West Indian Marsh Grass

Hymenachne amplexicaulis, also known as West Indian marsh grass in the US, Olive hymenachne in Australia, and hereafter referred to as hymenachne, is New World species^[1] in the genus Hymenachne. Hymenachne is from the Greek hymen meaning "membrane" and achne meaning "chaff, glume, scale" and amplexicaulis is from Latin, literally "embracing the stem" or "stem-clasping.^[2][3] This freshwater species is native to the tropical and subtropical regions of the West Indies, Northern South America, and Central America.^[4]

Biology

Hymenachne is a perennial grass that primarily reproduces via stolons.^[4] This robust species is commonly found growing 1-2.5m tall.^[4] The stems are hairless and float prostrate across the top of the water, then grow upright from these bases to support leaves.^[4] The flat, triangular leaves (10–45 cm long, up to 3 cm wide) have a prominent clasping bases, or auricles.^[4] The auricles help to distinguish it from other invasive wetland grasses and the native H. acutigluma in Australia.^[5] Hymenachne has been shown to increase the stem length between nodes to keep leaves above water during periods of flooding.^[6] Its stem is packed with airy tissue called aerenchyma to help the stems float and to aid in flood tolerability.^[5]

Hymenachne flowers in the early fall and seeds in late fall.^[7] The flower is a panicle^[8] that can grow to half a meter.^[5] Researchers at the University of Florida have seen multiple panicles per plant originating from branching at a single node.^[9] Hymenachne can produce thousands of seeds with up to 98% germination right off the panicle.^[10][11][12]

Habitat

Hymenachne prefers constantly inundated soil and is known to survive comfortably in about a meter of water for 20 years.^[5][13] This species was shown to survive poorly in Australia where flooding exceeded 1.2 m.^[14][12] In its native range in Venezuela, and in invaded areas of Florida, hymenachne was documented to survive in>1.2m of water for nearly a year.^[15][16] Hymenachne has also been found in water greater than 4m deep persisting in floating mats, likely dislodged after a storm event. Conversely, hymenachne is not recorded to survive sites that entirely desiccate.^[17]

Hymenachne is known to colonize and form dense monocultures in floodplains, roadside ditches, and lake margins in invaded and native areas.^{[14][18][19][4]} It is often found colonizing areas with little shading and competition from other species.^[5] Although limited information is available on soil preferences of hymenachne, it has been found in deep, poorly drained muck soils in Florida.^[20] It is not found in waters with a significant salt concentration.^[21][5]

Invasion

There have been three introductions of hymenachne into Australia, one in Florida, and one in Louisiana.^[22][23][5] Australia now lists hymenachne as one of the top twenty worst weeds.^[24] Hymenachne invasions have been shown to decrease species richness, increase invasive fish populations, and overall cause fundamental changes in floral and faunal composition in Australia.^[13] Hymenachne was introduced in Australia as forage for cattle, and likely for the same reason in Florida.^[5] The species was used in ponded pastures, or artificial ponds created to hold water for use in the dry season.^[5] High reproducibility from stem fragments aids in the dispersal and invasion of this species.^[10] The first herbarium record of hymenachne in Florida is from 1957.^[23][25] The second specimen is not recorded until 1977.^[25] The cause for this gap is unknown; however, today it is recorded in 28 counties across the state.^[26][27]

References