Salt Marsh Migration Work

BY LINDA VANDERVEER

Being a child of the 1980s, Blondie’s The Tide is High is on a relatively frequent playlist at my house.  But these days the tune is in my head more often for another reason – the tide is in fact high.  And getting higher.  The effects of climate change are unspooling before my eyes.

As the owner and steward of approximately 125 acres of salt marsh habitat in Dartmouth, DNRT is on the front line of climate change.  While all of our properties are expected to be impacted to some extent, our coastal properties are already beginning to show the effects of climate change on the ground.

At DNRT’s Ocean View Farm property we are responding to these impacts and working with our partners at the Buzzards Bay Coalition, Bristol County Mosquito Control, and Mass Audubon to use cutting edge research tools and techniques to try and make our marshes more resilient to these impacts.   We hope to share what we learn with other coastal landowners, including municipalities, land trusts, educational groups, and private landowners in order to create a collective effort to prepare for climate change.

Before launching into the great research we are doing, let’s review some basic marsh ecology.  Salt marshes can be broken down into two ecological zones: the low marsh, which is the part closest to the sea, and the high marsh, which is the part at the “back” of the marsh that ultimately borders the upland.

In a perfect world the low marsh is flooded daily by the high tide, and can have salt water on it up to 50% of the time.  Plants that grow there must be able to handle this frequent flooding and the very saline conditions.  The highly salt-tolerant Spartina alterniflora, or salt marsh cordgrass, grows in this zone.  The high marsh, on the other end, is meant to be only flooded by the higher high tides and typically only has salt water on it 10-15% of the time.  As a result, this part of the marsh is subject to much less salinity and tends to be dominated by plants with less salt tolerance like salt marsh hay (Spartina patens) and saltgrass (Distichlis spicata).

As most people are aware, climate change is resulting in rising sea levels. With more water in the ocean, the high tides are getting higher, and the low tides are also getting higher.  With higher tides on all fronts the marsh is getting flooded more deeply into the marsh than was previously typical, and the water is sitting on the marsh for longer periods of time.  While all salt marsh vegetation can withstand some degree of salinity, there is a limit to the tolerance.  With enough increased inundation and salinity, the marsh will ultimately either drown or attempt to retreat inland.

At Ocean View Farm we are already seeing places where low marsh plant species are creeping into the high marsh.  This tells us that sea level rise is already having an impact on this site.  The high marsh is getting more salt water and for longer periods of time than in the past.  Meanwhile, the high marsh plant species, which are less salt tolerant, are forced to move inland to survive.  At Ocean View Farm that move, or retreat, is at least theoretically possible because the upland area adjacent to the salt marsh is not developed, rather it consists of a thin band of shrubs that give way to a hayfield.  Were that area a parking lot or a sea wall, retreat would not be possible.

Through our partnership with Mass Audubon and the help of an EPA grant and DNRT’s amazing volunteers, we spent more than 300 hours this spring clearing more than 2 acres of invasive plants out of the shrub zone adjacent to the marsh.  By removing invasive plants, we are helping to clear the way for the high marsh vegetation to retreat, or move into the upland.  The upland, in turn, will eventually transition to be high salt marsh.

In addition to easing the way for marsh migration through the shrub layer, we are preparing the adjacent hayfield at Ocean View Farm for the transition to marshland as well.  We will be transitioning the plants from non-native, cool season hay species to native, warm season grasses because they are more salt tolerant.

In 2022 we will be seeding approximately 3.5 acres of the southernmost portion of the field with warm season grasses and salt-tolerant flowers.  This will allow the field to remain vegetated longer as the higher tides eventually drive salt water into the field.  The hope is that the salt marsh grasses can move in while the warm season upland grasses are transitioning out, thereby avoiding large non-vegetated areas that will would be subject to erosion.

Another issue we are seeing on the ground at Ocean View Farm is areas of marsh vegetation dieback.  These dieback areas are locations where water from higher tides is getting trapped on the marsh, or impounded, and not able to drain when the tide recedes.  As a result, there is standing water on the marsh that eventually drowns the vegetation in that area.  Because of the higher tides, dieback areas are spreading.

 

DNRT is partnering with the Buzzards Bay Coalition and Bristol Country Mosquito Control to investigate a relatively new restoration technique to combat marsh dieback.  The technique, known as runneling, has shown promise in Rhode Island and other parts of New England.  The process is relatively simple, but requires careful analysis of the marsh.

Runnels, which are essentially very shallow channels (1 foot or less), are dug either by hand or by a special low ground pressure excavator to drain areas of impounded water so that they can dry out.  Theoretically the areas can then re-vegetate if not constantly submerged.  Guidance for this project comes from Save the Bay in Rhode Island, who has deployed the technique with good results.  It is our hope that we can reverse or at least slow the dieback in many areas so that the marsh may remain intact long enough to allow a full-scale retreat into the upland.  The news about climate change can be very daunting and unsettling, but I’m hopeful that we can report back some positive results from our efforts in the years to come.  Stay tuned!

All of this work at Ocean View Farm has been made possible by a Watershed Grant from the Southeast New England Program (SNEP), which is funded by the U.S. Environmental Protection Agency (EPA).