The role of environmental variables on interannual variation in species composition and biomass in a subtropical minerotrophic floating marsh

Floating marshes supporting emergent vascular vegetation occur in expansive areas in many parts of the world. We analyzed the long-term variability in species composition and related plant biomass to environmental variables in a subtropical minerotrophic floating marsh, Louisiana, U.S.A. Panicum hemitomon was the dominant plant species, representing 76% of the total mean end of season aboveground dry weight of 840 g∙m−2. Multivariate analyses showed that community structure in the Lake Boeuf floating marsh has changed little during the 11 years included in this study. Individual species occurring in varying frequency with the dominant, Panicum hemitomon, form two marginally distinct assemblages. Mean live end of season biomass varied from a low of 602 g dry wt∙m−2 to a high of 1173 g dry wt∙m−2 during the period of the study. Ninety-nine percent of the variation in total aboveground biomass can be predicted by environmental variables related to temperature, precipitation, evaporation, and water level. Mapping of the area for the years 1945, 1952, 1981, and 1992 show that a net loss of about 4% of marsh has occurred between 1945 and 1992. Key words: vegetation, stability, freshwater floating marsh, Panicum hemitomon, Louisiana, climate.

Chromosome numbers of Texas grasses

Records for 409 of 572 native and introduced grass species of Texas show that 30% are diploid, 30% are tetraploid, 11% are hexaploid or of a higher level of ploidy, and 29% have two or more ploidy levels. The relatively high percentage of diploids is due to the large number (40 species) of diploid panicums. In the present report a total of 304 chromosome counts are presented for 140 species. Chromosome counts for the following 22 species are the first for the taxon or different from previous records: Sitanion longifolium, Agrostis hiemalis, Axonopus affinis, Echinochloa walteri, Panicum hemitomon, P. thurowii, Eragrostis intermedia, E. swallenii, Neeragrostis reptans, Sporobolus asper, Tridens ambiguus, T. muticus, T. strictus, Triplasis purpurea, Bouteloua hirsuta, Chloris latisquamea, C. verticillata, Gymnopogon ambiguus, Hilaria mutica, Spartina alterniflora, S. patens, and Trichloris pluriflora. The records of 2n = 60 for Neeragrostis reptans, 2n = 40 for Triplasis purpurea, and 2n = 40 for Gymnopogon ambiguus are the first for the genera Neeragrostis, Triplasis, and Gymnopogon.