Regeneration of Phragmites australis from rhizome and culm fragments: an experimental test of environmental effects, population origin and invasion status

Regeneration from vegetative fragments is common in plants that occur in disturbed and wet habitats but quantitative data comparing regeneration of different plant parts under various environmental conditions are still scarce. Phragmites australis is a dominant and a keystone wetland species that is widespread all around the world. It spreads both vegetatively by rhizomes and stolons and generatively by seed. Detached vegetative fragments of culms and rhizomes can support local regeneration after disturbance and spread populations over considerable distances when transported with water, soil and other means. In P. australis, there is no information on culm regeneration and how regeneration differs between the clones of different origin and what is the effect of the environment. Here we studied the regeneration of P. australis from culm and rhizome fragments over six weeks (mid-June to late July 2017) in a common-garden pot experiment. To simulate various scenarios that can happen in nature, we placed cuttings of culms and rhizomes (representing propagules) in 6-l pots in water, on the sand surface, and buried them 5 cm deep in the sand. We included 19 distinct clones representing populations from three phylogeographic groups (North American invasive, North American native, and European). We tested the effect of phylogeographic group, plant part (culm, rhizome), environment (water, surface, buried), ploidy and genome size on clone regeneration using generalized mixed-effect models. A higher percentage of culms than rhizomes regenerated (69% vs. 37%, respectively). Regeneration was better in fragments placed in water than in those buried and on the sand surface (65%, 50%, and 44%, respectively). Although we found considerable differences in regeneration among particular Phragmites populations (ranging from 31% in one of the North American native tetraploids to 90% in a North American invasive octoploid), the effect of the phylogeographic group was not statistically significant. However, phylogeographic group interacted with plant part – culms of the North American invasive populations regenerated better than those of North American natives, while rhizomes did not differ among phylogeographic groups. This difference was most pronounced in the sand-surface treatment. Rhizome fragments produced greater culm- and root biomass than culm fragments and North-American native clones produced the least new biomass of all groups. Lastly, rhizomes regenerated more slowly than culms (16 and 13 days to produce new shoots, respectively), and regeneration was fastest in water. Our results point to a great regeneration ability of culm fragments, which can cope with a wide range of environmental conditions and grow rapidly to produce new plants. We suggest this played an important role in spreading the invasive populations in North America, whose culm fragments regenerated better than those of native populations. Culms of invasive populations did not require to be permanently exposed to water for regeneration, which may have also facilitated their spread to drier habitats and their niche expansion beyond wetlands.