Simultaneous disturbances during secondary succession can cause plant community shifts, facilitating exotic species invasions. However, it remains unclear if exotic invasion is facilitated by addit...

Few studies have incorporated the evolutionary insights provided by analysis of phylogenetic structure along with community composition to assess the effects of exotic invasion on freshwater wetlands. Here, we assess the taxonomic and phylogenetic relationships among acid seep springs to investigate the potential homogenization or resistance of communities due to invasion of an exotic grass. Multivariate community analyses indicated differences in community and phylogenetic composition and dispersion among acid seep springs, associated with gradients in soil moisture, canopy cover, and phylogenetic diversity. By contrast, univariate analyses showed differences in taxonomic diversity but not phylogenetic diversity among acid seep springs. Despite exotic invasion, individual acid seep springs remained taxonomically and phylogenetically distinct from each other. Taxonomic and phylogenetic diversity metrics revealed different aspects of composition, reinforcing the importance of including both in analyses of plant communities for understanding community assembly following exotic invasion and for management purposes. Within acid seep springs, taxonomic and phylogenetic composition appear to be driven more through environmental filtering by light and moisture than by the competitive effects following invasion of an exotic grass in support of Elton’s biotic resistance hypothesis.

— The number and identity of species within Euthamia (Asteraceae, Astereae) have varied considerably among taxonomic treatments. Euthamia graminifolia (L.) Nutt. is often treated broadly, including plants from the northern and eastern United States and Canada, including the Atlantic and Gulf Coasts. Broad-leaved, largely glabrous plants from New Jersey to the Florida Panhandle have been inconsistently treated as E. graminifolia, E. graminifolia var. hirtipes (Fernald) C.E.S. Taylor & R.J. Taylor, E. hirtipes (Fernald) Sieren, or a hybrid between E graminifolia and E. caroliniana (L.) Greene ex Porter & Britton. Similarly, plants from the Florida Panhandle to eastern Louisiana have been incorporated into E. graminifolia or E. graminifolia var. hirtipes with only Greene in 1902 recognizing these plants as a distinct species, E. scabra Greene. To determine the identity and proper rank of these entities, morphological and phylogenetic analyses were performed to evaluate relationships within Euthamia. Plants from the Atlantic Coast most morphologically resemble Gulf Coast plants which similarly resemble E. gymnospermoides Greene. The Gulf Coast plants and E. gymnospermoides share similar DNA sequences while the Atlantic Coast plants represent a unique clade. Neither Gulf Coast nor Atlantic Coast plants contain highly polymorphic sequences, indicating that they are not hybrids. Occasional plants found within southernmost Alabama and the Florida Panhandle have polymorphic sequences and intermediate morphology however, suggesting that putative hybrids exist between Gulf and Atlantic Coast plants. This study concludes that both entities deserve specific rank as E. scabra Greene for scabrous plants along the central Gulf Coast and E. hirtipes (Fernald) Sieren for largely glabrous plants mostly along the Atlantic Coast. Ecological niche modeling indicates that precipitation, especially during summer months, and soils, namely coarse fragments and sand content, drive the distribution of these organisms, largely restricted to either side of the Apalachicola River serving as a distributional barrier.