Effects of prolonged flooding on the distribution and biomass of emergent species along a freshwater wetland coenocline

Abstract

Water levels were raised 1 m for two years in 10 cells of an experimental wetland complex located in the Delta Marsh, Manitoba, Canada. The mean area covered by each of the 5 dominant emergent species in these cells declined significantly during the first year of flooding. There was no significant difference in the total acreage of ?mergents between flooding years. Three species were completely {Carex atherodes Spreng, Scolochloa festucacea (Willd.) Link) or almost {Scirpus lacustris L. spp. glaucus (Sm.) Hartm.) eliminated from the cells. Typha glauca Godr. and Phragmites australis (Cav.) Trin. still covered in the second year of flooding about 40 and 25%, respectively, of the area that they had occupied in the cells in the preflooding year. The mean elevation at which the two surviving species were found both increased 0.1 and 0.2 m over preflooding elevations during the first and second year of flooding, respectively. There is no evidence that during the flooding years any species migrated upslope toward more optimal water depths. Aboveground standing crop for all five species declined significantly during the first year of flooding, but was not significantly different between the two flooding years. Total belowground biomass did not differ between spring and fall, but declined significantly (about 20%) during the first year of flooding and remained about at that level during the second year. In the spring following the two years of flooding, mean total belowground biomass was only 40 g m-2, less than 10% of the belowground biomass dur ing the preflooding year.