The Role of Phragmites australis in Mediating Inland Salt Marsh Migration in a Mid-Atlantic Estuary

Joseph A M Smith,Andrew Davies

doi:10.1371/journal.pone.0065091

Abstract

Many sea level rise adaptation plans emphasize the protection of adjacent uplands to allow for inland salt marsh migration, but little empirical information exists on this process. Using aerial photos from 1930 and 2006 of Delaware Estuary coastal habitats in New Jersey, I documented the rate of coastal forest retreat and the rate of inland salt marsh migration across 101.1 km of undeveloped salt marsh and forest ecotone. Over this time, the amount of forest edge at this ecotone nearly doubled. In addition, the average amount of forest retreat was 141.2 m while the amount of salt marsh inland migration was 41.9 m. Variation in forest retreat within the study area was influenced by variation in slope. The lag between the amount of forest retreat and salt marsh migration is accounted for by the presence of Phragmites australis which occupies the forest and salt marsh ecotone. Phragmites expands from this edge into forest dieback areas, and the ability of salt marsh to move inland and displace Phragmites is likely influenced by salinity at both an estuary-wide scale and at the scale of local subwatersheds. Inland movement of salt marsh is lowest at lower salinity areas further away from the mouth of the estuary and closer to local heads of tide. These results allow for better prediction of salt marsh migration in estuarine landscapes and provide guidance for adaptation planners seeking to prioritize those places with the highest likelihood of inland salt marsh migration in the near-term.

Highlights

Coastal habitats are dynamic by nature, but sea level rise resulting from climate change has the potential to dramatically alter the pace of these dynamics [1]
Over a 76-year time frame, I found that at the salt marsh-forest interface, forest has been lost at more than three times the amount (1.8 m/yr) that salt marsh has been gained via inland migration (0.54 m/yr)
On the the adjacent Delaware Bay coastline at this study site, loss of salt marsh to erosion is approximately 3 meters per year [37], an amount that is 5.5 times greater than the amount being gained by inland migration (0.54 m/yr)

Summary

Introduction

Coastal habitats are dynamic by nature, but sea level rise resulting from climate change has the potential to dramatically alter the pace of these dynamics [1]. The process of habitat change at this upland interface is wellunderstood over geologic time-scales [9,10,11], but there are few studies that document this process at a contemporary time-scale [12,13] Such information is needed to gain a realistic understanding of the contribution these upland areas will make toward offsetting salt marsh losses due to sea level rise. On the east coast of North America, there has been a steady pace of sea level rise that gradually overtakes forested uplands and wetlands [14] Over time, these areas become part of the salt marsh as salinity and moisture increase at upland edges [3]. These recent losses of terrestrial habitats to sea level rise present an opportunity to examine how the change from forest to salt marsh proceeds over time so that future changes can be better anticipated

Methods

Results

Conclusion