Managers should focus on three major aspects of local marsh conditions to
understand the role of and response of plants to salt marsh restoration. Here in this
report I will discuss these for the Neponset river marsh restoration project. First,
we must know what the current vegetation community composition is and how that
compares to the targeted ‘restored’ vegetation community composition. For
example, we need to know what proportion of the marsh is Phragmites compared to
Spartina (i.e. biomass or percent cover), and what is the restoration’s targeted
percent cover of these species over time. Second, we must evaluate some aspect of
the hydroperiod (i.e. elevation, tidal height, inundation time) of the Neponset marsh.
Finally, we must determine the current salinity and project how porewater salinity
will change with restoration efforts.
To address these aspects of Neponset marsh restoration, I will answer the following
questions: (1) What do we know about the Neponset marsh restoration site that will
influence the role of and responses of plants in the restoration process? (2) How can
we expect the Neponset marsh to respond to restoration? and (3) What is the role of
plants in Neponset marsh restoration?
What important aspects of the Neponset marsh restoration site will contribute
to the role of and responses of plants in the restoration process?
What is known about the present vegetation community at the Neponset marsh
restoration site?
The major aspect of marsh vegetation of interest in most New England marsh
restorations is the extent of Phragmites and how it compares to the cover of
Spartina. For the Neponset site we have three major sources of information on this:
historical imaging from Google Earth, personal observations from visits to nearby
marshes, and a floral survey done by Tom Palmer for the Milton Garden Club in
1997 (Tom Palmer pers. comm. 2014). Here, Palmer surveyed the Neponset
marshes and qualitatively described the patterns in cover and diversity of the site.
Palmer noted that, by 1997, Phragmites had expanded in total coverage of the marsh
and occupies anywhere from 75-85% of the total marsh surface. Specifically,
Spartina alterniflora had been limited to the edges of creek banks and only tall form
Spartina was able to persist, as Phragmites held ground just at the edge of the tall
Spartina zone. Palmer reported intact but small bands of Spartina which are visible
from the other side of the Neponset River (MH personal observation), the presence
of which should be important for recolonization of Spartina following restoration
efforts.
Palmers report combines with satellite imagery from Google Earth (ground truthed
by our actual visit to this marsh) to show the extent of Phragmites in nearby
marshes. This imagery (Figure 1) shows darker Phragmites patches as very
extensive and somewhat permanent fixtures over the last 15 years. Palmer
reported that, in the across the river Dorchester marshes, despite creek digging and
Spartina planting, Phragmites has either returned to the marsh or has been
restricted to large patches throughout the marsh (see picture). Lastly, personal
observations and Palmer’s report indicates that much native marsh plant diversity
is present in the system but it may be restricted to small zones throughout the area.
What aspects of the hydroperiod are known for the Neponset marsh?
We can use NOAA bouy data to estimate the tidal height for the Neponset marsh in
the absence of on the ground tidal topographic maps. NOAA data indicates that the
normal maximum tidal height of the Neponset in this area is ~9 -11 feet. This tidal
elevation is similar to many other restoration marshes throughout New England
that lie near openings to a larger oceanic body of water (Warren et al. 2002, Smith et
al. 2009). Local inundation time should be calculated in the marsh to determine if
the tidal elevation varies throughout the marsh, as local variation in elevation could
explain the patchy stands of Phragmites or the narrow band of Spartina in the
Neponset marsh.
What is the salinity of the Neponset river at this site?
There is no porewater salinity data for the Neponset marsh but a paper from
Gardner et al. (2005) measured salinity at different locations throughout the
Neponset. Salinities, which vary annually depending on precipitation and time of
year, reached around 30-32 ppt in August 2002 in the section of the Neponset
nearest to our restoration site. Using information from other studies that indicate a
ppt of 25 is what begins to kill Phragmites (Warren et al. 2002), we can assume that
the porewater salinity of the Neponset marsh is below this. However, because
channel salinities are in the 30-32 ppt range, porewater salinities will likely increase
with the expansion of marsh creeks from restoration.
How can we expect plants in the Neponset marsh to respond to restoration
efforts?
Salinity response and Phragmites die back
There are a number of restoration projects that have similar tidal heights and
salinity levels to that of the Neponset system (Burdick and Dionne 1996, Warren et
al. 2002, Buchsbaum et al. 2006, Smith et al. 2009). As mentioned, since salinities in
the Neponset near our site have been found to be around 30 ppt (Gardner et al.
