High marsh dieback
High marsh dieback (primarily Spartina patens, but also Distichlis spicata and Juncus gerardii) always occurs along the seaward-most edge of the high marsh zone (i.e., at the lowest elevations). <click for enlarged image> This is distinctly different than low marsh dieback, which occurs in many places throughout the zone between mean low and mean high tide.
Moreover, standing dead foliage is observed in high marsh species - something that does not happen with Spartina alterniflora.
The fact that high marsh losses only occur along the seaward edge suggests a link with hydrology (duration of flooding). However, the extent of loss also correlates with the presence of high densities of crabs (both Sesarma and fiddler crabs). We have, in fact, witnessed Sesarma crabs feeding on S. patens in addition to S. alterniflora, so high marsh dieback may be linked to herbivory as well.
Field experiments over the last 2 years have revealed a few things about high marsh dieback:
1) there is significant but highly variable herbivory on S. patens
2) plant mortality also can occur in the absence of herbivory (i.e., caged plants can die)
3) the seaward edges of high marsh dieback zones experience more high tides, higher water levels during high tide, and slower drainage after high tides than the seaward edges of healthy high marsh zones
4) plants along the seaward edges of high marsh dieback zones have a much reduced ability to resprout after similulated disturbance (clipping of foliage) compared to plants at higher elevations or plants in other marshes where dieback has not occurred
As such, high marsh dieback may be the result of a combination of different factors, including hydrology (flood duration related to sea level), disturbance (herbivory, wrack smothering), and possibly soil properties (very low nutrient, organic matter content).