Abstract
Understanding the process of marsh migration into coastal areas, particularly low-lying old fields, will help to inform adaptive strategies that facilitate ecosystem change from upland to salt marsh. This study sought to understand better the drivers of old-field plant community response to sea-level rise and to evaluate how two types of disturbance, tilling the soil and introducing a new species, alter community response.
From 2017-2019, water levels and salinity were measured in seven shallow wells in an abandoned agricultural field adjacent to a salt marsh. Water-table fluctuations greater than 10 cm were associated with flooding events from storm surges, precipitation, or a combination of the two. Groundwater elevation and salinity showed a temporal response to freshwater flooding from precipitation and saltwater flooding from storm surge and extreme tides that suggest overland flooding with saline water from nearby tidal creeks is the source of salinity in the groundwater. However, salinization of the groundwater may not persist if seawater flooding events are followed or accompanied by freshwater inputs from precipitation. These findings show that the magnitude and timing of precipitation events can reduce salinity stress to the plant community by retarding the salinization of old fields experiencing high rates of sea-level rise.
Greenhouse studies of Panicum virgatum (switchgrass) and Kosteletzkya pentacarpos (salt marsh mallow) seed germination and seedling growth in three levels of saline water showed that salt marsh mallow germination and survival was greater than that of switchgrass at all treatment levels, and that mallow began linear growth sooner than switchgrass. Mallow seed germination and seedling growth appeared more tolerant of saline water than switchgrass, and these findings suggest that mallow should become established more rapidly than switchgrass in abandoned agricultural fields experiencing increased frequency of flooding with saltwater.
A field experiment was conducted to determine the effect of increased frequency of flooding on the establishment and growth of monocultures and mixed cultures of salt marsh mallow and switchgrass. Mallow cover was higher in the wet treatment plots, which received supplemental fresh water every week during the summer, than in the dry plots, while switchgrass cover was unaffected by watering. Additionally, seeds of the two plants were mixed in different ratios and then planted together. Regardless of the seed mixture ratio in a plot, mallow cover was greater than that of switchgrass. Thus, salt marsh mallow may be a more suitable species than switchgrass for cultivation in abandoned agricultural fields where groundwater elevations are increasing or the fields experiencing more frequent overland flooding with saline water.
In 2015, K. pentacarpos seeds were planted in experimental plots at the field site to determine if the recruitment of a native high marsh species, Distichlis spicata, was enhanced by disturbing the existing plant community by tilling and adding marsh mallow. Species richness and percent cover at the site were measured at the end of each growing season from 2015 to 2019. After two years, coverage by high marsh species, including D. spicata, and transition species, including K. pentacarpos, increased in all treatment plots. Disturbance of the native plant community by tilling increased D. spicata cover in the plots one year after the initial treatment. Changes in hydrological conditions in response to seawater flooding in three distinct zones within the field resulted in different plant communities. These results suggest that cultivating K. pentacarpos in salinized agricultural fields may improve the natural recruitment of high marsh and transition plant species in coastal agricultural land experiencing sea-level rise.
Based on the results of these experiments, a new conceptual model was developed to explain the drivers of ecosystem transition, especially groundwater elevation and salinity, from abandoned agricultural fields to high marsh. This model decouples those drivers, which occur at different rates, and emphasizes the role of precipitation in plant community change. In fields experiencing high rates of salinization, disturbance facilitates the transition from upland to salt marsh. Thus, the rate of inland marsh migration depends on the abiotic conditions at a given site and the resiliency of the plant community to environmental change, which vary at different spatial and temporal scales. Furthermore, the addition of new species, such as salt marsh mallow, that thrive under dynamic environmental conditions may preserve important ecosystem services that otherwise may be lost, at least temporarily, during transition.
