Revegetation of Salt Marsh Dieback Areas
On Cape Cod (Massachusetts, USA), high densities of purple marsh crabs (Sesarma reticulatum) have decimated large areas of salt marsh vegetation (Holdredge et al. 2008). Intense herbivory by this species, primarily on Spartina alterniflora (smooth cordgrass) and S. patens (salt marsh hay),has resulted in the formation of large bare areas that are almost completely devoid of vegetation. As such, they are subject to considerable erosion and subsidence (Smith 2009). However, recent field observations have revealed that certain halophytic forbs such as Salicornia virginica (perennial glasswort), Suaeda maritima (sea-blite), and Limonium carolinana (sea lavender) are slowly colonizing some of these locations. Plants of the abovementioned taxa that have managed to become established in dieback areas do not exhibit any signs of herbivory, suggesting that it is the perennial grasses (S. alterniflora, S. patens, and Distichlis spicata) that are much preferred by S. reticulatum. While S. maritima tends to be restricted to sites near mean high tide elevations or above, S. virginica and L. caroliniana, are capable of withstanding abiotic conditions (i.e., salinity, hydrology, redox potential, etc.) from the lowest reaches of the low marsh to the upper extremity of the high marsh. However, these taxa are not often found in the low marsh zones, most likely due to the dynamics of seed dispersal. Their seeds are very small, round, and buoyant. As such, they are often transported far upslope by tides, which is why they are much more abundant in the high marsh. The main objective of the project described here was to test various methods of re-vegetation with transplants and seeds of low or unpalatable species. Successful establishment and persistence of such vegetation, although representing a change in species composition, could greatly enhance the process of marsh stabilization. The naturally low rate of re-vegetation with these species is probably due to a lack of seed source populations, seed availability, and dispersal mechanisms - all of which can severely limit the extent of their distributions across salt marshes, irrespective of habitat suitability.
Two study sites (Lt. Island and the Gut) with extensive vegetation losses were selected for this study. Limonium and Salicornia plants were harvested from a local marsh (Hatches Harbor), where they grow in abundance in the high marsh. The plants were transplanted into dieback areas in Lt. Island and the Gut. The transplant sites generally occurred within the mid-marsh between the landward edge of S. alterniflora and the seaward edge of S. patens.
Seeding with wrack
Treatments were wrack + jute, jute alone, wrack only, control (no wrack or jute) (n=3 replicates per treatment). Wrack was deposited in twenty 1-m2 plots spaced at least 5 m away from each other. One half of the wrack deposits that were placed along creekbanks were covered with jute which was fastened to the substrate using lawn staples. The other half of the wrack deposits had no fabric covering the wrack. Jute was also fastened to areas that did not receive wrack deposits to assess whether jute alone could facilitate natural recruitment from the any seed bank or naturally dispersing seeds. The control plots had no wrack and were not covered with Jute.
In December 2011, senescent plants of Limonium caroliniana and Salicornia virginica (with seeds still attached) and wrack material was collected from Hatches Harbor. Wrack from this location is known to contain seeds of Spartina alterniflora, Spartina patens, Salicornia maritima, Suaeda spp., and L. caroliniana (citation). Seeds were separated from vegetative material by shaking them off the adult plants and then sieving this material through 5 mm mesh screen. The seed mixture was then poured into a 1-m long planting constructed out of PVC. A handle releases the door, which deposits approximately 100-ml of seed.
To plant the seeds, a ~5 cm deep hole was made in the substrate and seed released into the hole, which was then covered up with sediment. A pin flag was inserted into the ground next to each planting site.
The results of this study suggest that there is promise for re-vegetating salt marsh dieback areas with native halophyte species that are unpalatable to S. reticulatum. As mentioned previously, this appears to be happening naturally. However, it is occurring at a relatively slow rate. The dynamics of seed dispersal are considered the primary factor influencing their distributions. However, other constraints on the establishment of these species in dieback areas include the proximity and size of extant populations that can provide seeds to the marsh as well as physical factors such as storms, waves, and fiddler crab bioturbation.
The methods in this study (i.e., use of mature plants with developed root systems, stabilizing effect of jute, burial and stabilization by seed sowing) may ameliorate some of these disturbances. Although active transplantation or seeding of the abovementioned halophytes will alter the plant communities of these marshes, all species are native. The principal objective of promoting the expansion of these plants into dieback areas is to limit marsh substrate and elevation loss. The extreme degradation is disconcerting and does not bode well for salt marsh resiliency to sea level rise. Although the root systems of halophytic forbs such as L. carolinianum and S. virginica may not be as effective as Spartina grasses in binding substrate, any increase is sediment binding by plant root systems will reduce sediment loss. The fact that some salt marshes are comprised almost entirely of Salicornia spp. (e.g., in California) suggests that they are an effective stabilizing entity. In addition, the mere establishment of any vegetation in these bare dieback areas will enhance marsh persistence through sediment deposition.
Numerous salt marshes along the Atlantic coast of the United States have been severely impact by crab herbivory, including Georgia, Delaware, New York, Connecticut, and Massachusetts. As such, this technique may have broad appeal to managers who wish to preserve salt marshes through revegetation.