Long-term monitoring of plant communities provides insight into mechanisms of community assembly and informs development of management plans. The acid seeps of southern Illinois are unique plant communities threatened by disturbance and exotic invasion. Floristic surveys of 13 acid seeps by Schwegmann (1968) and Basinger (2008) documented invasion of exotic grass Microstegium vimineum in all acid seeps by 2008, extensive stream down-cutting, and predicted species richness declines (S). Land managers are concerned with how these disturbances will affect communities. Objectives of this research were to test Basinger’s prediction and investigate changes in taxonomic and phylogenetic diversity to determine how plant communities changed following exotic species invasion and stream down-cutting.
Species occurrence in ten acid seeps surveyed in 1968 and 2008 were resurveyed in 2017 and differences in S were tested using repeated measures analysis of variance (rANOVA). Species richness from the 13 seeps surveyed in 1968 and 2008 were compared using paired-ranked t-tests to test Basinger’s prediction. A maximum likelihood phylogenetic tree of all taxa recorded over time periods was constructed based upon concatenated rbcl and matk sequences retrieved from Genbank. Trees pruned for location and survey were constructed following Muscle sequence alignment. Adiantum pedatum was selected as the outgroup. Phylogenetic diversity was calculated as Net relatedness indices (NRI) and nearest taxon indices (NTI). Effect of acid seeps and time period and their interaction on NRI, and NTI were tested using rANOVA. Tukey’s Least Significant Differences (LSD) post-hoc tests were calculated for significant relationships.
Results/Conclusions
A paired t-test showed species richness not significantly different between the 13 seeps surveyed in 1968 and 2008 (t12 = 1.2582; p = 0.2322). The maximum likelihood phylogenetic tree contained 235 taxa. There was a significant difference in species richness among acid seeps (F9,2= 9.4706; p=7.927x10-4), and a difference in NTI among time periods (F2,9 38.7257; p= 9.845x10-5). Tukey’s LSD tests showed one seep had significantly higher species richness compared to seven acid seeps, but not two other seeps. NTI increased from 1968 to 2008, but not from 2008 to 2017.
Species richness of acid seeps did not change over 49 years despite biotic and abiotic disturbance. However, increase in NTI indicates that acid seep communities changed with species co-occurring more with more often with closely related species than expected. This phylogenetic clustering may have been caused by environmental filtering from steam down-cutting decreasing soil moisture and facilitating invasion of the exotic species.