Rapid spread of Phragmites australis in an abandoned limestone quarry

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Diana Borse and Dana A. Dudle, Biology, DePauw University, Greencastle, IN, M. Beth Wilkerson, GIS Center, DePauw University, Greencastle, IN

Background/Question/Methods
Abandoned limestone quarries in the midwestern United States host a surprising diversity of plants native to wetlands, glades, and other habitats of concern; in some quarries, these native plant populations may be threatened by the expansion of introduced species. From 2009-2020 we mapped the expansion of a large, patchily-distributed population of Phragmites australis, an introduced hydrophytic grass that has established in an abandoned limestone quarry at the DePauw University Nature Park in Putnam County, Indiana. We compared the density of growing shoots at the edge of patches of P. australis to patterns of flowering at the edge of the patches, and the rate of patch expansion. We investigated the possibility of resource partitioning between P. australis and another introduced wetland grass, Typha angustifolia by comparing the elevations of the two populations in temporary wetlands on the quarry floor. Finally, we identified native plant species growing near expanding P. australis patches whose populations might be obstructed by continued expansion.
Results/Conclusions
The Phragmites australis population on the quarry floor expanded significantly during our study; from 2009 to 2020, the area covered by Phragmites australis increased by approximately 64%, or on average 5.8% yearly (P = 0.007). We found evidence that the P. australis population is spreading via both vegetative and sexual reproduction. In some recently-established patches, the pattern of vegetative spread can be predicted by examining the density of growing shoots in prior years. Contrary to our expectation, we found no evidence that T. angustifolia occupied areas of lower elevation than P. australis; it may be that our topographical data is not precise enough to rule out the possible influence of topography on the distributions of these plants. Among the native wetland species growing within five meters of P. australis patches in 2020 were bulrushes (Scirpus atrovirens; S. pendulus), sedges (Carex vulpinoidea; C. granularis), and small populations of milkweed (Asclepias verticillata), and orchid (Spiranthes sp.). The rapid growth of P. australis in the quarry indicates that the site’s ability to serve as refuge habitat for native wetland species may be limited.