Authors: John Gerlach, ed. Barry Rice, Global Invasive Species Team, The Nature Conservancy
Latin Names: Glyceria declinata Brébiss., Glyceria fluitans subsp. declinata (Brébiss.) O. Bolòs, Masalles, & Vigo, G. fluitans var. declinata (Brébiss.) Ghisa, G. notata subsp. declinata (Brébiss.) E.J. Weeda, G. plicata subsp. declinata (Brébiss.) E.J. Weeda, Glyceria plicata var. declinata (Brébiss.) Druce
Common Names: Eurasian waxy mannagrass, small mannagrass, sweet grass
The genus name Glyceria is derived from the Greek name for sweet, for the taste of the grain which is relished by waterfowl and which has been harvested by humans for millennia.
Glyceria declinata has invaded deep vernal pools, swales, ditches, and stock ponds along the entire eastern side of California's Central Valley, from Shasta County south to Fresno County. It is also present in vernal pools in Mendocino County. It is adapted to long periods of inundation and typically grows from the Lolium multiflorum ring that lines the high water mark, to the bottom of the pools where Eleocharis, Eryngium, Psilocarphus, and Downingia are found.
The invaded vernal pools are some of the most important natural resources in California and contain both federal and state listed endangered plant and animal species. Dense G. declinata invasions appear to eliminate or significantly reduce populations of all native annual plant species, such as endangered Orcuttia viscida (Sacramento orcutt grass) from the vernal pools. It was rated a weed of "Moderate" impact by the California Invasive Plant Council (Clayton et al. 2006).
Its population dynamics in vernal pools are erratic but it often establishes dense stands of large plants which both shade-out endemic species and eliminate the bare ground they require for germination and establishment. It produces an enormous amount of fine root mass on or just under the surface of the soil and a large leaf mass. These biomass changes probably effect nutrient cycling in the vernal pools and negatively impact vernal pool hydrology through increased transpiration.
While it has been collected from rice fields in Butte County for 20 years, it has recently become problematic enough to be considered a winter and spring weed and its dense populations make it more difficult to cultivate the fields for planting.
Perennial, culms 10-45 cm long. Leaf-sheaths tubular for much of their length; ligule an eciliate membrane 4-9 mm long, acute; leaf-blades flat, or conduplicate; 3-18 cm long; 1.5-8 mm wide. Leaf-blade surface smooth. Leaf-blade margins scabrous. Panicle open; linear, or lanceolate; equilateral, or nodding; 4-30 cm long. Spikelets ascending, or appressed; solitary. Glume apices obtuse. Lemma and palea apices dentate. (Summarized from (Clayton et al. 2006.)
While G. declinata is described as a perennial species, all observations from the Central Valley of California indicate that it is either an annual genotype or growing as a facultative annual. In the Central Valley the spikelets mature from late April through May and shatter at maturity coating the ground below the plant with seed.
To the casual eye, G. declinata can easily be mistaken for the non-native Lolium multiflorum.
Because the Jepson Manual (Hickman 1993) omits G. declinata and incorrectly describes the lemma tip of G. occidentalis as jagged or irregular, there has been some confusion regarding whether G. declinata from the Central Valley is in reality G. occidentalis. Glyceria declinata is correctly described in Munz & Keck (1968). Another complication with correctly identifying Glyceria species is that the morphological characteristics used to identify them are very similar and variable. As part of the development of the Poaceae section of The Flora of North America Barkworth (in prep), Ian Wipple, Mary Barkworth, and Shaun Bushman developed chloroplast molecular markers to help distinguish species among herbarium collections of several Glyceria species (Whipple et al 2006). They tested herbarium samples from Humboldt County, the Sierra Nevada foothills, and a degraded vernal pool in Sacramento County and all samples were determined to be G. declinata. Using their markers, Harald Meimberg and John Gerlach recently completed an analysis of six vernal pool populations from Sacramento County and Stanislaus County and all populations were determined to be G. declinata (unpublished). From these two studies and from observations of lemma characteristics of field samples from other counties, it appears that the only Glyceria species present in the Central Valley is G. declinata.
Glyceria declinata was first identified in California in 1953 by Beecher Crampton from a plant he collected from a vernal pool in Stanislaus County (DAV UCD36270) and, according to vernal pool botanists (Aheart et al. 2006), during the last twenty years it has rapidly spread through rice fields, vernal pools, cattle ponds, and roadside ditches.
In 2001 during a vernal pool vegetation classification project, a survey crew found it in a large vernal pool on the Lane Ranch in San Joaquin County but identified it as G. occidentalis (Barbour, et al. 2005). In 2001, the cover of Glyceria in the pool was 2%. When John Gerlach surveyed the pool in May 2006 the plants had lemma morphology consistent with that of G. declinata and the cover was greater than 90% in spite of being recently grazed. The extensive and complete cover and the lack of flowering native species suggests that this is not just an annual fluctuation in population size.
Waterfowl are strongly attracted to maturing plants and strip the seed from the culms with their bills and they probably function as primary long distance seed dispersal vectors (Gerlach, pers. observation). Seed dispersal by waterfowl has probably increased dramatically as nonmigratory populations of Canada geese (Branta canadensis) have exploded in California during the last 20 years. Humans and wildlife disperse the seed over shorter distances as seed readily adheres to wet pants and boots.
Europe: northern, central, southwestern, southeastern, and eastern. Africa: Macaronesia. Glyceria declinata is widespread in Europe and in areas with temperate climate it is considered a colonizer of mud flats and banks along slow moving rivers and streams and along the shores of lakes and ponds. It generally grows in the less inundated areas between the more inundation-tolerant G. fluitans and upland vegetation.
In the Mediterranean climate areas of Spain and Portugal G. declinata is often a dominant native species of vernal pools (Molina 1996) and playas and a weed of rice fields in Spain and Portugal where it appears to behave as an annual species.
As an invasive in California, it is known to occur in Shasta, Tehama, Butte, Yuba, Placer, Sacramento, San Joaquin, Stanislaus, Fresno, Mendocino Sonoma, Alameda, and counties. In the USA it is also recorded from Louisiana and New York (Plants 2006).
Control methods in vernal pools with sensitive status species will be a complex regulatory problem and field eradication efforts will require persistence. All methods should target existing seed and adult plants and reduce the spread of seed by animal and human vectors. Hand weeding prior to spikelet shattering is effective for small populations in high value vernal pools and must be continued annually until the seed bank is completely exhausted. Surrounding vernal pools and swales should also be weeded to prevent the rapid reintroduction of seed. Reducing seed spread can be accomplished by excluding humans and domestic animals from G. declinata populations from the beginning of spikelet shattering until all seed has been dropped and the vernal pools are completely dry. It is critical that all equipment used in both natural and created vernal pools is absolutely free of adhering seed or soil. Additionally, artificial ponds that attract waterfowl should be eliminated to reduce site desirability. Stock ponds may need to be replaced with other water sources that do not attract waterfowl or support G. declinata. Glyceria species are generally regarded as species with high soil and water fertility requirements so precautions should be taken to prevent the addition of nutrients in any form.
Aheart, L., Dittes, J., and Witham, C. 2006. pers. communication.
Anonymous 2006. broken link
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