Invasive Spartina in Humboldt County

Summary of Humboldt County Spartina Summit 2008

Introduction
Invasive Spartina around the World
Worldwide Invasive Spartina Impacts
Large-scale Control Efforts on the West Coast U.S.
The Humboldt County Spartina Invasion
Humboldt Bay’s Salt Marshes
Invasive Spartina Impacts in Humboldt County
Experimental Spartina Control Efforts in Humboldt Bay
News and Events
Want to Help?
Contact Us
References
Links

Introduction
Invasive dense-flowered cordgrass (Spartina densiflora) has invaded an estimated 90% of salt marshes in Humboldt Bay, and is abundant (although unmapped) in the Eel River and Mad River estuaries. Cordgrass is most abundant at mid-marsh elevations, where it has displaced native pickleweed throughout Humboldt Bay.  It has recently been found to be increasing in the high marsh, where it threatens a diverse native plant community that includes the salt marsh plants Humboldt Bay Owl’s Clover  and Point Reyes Bird’s Beak, both ranked as rare, threatened, or endangered by the California Native Plant Society. Spartina densiflora has also recently been observed to be colonizing bare mudflats in some places around Humboldt Bay. 

Invasive Spartina around the World
Dense-flowered cordgrass is one of 17 species of cordgrass, several of which are highly aggressive invaders that significantly alter both the physical structure and biological composition of our tidal marshes, mudflats and creeks. Over the last two hundred years, non-indigenous cordgrasses (Spartina spp.) have invaded marshes around the world, including marshes from California to British Columbia, Australia, New Zealand, China, Morocco, and Europe (UK, the Netherlands, France, Denmark, Germany, and Spain).  In many cases, these invasions have dramatically changed tidal marsh characteristics. 

Invasive Spartina Impacts Worldwide

• Hybridizes with native cordgrasses where present, such as in San Francisco Bay and in England. This can result in “genetic swamping”, where the genes of the native species are overwhelmed.
•  Outcompetes indigenous plant species, including rare and endangered plant species, reducing marsh biodiversity and ecological functions. In China, invasive Spartina marshes threaten to displace coastal mangrove forests.
• Converts mudflats to vegetated marsh, eliminating important foraging habitat for shorebirds, including endangered species. (Applies to the primary species of Spartina invading SF Bay and Willapa Bay, Washington. The species of Spartina in Humboldt Bay does not colonize mudflats.)
• Produces large amounts of persistent detritus. Large rafts of Spartina detritus (wrack) are deposited on the upper marsh, producing bare areas (often in native marsh) that are then colonized by Spartina.
• Changes habitat for marsh animals (vertebrates such as birds and rodents, and invertebrates such as crustaceans and gastropods) by increasing stem and root density.
• Threatens estuarine restoration projects by increasing the difficulty of establishing indigenous saltmarsh vegetation.

Large Scale Control Efforts on the West Coast U.S.
Large-scale efforts to control invasive Spartina are underway in a number of areas on the west coast, including San Francisco Bay and Willapa Bay in Washington. These efforts have employed a mixture of mechanical and chemical control methods, including mowing, covering, and spraying with herbicides.  Biological control using a planthopper (Prokeleisa marginata) has been used in Washington on Spartina alterniflora.

The Humboldt County Spartina Invasion
Dense-flowered cordgrass, the invasive species of Spartina currently found in Humboldt County, is native to South America. It probably arrived in Humboldt Bay around 1850 with ballast on ships carrying lumber toChile, and was not recognized as a non-indigenous species until the 1980s (Spicher and Josselyn 1985). There are few data on the spread of the species in the region over time. Cordgrass was not mapped in Humboldt Bay until 1998. Monitoring at one site in the Mad River Slough from 1989 to 1997 has tracked the species’ spread into the high marsh. Aerial photos from the 1970s and 1980s indicate that cordgrass did not yet dominate the Jacoby Creek marsh or marshes on Indian Island. Ground photos document the continued spread of the species in many sites around Humboldt Bay.

The only comprehensive inventory of Spartina densiflora in Humboldt Bay was conducted in 1998, by mapping stands of Spartina in two abundance classes over aerial photos in the field. This detailed mappingproject (Pickart 2001) revealed that a decade ago, Spartina was present in a total of 812 acres, or 94% of Humboldt Bay’s salt marsh. It occurred at cover values of over 70% of the marsh in 479 acres, or 55% of our salt marshes. The Mad River Slough at the north end of Humboldt Bay had thegreatest proportion of salt marsh with respect to bay acres, the least severe Spartina invasion (based on both the percent of total salt marsh invaded and the proportion of salt marsh in low vs. high cover classes), and the highest densities of rare salt marsh plants. Although no recent bay-wide inventories have been conducted, Humboldt Bay National Wildlife Refuge conducted detailed on-the-ground mapping of Spartina populations within its holdings on the bay in 2006-2007. Between 1999 and 2007, Spartina-invaded mainland marsh on the Mad River Slough increased from 14 to 20 acres. Although no mapping has been conducted in the Eel River estuary, Spartina is observed to be abundant in salt marshes there as well as at the mouth of the Mad River.

