Range-Wide Quantitative Habitat Characterization of Critically Imperiled Ludwigia ravenii (Onagraceae).

Ludwigia ravenii (G1G2) is a wetland-obligate perennial, endemic to the Atlantic Coastal Plain. The species appears to be relegated to roadside ditches and ruts, and is under review for federal listing. The objectives of this study were to quantitatively assess biotic and abiotic habitat characteristics of extant populations and augment knowledge of the species ecology. Two congeners were selected for comparative analysis: L. alternifolia and L. pilosa. Over two field seasons, data were collected from 108 1 m2 quadrats and variable length transects across the species range. All 17 element occurrences of L. ravenii on record during the 2020--2021 sampling period were visited and 15 (nplots=62) were sampled. Forty-six plots at 14 sites were collectively sampled for the target congeners (L. alternifolia, nplots=27; L. pilosa, nplots=19). Variables assessed included: physical ditch dimensions, aspect, elevation, canopy cover, abiotic and vegetative cover, and edaphic variables (nutrient composition and pH). Data were visualized using NMS and PCA, and analyzed using ANOVA and post-hoc tests. Only PCA showed distinct clustering of quadrats of L. ravenii along soil fertility gradients. Quadrats of L. ravenii exhibited median values compared to congeners in many measured variables, including canopy cover, species richness, and Shannon's diversity index. Quadrats of L. ravenii differed most strongly from congeners by more acidic mean soil pH (4.68±0.05). Although no single species emerged as an ideal indicator, the combination of L. linearis and Kellochloa verrucosa was found highly suggestive of suitable habitat for L. ravenii. [ABSTRACT FROM AUTHOR]