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Why Carex kobomugi should be removed from New Jersey's Coastal Dunes

Coastal dunes form the first line of protection for the communities behind them (e.g. uplands and wetlands such as interdunal swales and bayside tidal marshes), by reducing the energy of storm waves.  Dunes play a vital role in protecting coastal areas from erosion, coastal flooding and storm damage, as well as sheltering properties and ecosystems behind them from wind and sea spray and protecting the tidal wetlands on the bayside of barrier islands. They also provide an important sand reservoir for the beaches themselves (Bureau of Land and Water Quality, ME 2005). Dune vegetation traps wind-blown sand, preventing it from being blown inland where it can form a problem for homeowners and coastal infrastructure (roads, drainage etc.), while helping build the dunes themselves. Dunes also provide habitat for specially adapted plants and animals, several species of which are currently listed as nationally or locally threatened or endangered (e.g. seabeach amaranth (Amaranthus pumilus)  and piping plover (Charadrius melodus)) and act as a filter for rain and groundwater.  In a recent economic analysis of different ecosystems (Costanza et al. 2006), dunes and coastal beaches were found to be, by far, the most valuable ecosystem in New Jersey on a per-acre basis.

In 2000, New Jersey had the highest population density among the 50 states (1,134.4 persons per sq mi.; City-data.com 2008). The desirability of scenic ocean views, combined with the high human population densities on the Atlantic Coast, mean that coastal dunes and their associated maritime forests have been particularly strongly impacted by development. Only 31.2 of the 130-miles of shoreline between Sandy Hook and Cape May Point have not been developed by humans (Richard Stockton College of New Jersey  ND).  Since many of the plant species found in these habitats are found nowhere else, contraction of these habitats means that those species are becoming increasingly rare. Thus, the additional threat posed by exotic and invasive species in these habitats is a matter of particular concern.

One plant species that is invasive in coastal dunes in the United States is Carex kobomugi,  a perennial sedge in the Cyperaceae family that is native to Japan, Korea, China and parts of eastern coastal Russia. The first documented population of this plant in the U.S. was at Island Beach State Park, New Jersey, in 1929 (Small 1954).  It is not known how the plant arrived in North America. Initially it was suggested that the N.J. population derived from seeds or propagules from plants that were used as packing material for china or other fragile objects being imported to the U.S. from Asia, that might have washed up on shore from shipwrecks off the coast of New Jersey (Small 1954).  However, there is no evidence that this plant was ever used in this way, nor is it particularly suitable for such a use. A more likely pathway for the introduction is transport of seeds in sand used as solid ballast in ships sailing empty from Asia in order to collect a cargo from New York.  However it got here, since its introduction C. kobomugi has expanded its range from North Carolina to Massachusetts (Merhoff et al. 2003). Part of the reason for this rapid expansion is that this species was propagated at the Cape May Plant Materials Center (PMC) and distributed to multiple sites along the Atlantic Coast under the name ‘Sea Isle’ (plant # PI-433953) for use in coastal plantings, starting in 1970. By 1977, production of C. kobomugi  at the PMC was up to 20,000 plants per year (Shisler et al. 1987).

Increased awareness of the ecological and economic problems caused by invasive species meant that C. kobomugi  sales ceased in the mid-1980s. However, many areas of the eastern United States that were not deliberately planted have since been invaded by C. kobomugi  via natural propagation. For example, the area of dunes occupied by C. kobomugi  at Island Beach State Park, NJ, doubled between 2002 and 2008 through natural propagation alone (Wootton 2003 and unpublished data). Carex kobomugi populations at the Sandy Hook Unit of Gateway National Recreational Area tripled in area over the same period (Wootton 2003 and unpublished data). Expansion of C. kobomugi  in both areas has been roughly exponential (Figure 1).

Figure 1. Increase in area invaded by Carex kobomugi  at Island Beach State Park and Sandy Hook, New Jersey over time (Modified from Wootton 2003 using unpublished 2008 survey data)

Stem densities of many native plants, including American beachgrass (Ammophila breviligulata), wormwood (Artemisia  spp.), saltmeadow cordgrass (Spartina patens), as well as species richness were significantly lower within areas invaded by C. kobomugi  than in the surrounding (uninvaded) dune areas (Wootton et al. 2005, Burkitt 2007).  In addition, there was a strong negative correlation between stem densities of C. kobomugi  and those of native species within the invaded areas, again confirming the negative effects of this species on native plants. Invasion by C. kobomugi  also negatively impacts seaside goldenrod (Solidago sempervirens ), the nectar of which is an important food resource for migrating monarch butterflies, Danaus plexippus  (Walton et al. 2005). As a result of its rapid expansion and its negative impacts on native plant species, C. kobomugi has moved from being considered an endangered species in the 1970s (Fairbrothers and  Hough 1973) to being one of the 10 most unwanted plant species in New Jersey today (Bennett-Chase 2001). 

