Salt marsh climate change adaptation: Using runnels to adapt to accelerating sea level rise within a drowning New England salt marsh

Abstract

Sea level rise within New England is accelerating at a rate faster than the global average, leaving salt marshes particularly susceptible to degradation. Hydrological alteration is a type of climate change adaptation technique that can be used to combat the effects of sea level rise within salt marshes. Runnels (shallow channels) are a type of climate adaptation strategy used to enhance drainage in drowning marshes. In this study, we investigated the impacts of runnel installations, 3–5 years post‐implementation, on soil properties, vegetation composition, and greenhouse gas fluxes. We studied two runnel treatments (Low Elevation Runnel and High Elevation Runnel) and found that in the Low Elevation Runnel areas Spartina alterniflora stem density significantly increased in the three growing seasons after runnels were installed, and the high marsh plant, Spartina patens, persisted in the High Elevation Runnel areas. There was a significant difference in carbon dioxide uptake rates among treatments, with the unmanipulated (Reference) areas having the highest uptake rates and an increase in CO2 uptake over time seen in the Low Elevation Runnel treatment. These findings highlight the potential use of a climate change adaptation strategy to combat sea level rise impacts and provide insights for future adaptation efforts.

Buying Time with Runnels: a Climate Adaptation Tool for Salt Marshes

“…Researchers and resource managers have recently pointed to the widespread increase of interior shallow water as an indicator of marsh loss (Watson et al 2017;Campbell and Wang 2019;Adamowicz et al 2020;Schepers et al 2020;Taylor et al 2020;Duran Vinent et al 2021;Himmelstein et al 2021), and have responded with management actions to stop or slow open water conversion (Wigand et al 2017;Raposa et al 2019;Adamowicz et al 2020;Babson et al 2020;Perry et al 2021;Wolfe et al 2021). However, in some marshes an increase in standing surface water could represent a recovery of "natural" hydrology after marshes were historically over-drained by ditches, and some marshes show potential for pool recovery (Wilson et al 2014;Smith and Pellew 2021).…”

“…After observing rapid expansion of shallow water within northeastern US marshes, restoration ecologists began working with mosquito control agencies to use runnels for the dual purpose of mosquito abatement and marsh adaptation to SLR. Practitioners used the technique to target shallow water features that were expanding, had formed within the last few decades, and where an anthropogenic topographic feature was impairing water flow (ditch spoils, plugged ditches, embankments) (Wigand et al 2017;Adamowicz et al 2020;Perry et al 2021;Wolfe et al 2021). True ponds that remained flooded throughout the tide cycle, with unconsolidated sediments in the basin, and that appeared stable in dimension on decadal timescales, were not targeted with this technique (workshop communications).…”

“…In our 2020 workshop, resource managers reported projects on dozens of marshes across six northeastern US states, and another half dozen northeastern and mid-Atlantic marshes were under consideration for runnel-adaptation by land trusts, NGOs, and government agencies (Supplemental File 1). The majority of projects from the workshop, and one recently published study on runnels (Perry et al 2021), have reported some vegetation recovery within 1-5 years (Table 2 and Supplemental File 1).…”

“…Water measurements from Winnapaug runnel sites show that the water table remained within a few centimeters of the soil surface (Supplemental File 2), suggesting minimal risk of over-draining from the shallow runnels used at this site. Further, one study found no increase in CO 2 emissions after installing runnels, suggesting that peat oxidation rates did not change (Perry et al 2021). Overdraining soils could limit recovery of target vegetation species as well.…”

“…A surge in restoration of Northwest Atlantic salt marshes (specifically, northeastern USA) occurred over the past decade, funded by the US government in the Communicated by Kenneth L. Heck aftermath of Superstorm Sandy in 2012 (Babson et al 2020). One technique involved digging "runnels"-small channels meant to drain standing water and promote revegetation (Wigand et al 2017;Raposa et al 2019;Babson et al 2020;Perry et al 2021;Wolfe et al 2021). As of 2019, knowledge on runnel efficacy as a tool to build marsh resilience to SLR (defined here as the ability of a marsh to resist a state-change to open water) had not been widely shared.…”