Phenology and autumnal accumulation of N reserves in belowground organs of wetland helophytes Phragmites australis and Glyceria maxima affected by nutrient surplus

Two co-occurring dominant wetland helophytes and potential competitors, Phragmites australis and Glyceria maxima, were cultivated under N, P availabilities simulating the trophic status of wetlands with different fertility (oligo- and eutrophic). The long-term outdoor cultivation was performed with the goal to characterise the extent to which the nutrient enrichment affects plant growth, phenology, and particularly, the accumulation of N storage compounds in belowground organs of wetland rhizomatous plants prior to the onset of winter dormancy. In the present study, both species responded similarly to nutrient surplus. The enhanced growth, delayed shoot senescence, and delayed retranslocation of N into belowground organs were found in both species in eutrophic treatment. Furthermore, N levels remaining in dry leaves were proportionally related to those in living ones, being significantly higher in eutrophic treatment. The efficiency of N retranslocation from senescing leaves varied around 60% in both species and treatments. The formation of N reserves was, however, not disrupted in either species. Although plants in eutrophic treatments accumulated N in their belowground organs significantly later in the season (in the September–December period), the amount of accumulated N was sufficient to reach high belowground N standing stock. Considering formation of N reserves, the differences in species response to treatments were negligible. Phragmites and Glyceria accumulated similar belowground N standing stock prior to the winter. Glyceria may, however, additionally profit from N standing stock of over-wintering green leaves and from the potential of growth and N assimilation during a mild winter period, which is not possible in fully dormant Phragmites.