In addition to the relatively common plant species described above, the dune community of the New Jersey coast is habitat for a number of rare, threatened or endangered species including, but not limited to, piping plover (Charadrius melodus ), least tern (Sterna antillarum ), black skimmer (Rynchops niger ), northeastern beach tiger beetle (Cicindela d. dorsalis ), seabeach amaranth (Amaranthus pumilus ), seaside knotweed (Polygonum glaucum ), sea sandwort (Honckenya peploides ), slender seapurslane (Sesuvium maritimum)  and seabeach evening primrose (Oenothera humifusa ) (NJDEP 2008a-e, NJDEP ND). Expansion of Carex kobomugi on the coastal dunes of New Jersey threatens plants like sea sandwort, slender seapurslane and seabeach evening primrose directly through competition. However, for T+E species, the greater impact of the Carex kobomugi comes through habitat elimination. Piping plover (federally threatened), least tern and black skimmer (state endangered) all nest in open sandy areas between the high tide mark and the vegetated portions of the dunes (Jenkins 2003, Liguori 2003, Liguori and Jenkins 2003, NJDEP 2008 a,b,e). These birds and other such as oystercatchers will not nest in highly vegetated areas. For instance, black skimmers are known to relocate colonies when dense vegetation takes over a previous nesting area (Liguori 2003). The northeastern beach tiger beetle (Federally threatened, State endangered) is also typically found on wide, relatively undisturbed expanses of sandy beach (Bowers-Altman 2003, NJDEP 2008d). It was reintroduced to the Sandy Hook Unit of Gateway National Recreation Area in 1997 (NJDEP 2008d). Seabeach amaranth, a federally endangered plant, is found in the sparsely vegetated area of sandy beach at the toe of slope of a dune, outside of the high tide line (NJDEP ND). All of these species are already threatened by the reduction of available open sandy habitat for nesting as a result of coastal development. In addition, the increased sea levels that are expected to occur as a result of global warming in upcoming years will further reduce the amount of high beach habitat that’s available for these species, regardless of the community composition of the vegetation on the dunes. However, the C. kobomugi  invasion may further exacerbate the problems experienced by these species, since the sedge appears to grow further down into the previously unvegetated regions of the high beach shore than does the native Ammophila breviligulata, thereby reducing yet further the area of open sandy habitat between the high tide line and the vegetated high shore (Lea and McLaughlin 2006). In addition, plover chicks are known to use open, sandy corridors between vegetated dunes as pathways to more interior habitat suitable for foraging (Loegering and Fraser 1995). If such pathways are lost to encroachment by C. kobomugi,  plovers and other shorebirds may exhibit reduced survival.

In recent years the importance of coastal dunes in protecting the mosaic of shrubland, grassland, and interdunal swale wetland as well as human coastal communities behind them from the effects of projected climate change has been increasingly recognized  (Nordstrom 2000, Spense et al. 2007). Cooper et al. (2005) suggest a relative sea level rise for the New Jersey coast between 1990 and 2100 of between 0.31 and 1.10 m, with a median expected value for sea level rise in this region of about 0.71m. If sealevel rise occurs within the median range of this estimate, the result would be that 100-year storms (storms creating the kind of flooding that used to happen only about one a century) will happen 3 or 4 times more often in the future and the resulting storm surges would inundate vast areas along the New Jersey coastline (Cooper et al. 2005, Lathrop and Love 2007). The presence of dunes has been shown to significantly reduce coastal flooding, even during extreme storm events (Houser et al. 2008). Thus, careful management of healthy dunes can reduce the need for expensive and time-consuming rehabilitation in the future. 