Although often overlooked in Humboldt Bay’s recent history, a Spartina alterniflora invasion was successfully thwarted in 1985, preventing potentially massive impacts to the mudflat habitats of the Bay.  After observing an increase in the size of the stand from 100 to almost 5,000 sq ft over a 3-year period, the California Department of Fish and Game eradicated the stand by diking the area, cutting the grass, applying salt and covering it with black plastic. This timely intervention prevented the massive invasions that have occurred in San Francisco and Willapa Bays. Unfortunately, a similar early detection and response strategy was not possible with the long-established Spartina densiflora. However, its continued spread in Humboldt Bay, and its availability as a source to other west coast estuaries, justifies a timely response to thisinvasion.

Humboldt Bay’s Salt Marshes
Salt marsh occupied approximately 9,000 acres around Humboldt Bay prior to Euro-American contact. Subsequent diking, draining, and conversion have left us with less than 900 acres, or 10% of the historic total (see enlarged map of Changes in the extent of salt marsh adjacent to Humboldt Bay between 1870 and the present). Similar losses to salt marshes have occurred in all estuaries in California. The rarity of this vegetation type amplifies the impacts of an invasion occurring over 90% of its total extent.

The species composition of salt marshes is strongly controlled by the influence of tides. In Humboldt Bay, salt marshes can be divided into three types based on their elevation, with corresponding differences in vegetation (Eicher 1987): low elevation marsh, mid-elevation marsh, and high-elevation marsh, also called mixed marsh. Salt marsh vegetation can only become established at or above the Mean Lower High Watermark (the average of the lower of two daily low tides). Low marsh is overwhelmingly influencedby theduration of salt water inundation. Both salinity and lack of oxygen during submersion limit plant life to only the most stress tolerant of species. In our bay, the low marsh is dominated by pickleweed (Salicornia virginica). At these elevations (<6.9 ft. MLLW), pickleweed forms homogeneous stands known as Pickleweed Marsh photo 7. Spartina densiflora has been found invading both low marsh and even bare mudflats in our Bay. However, it reaches its peak at mid to high elevation marshes (6.9-7.3 ft MLLW), where it dominates to such an extent that the resulting vegetation type has been termed Spartina Marsh. Our highest elevation marshes (>7.3 ft. MLLW) are known as Mixed Marsh and contain the greatest diversity of species (22 species). These include pickleweed, salt grass (Distichlis spicata), jaumea (Jaumea carnosa), marsh rosemary (Limonium californica), arrow-grass (Triglochin concinnum and T. maritima), as well as two rare salt marsh annuals, Humboldt Bay owl’s clover (Castilleja ambigua ssp. humboldtiensis) and Pt. Reyes bird’s beak (Cordylanthus maritimus ssp. palustris). Although Spartina is less vigorous on these high elevation marshes, it was found to be increasing dramatically on one high marsh (from <5% to >40% frequency  from 1989 to 1997, Pickart 2001).

In high marshes, submergence is less frequent, and physical stresses are replaced by interspecific competition as a limiting feature. Thus, Spartina can exert a greater influence. The upper limit of salt marsh is not necessarily fixed by tidal elevation but depends on terrestrial influences. In Humboldt Bay, saltmarshes occur both as islands, where there are no terrestrial influences and salinity is high and limiting, andmainland salt marshes, where freshwater runoff can dilute salinities and form brackish marshes at the upper edge of salt marshes. Brackish marshes around our bay are characterized by a diverse group of species including Lyngbye’s sedge (Carex lyngbyei), hardstem bulrush (Scirpus acutus), salt rush (Juncus lesueurii), sea watch angelica (Angelica lucida), narrow-leaved bur-reed (Sparganium angustifolium) and others. Spartina densiflora appears to flourish in our brackish marshes, as it does in other countries where it has invaded.

Invasive Spartina Impacts in Humboldt County

Outcompetes indigenous plant species 
Cordgrass displaces pickleweed (Salicornia virginica), saltgrass (Distichlis spicata), and seaside arrow grass (Triglochin maritima) in mid-elevation marsh. In the high marsh, it threatens a diverse plant community that includes the rare Humboldt Bay owl’s clover and Point Reyes bird’s beak. It is unknown what the ultimate extent of the cordgrass invasion could be if it is not controlled. In Spain, which has also undergone a major invasion by dense-flowered cordgrass, the species has spread beyond intertidal environments to occupy extensive areas of terrestrial habitats (Bortolus 2006). In Humboldt County, Spartina densiflora can be found not only in salt marshes, but in brackish marshes, along slightly brackish river channels, on sandy substrates in dune wetlands, and colonizing bare mudflats.

