Plant community and phylogenetic shifts in acid seep springs over 49 years following Microstegium vimineum invasion

Community composition and diversity can change following exotic invasion. We tested the extent to which acid seep spring communities in southern Illinois had changed in composition and community heterogeneity based on three species surveys over 49 years. We also investigated the extent of changes in taxonomic and phylogenetic diversity since the invasion of Microstegium vimineum, an exotic grass. Floristic surveys of ten acid seep springs in southern Illinois were conducted in summer 2017 and compared to two previous surveys from 1968 and 2008. We investigated changes among acid seep springs and between surveys in species composition and community heterogeneity with multivariate ordination based on site dissimilarity, and univariate analyses comparing changes in taxonomic diversity (species richness and exotic species richness) and phylogenetic diversity [net relatedness (NRI) and nearest taxon indices (NTI)] determined from a species phylogeny based upon two plastid sequences (rbcL and matK). Acid seep spring communities became more compositionally similar to each other following woody plant encroachment and M. vimineum invasion. Total species richness declines were observed between surveys while exotic species richness increased. While NRI did not exhibit any temporal patterns, NTI significantly increased over time indicating that a decrease in phylogenetic diversity accompanied exotic invasion over almost half a century within these plant communities.