Figure 2.  Typical dune profiles for dunes stabilized by native plant communities (dominated by American Beach Grass, Ammophila breviligulata ) versus dunes stabilized by the invasive Carex kobomugi  (Data from paired dune locations in the southern portion of North Beach, Sandy Hook NJ. Wootton unpublished data)

Dune vegetation plays an important role in maintaining healthy dunes. The plants growing there trap blowing sand, thus building the dune. Their presence reduces future erosion by creating friction through their leaves which slows air movement and consequently reduces erosion. They also stabilize sand within the dune by holding it in place with their roots and rhizomes.  Because C. kobomugi  is shorter in stature than native dune plants like A. breviligulata,  it scavenges less sand at the front of the dune (Shisler et al.1987, Pronio 1989). As a result, C. kobomugi  supports the formation of lower, wider dunes than does the native Ammophila breviligulata  (Figure 2). The diminished height of C. kobomugi -  stabilized dunes would clearly mean that they are less able to protect the communities behind them from the rising sea levels and increased frequency of storm surges associated with global warming that were discussed earlier. Carex kobomugi - stabilized dunes are also believed to be more vulnerable to wind blowouts and storm erosion than are those stabilized by A. breviligulata  (Lea and McLaughlin 2006). In addition, while growth of native dune plants is actually stimulated by sand burial, C. kobomugi  is less tolerant of sand burial. Thus, when over-wash does occur, the plant is likely to be less able to grow back rapidly (Shisler et al. 1987), resulting in a dune that is less resilient to, and slower to recover from, storm events. The proposed removal of this species, combined with a restoration of the native plant community, will restore the native dune structure and thus return the normal geomorphology and function of the dunes. This in turn will allow the dunes to better protect the communities behind them from the negative effects of expected changes in sea level and storm frequency associated with climate change.

Ecosystem resilience is defined as the ability of that system to maintain its function when faced with novel disturbance (Webb 2007). Biological diversity appears to be an important factor in determining the resilience of ecosystems and thus in their ability to sustain themselves in the face of environmental challenges. The relationship between diversity and resiliency appears to derive from the apparent redundancy of species within healthy ecosystems (Peterson et al. 1998, Elmqvist et al. 2003). The presence of species with different, but overlapping, functions within an ecosystem, as well as of species operating at different scales, results in a variety of tolerances and responses to new conditions.  The result is that, when diversity levels are high, at least some of the species are likely to be able to thrive and thus to maintain ecosystem services under whatever set of conditions prevail in that area at any point in time. Thus, a more biologically diverse ecosystem generally displays more resilience, or a greater ability to overcome a natural disaster or human-caused destruction (Naeem 1998). As previously discussed, invasion by Carex kobomugi  decreases both the abundance and the number of native species (species richness and diversity) that characterize healthy dune ecosystems.  As the invasion matures, the plant forms more or less a monoculture on the dunes (Wootton 2003, Wootton et al. 2005, Burkitt 2007).  Thus, invaded dunes are likely to be less resilient to future environmental changes or other challenges.

 While populations of Carex kobomugi  have been documented in New Jersey from Sandy Hook to Cape May, the dunes within Island Beach State Park (IBSP) and the Sandy Hook Unit of the Gateway Recreational Area (SHU) are suffering from particularly high levels of infestation by this species.  Thus these two areas have been targeted as the focus for this project.  Mapping of all areas affected by invasive Carex  species (C. kobomugi  and C. macrocephala ) within the two parks was completed in summer and fall of 2008. Carex kobomugi   was found to be widely distributed in both parks (Unpublished data, results of which are summarized in Supplemental Figure 2 in the Associated Materials submitted with this proposal). In 2008 there were 39.9 acres (161,534 m ˛) of dune infested by C. kobomugi  at IBSP. That is almost exactly double the amount that was present in 2002 (88,042 m ˛; Wootton 2003). Similarly, there were 54 acres (218,713 m ˛) of C. kobomugi  at SHU, which is more than triple the area that was infested in that park in 2002 (66,053 m ˛; Wootton 2003).  The areas occupied by C. kobomugi  in 2008 represent approximately 2.5% of the land area of SHU and 1.5 % of the total area of IBSP.  In most of these areas, the plant is growing very densely (typically > 150 stems m -˛  and up to 675 stems m-˛ in some areas Wootton et al. 2005). Only one small (<100 m ˛) population of C. macrocephala  is known within the parks, located in the back dune swale near the Fisherman’s Trail at SHU (Wootton 2007).  This population shows also much less dense growth within the affected area (typically < 20 stems m-˛). 

The ever-increasing rate of spread of Asiatic sand sedge and the potential for hybridization between these two sister species of sedge (Wootton 2007) makes clear that intervention is urgently needed.   We thus propose that this species should be removed from the dunes in New Jersey as soon as possible.

 

Author: Louise Wootton. Ph.D.  Last updated June 17, 2009

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