Flora of six Lower Mississippi River islands (U.S.A.)

Lower Mississippi River islands are part of the last remaining floodplain habitat of the river, and studying their biodiversity is important to understand the river ecosystem. We conducted a floristic survey of six Lower Mississippi River islands between river miles 668 and 526 in southern Arkansas and northern Mississippi. These islands contain a variety of habitats and span a range of sizes, from 106 to 697 vegetated hectares. We identified 491 taxa (485 species plus six infraspecific taxa) from 2,310 specimens collected from June 2020 to December 2021. These taxa are within 89 families and 282 genera, and 100 (20.6%) species are not native to the Mississippi River drainage basin. Nine taxa found on Arkansas islands are tracked by the Arkansas Natural Heritage Commission for conservation. Five taxa found in Mississippi are tracked by the Mississippi Natural Heritage Program. We found five taxa that had not previously been reported in Arkansas (Equisetum variegatum, Achyranthes japonica var. hachijoensis, Stellaria aquatica, Potentilla paradoxa, Cyperus diandrus) and seven previously unreported in Mississippi (Euploca convolvulacea, Sisymbrium loeselii, Chaiturus marrubiastrum, Mentha canadensis, Potentilla norvegica, P. paradoxa, Physalis mollis). Additionally, we collected 266 Arkansas county records, including 132 in Phillips, 64 in Desha, and 28 in Chicot counties, respectively, and 42 in Washington County, Mississippi. The high number of new encounters and tracked species found emphasizes the need for more botanical study on Lower Mississippi River islands. Of the 485 species in the flora, we found 110 species on all six islands (22.7%), 43 on five of the six islands, 47 on four islands, 75 on three islands, 72 on two islands, and 138 on only one island (28.5%). The number of unique species found on only one island illustrates how variable and dynamic these islands are in this riparian zone.