Expansion of Phragmites australis (Cav.) Trin. ex Steud. (Common Reed) into Typha spp. (Cattail) Wetlands in Northwestern Indiana, USA

Expansion of Phragmites australis (Cav.) Trin. ex Steud. (common reed) into stands of Typha spp. (cattail; Typha australis L. and T. x glauca) is common in the wetlands of northwestern Indiana (USA). To understand this phenomenon better, we investigated the production of shoot sprouts and proportional allocation of biomass as well as a potential role for the water table in the relative dominance of each species. The reduction in sprouts from rhizomes upon vegetative expansion of Phragmites appeared to be the most likely process causing the decline of Typha. The latter had a shoot density of 39/㎡ in plots without Phragmites, but this dropped to 13 shoots m-² in plots that had been invaded by Phramites. Such a decline was likely caused by reduced reserves; e.g., the belowground biomass of Typha decreased from 11.3 g m-² without Phragmites to 8.1 g m-² with Phragmites. The latter also reduced its belowground biomass but not its shoot density in the presence of Typha. The mean weight of Phragmites shoots was 2.9 g, and nearly all produced inflorescences. Meanwhile, Typha failed to develop spadices despite its shoots having a greater biomass (7 g). This suggests that Phragmites is more efficient than Typha in shoot growth. Springtime flooding appeared to promote the sprout of Typha shoots from shallow rhizomes (almost equal to 18 cm below the soil surface), whereas the shoot density of Phragmites showed no correlation with water level in that season. Deep-rooted Phragmites (almost equal to 39 cm) occurred on both high and low water-table sites, whereas the shallow-rooted Typha was limited to only the former. Phragmites will likely continue its expansion, by vegetative sprouts from rhizomes, into Typha wetlands.