2005), restoration of creek flow into the marsh should increase marsh porewater
salinity. The timing of this salinity restoration depends on tidal volume and
precipitation. In similar systems, salinity levels returned to that of non-restored
reference sites in 1-5 years (Warren et al. 2002). In that study, shallower sediments
recovered their porewater salinities faster than deeper sediments but once
porewater salinities reached 25 ppt, Phragmites will start to die back.
Smith et al. (2009) found that the die back of Phragmites varied not only with the
initial salinity but also the ability for higher salinities to be maintained. In this
study, precipitation varied and thus slowed salinity recovery to 3-5 years. In both
studies however, Phragmites cover decreased and Spartina cover increased,
responses we can expect in the Neponset marsh site in relation to increases in
porewater salinity. We can expect this in Neponset because of the already oceanic
levels of salinity (i.e. no tidal flow restriction) and the remnant population of
Spartina which can serve as a source for eventual expansion.
Changes in plant community distribution throughout the marsh
Because of the presence of the thin band of Spartina near creek banks at our site, it
is likely that this will provide the nucleus of recovery. Like in Smith et al. (2009)
(Figure 2), as restoration progresses and porewater salinities increase, Phragmites
will likely move back into the higher marsh. Over time, we would expect the
remnant Spartina to recolonize the area abandoned by Phragmites. Like in the
Dorchester Marsh across the river from our site, we should expect patches of
Phragmites to be a relatively permanent fixture post-restoration due to elevation
heterogeneity and persistence of Phragmites.
Nutrients, development, and Neponset restoration
Restoring a marsh from Phragmites dominated to Spartina dominated in the
Neponset will also require some caution regarding nitrogen loading in the system.
Research in New England has shown a high correlation with the amount of
developed coast, nitrogen availability, and the presence of Phragmites (Bertness et
al. 2002, Silliman and Bertness 2004). Returning salinity levels to 26ppt in the
marsh may not be enough to reduce Phragmites stands because our marsh site is
likely heavily impacted. Besides the large interstate that goes through our system,
the whole Neponset River is surrounded by development. Specific data on this is
lacking but it is possible that just changing salinities may not yield the desired plant
community.
What is the role of plants in Neponset restoration?
First and foremost, the role of plants in the overall restoration process is to use the
plant community as a gauge for the success of the restoration. The ultimate goal of
this and most restorations is to return the plant community to a Spartina dominated
marsh with a diveristy of high marsh species (e.g. Spartina patens, Iva frutescens,
Juncus gerardii). In this light, an important role of plants in the Neponset
restoration is to provide a source population for clonal or sexual recovery and to
facilitate the colonization of other marsh area. Recovery of marshes to other
stressors has been shown to be dependent on remnant patch size and positive
interactions (Angelini and Silliman 2012).
Another role of plants in the Neponset restoration is to increase species richness
and ecosystem functioning. Once the restoration takes place and plant communities
begin to shift from Phragmites to Spartina, we should see the marsh food web also
recover(Roman et al. 2002, Buchsbaum et al. 2006). Healthy Spartina marsh
supports a number of resident and transient marsh species that Phragmites marshes
do not (Roman et al. 2002). Because the Neponset marsh has been dominated by
Phragmites for so long, the food web may have to be replenished from outside
source populations. A number of intact salt marshes are located in the vicinity of
the Neponset marsh and could provide larvae for recolonization. Additionally, the
Spartina food web that is present in the remnant band along the creek banks may
provide a source for the rest of the marsh. Observations at the Dorchester marsh
across from our site indicated low populations of fiddler crabs and Littorina snails,
which would be expected to colonize new Spartina as it expands. Basically, as
Spartina expands, the marsh food web and marsh ecosystem functions such as
increased sedimentation and carbon sequestration will recover as well.
Figure 1. Google earth images of Neponset marsh restoration site and surrounding
areas from 2003 (top) and 2013 (bottom). Dark or brown patches are patches of
Phragmites confirmed by ground truthing
Figure 2 (adapted from Smith et al 2009). In certain cases, Phragmites total cover
may not change on the whole marsh scale but it can be pushed back from the creek
bank, as in the transects shown in (a). The Neponset river site will likely undergo a
similar shift in plant community because of the thin band of Spartina currently on
the creek banks, providing the nucleus for recovery
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