Alters wildlife habitat
Cordgrass changes habitat for marsh animals (vertebrates such as birds and rodents, and invertebrates such as crustaceans and gastropods) by increasing stem and root density, plant height, and shading, compared to pickleweed, saltgrass, and seaside arrowgrass. These structural changes may affect colonization of vascular plants by algal species which can serve as a source of food for invertebrates. More research is needed to understand the effects of this change in the Bay ecosystem.

Alters estuarine food web
Impacts of cordgrass on the estuarine foodweb have not been closely studied. However, west coast salt marshes are known to supply large amounts of nitrogen via the algae that grow on the surface of the salt marsh. In spite of the large standing biomass of cordgrass stands, the species could actually lower productivity by shading out the fast-cycling algal species in favor of the persistent detritus formed by cordgrass. Dead tissue from the deciduous, fleshy pickleweed appears to decompose rapidly, and may be more available to other species in the estuaries. Large rafts of persistent cordgrass wrack can be deposited on the marsh. These masses of wrack kill the vegetation beneath them. The wrack typically includes cordgrass seeds, allowing cordgrass to colonize these newly unvegetated areas. The vegetation that regrows in areas affected by wrack deposition typically has greater dominance by Spartina (Kittelson and Boyd 1997).
 
Threatens estuarine restoration projects
Cordgrass colonizes areas opened to tidal influence, making it much more difficult for restoration projects in the region to establish indigenous saltmarsh vegetation. Cordgrass is usually the first species to colonize in bare areas created when new tidal flow kills off existing fresh or brackish vegetation. Many people in the region question the value of tidal marsh restoration projects because of the threat of cordgrass colonization.

Source for cordgrass invasions of other areas
In addition to its impacts locally, cordgrass in the Humboldt Bay region threatens to colonize other west coast estuaries via ocean dispersal of its seeds, as demonstrated by the preliminary results of a drift card study carried out by Portland State University. Drift cards from Humboldt Bay in 2004 and 2005 were found within a month of their release in numerous locations along the Oregon Coast, as well as in southwest Washington.

Experimental Spartina Control Efforts in Humboldt Bay
Spartina control efforts in Humboldt Bay began with an experiment carried out from 2004-2005 at Humboldt Bay National Wildlife Refuge and funded by the State Coastal Conservancy (Pickart 2005). Spartina on a high elevation marsh in the Mad River Slough was mowed repeatedly in square-meter plots using a weed-whip. After two years, all Spartina plants were killed (compared with minor Spartina cover reduction in control plots), and native plants had recolonized in many areas. No Spartina had recruited as seed during this time.

Following this success, a 15-acre mainland salt marsh was treated on the refuge, with combined funding from the Conservancy and the FWS, beginning in Fall 2006. A metal-bladed weedeater was substituted for the weedwhip, and proved to accelerate mortality of Spartina, requiring only a few applications in most areas. However, seedling recruitment on the mainland marsh, where freshwater input is more abundant, proved to be a significantly limiting factor. Seedlings emerging on bare areas of the marsh in Spring 2007 were “flamed” using propane torches. Native plants colonized well over the summer but a second flush of seedlings emerged with early rains in September. These seedlings were treated using weedeaters, but in areas with high native recovery limiting impacts to targeted plants was difficult. This effort pointed to the need for a regional approach to eradication, in which seed production is entirely halted. Restoration efforts at this site have shifted to revegetation, with the goal of identifying the most efficient and successful methods.

News and Events

See a summary of the Humboldt County Spartina Summit 2008. 

Want to help?
Volunteer for a Spartina control workday at Humboldt Bay National Wildlife Refuge.  Workday activities include replanting areas from which Spartina has been removed. 

Email for more information

Concerns or questions? Email us!

References

Bortolus, A. 2006. The austral cordgrass Spartina densiflora Brong.: its taxonomy, biogeography and natural history.

Eicher, A.L. 1987. Salt marsh vascular plant distribution in relation to tidal elevation, Humboldt Bay, California. M.A. Thesis, Humboldt State University, Arcata, California.

Falenski, H.D. 2007. Spartina densiflora, an invasive species in the marshes of Humboldt Bay. M.S. Thesis, Humboldt State University, Arcata, California.

Kittelson, P.M. and M. Boyd. 1997. Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California. Estuaries 20:770-778.

Pickart, A. 2001. the distribution of Spartina densiflora and two rare salt marsh plants in Humboldt Bay 1998-1999. Unpublished report, U.S. Fish and Wildlife Service, Humboldt Bay National Wildlife Refuge, Arcata, California.

Pickart, A. 2005. Control of invasive Spartina densiflora in a high elevation salt marsh, Mad River Slough, Humboldt Bay National Wildlife Refuge. Unpublished report, U.S. Fish and Wildlife Service, Arcata, California.

Spicher, D. and M. Josselyn. 1985. Spartina (Gramineae) in Northern California: distribution and taxonomic notes. Madroño 32:158-167.

Links
Oregon’s Spartina Monitoring and Response Plan
San Francisco Invasive Spartina Project
Willapa Bay Spartina Control Program
California Invasive Plant Council