Coexistence of typha Angustifolia and Impatiens capensis in a tidal freshwater marsh

Several salt marsh plant species can oxygenate soils through their aerenchymous tissue and thereby facilitate the growth of neighboring species. Such positive interactions remain poorly understood for tidal freshwater marshes, yet may explain why species adverse to submerged roots are able to thrive in these marshes. Field observations showed a positive association between Typha angustifolia L. and Impatiens capensis Meerb., the two co-occuring dominant species in our study system, suggesting either that T. angustifolia facilitates I. capensis or that both species respond to environmental gradients the same way without interacting directly. We tested the facilitation hypothesis in a greenhouse experiment that measured the growth response of I. capensis with and without T. angustifolia as a neighbor species. Both species affected reduction-oxidation potential of the substrate, but facilitation of I. capensis was not observed. Typha angustifolia depleted soil nutrients in experimental mesocosms and negatively affected belowground root growth but not aboveground biomass of I. capensis; however, nutrient replenishment by tidal flushing may decrease the likelihood for competition in natural systems. Our greenhouse study suggests that abiotic conditions, dispersal events, or competitive biotic interactions, and not facilitation, are predominantly influencing the distribution of these two species in tidal freshwater marshes. Indeed, field measurements demonstrate that both species are positively associated with marsh surface elevation, which is related to frequency and length of water inundation of the marsh surface. © 2007, The Society of Wetland Scientists.