatened by accelerated rates of sea level rise (Raabe and Stumpf, 2016; Schieder and Kirwan, 2019)
The Girdled forest initially had similar ranges of light levels as the Control increased to maximum levels in 2018 as dead tree limbs fell and opened the canopy further, until it began to decrease as new vegetation re-grew and shaded the sensors in 2019
The trends show that the Cut forest resulted in an immediate increase in available light reaching the understory vegetation, while the Girdled forest resulted in a gradual increase in available light over several years

Summary

Introduction

Transgression into adjacent uplands is an important response of coastal wetlands that are threatened by accelerated rates of sea level rise (Raabe and Stumpf, 2016; Schieder and Kirwan, 2019). Humans create socio-economic barriers to transgression such as private landowners mowing areas with potential wetland vegetation (Anisfeld et al, 2017) and agricultural practices of altering natural hydrology via digging ditches and building impoundments (Smith et al, 2017). Even in a natural setting with upslope areas within the range for potential transgression, ecological barriers are present in the form of competition with invasive species with low habitat value (i.e., Phragmites australis) (Smith, 2013) and the resiliency of preexisting forest ecosystems where mature trees experience low mortality rates (Field et al, 2016)

Methods

Results

Conclusion