Abstract:
It is pointed out that there are available no detailed quantitative accounts of the vegetation of fresh water marshes in North America. An opportunity to undertake such a study presented itself at Delta, Manitoba. Brief accounts of the geology, geography, climate and the influence of water fluctuations in Lake Manitoba on the marshes south of the lake are given, and the vegetation met with is described. In 1955, the water in Lake Manitoba reached an unusually high level, and this resulted in the flooding of approximately twenty one per cent of the emergent marsh. Since 1955, the water has been diminishing, and 1958 saw the recolonization of considerable tracts of dead vegetation and mud, though only a small proportion of the bare areas bore vegetation. Early in the season a distinct zonation was apparent in sites with a slight gradient. Zone 1 was characterized by Ranunculus sceleratus and later by any or all of the following species: Chenopodium rubrum, Atriplex patula, Scolochloa festucacea, and Rumex maritimus var. feuginsm. Zone 2 was usually dominated by Aster brachyactis or Epilobium glandulosum var. adenocaulon, and Zone 3 by Phragmites communis, Cicuta maculata, Typha latifolia, Scirpus validus and a number of minor species. In other sites the vegetation consisted of a mosaic, important species being Ranunculus, Atriplex, Chenopodium, Scolochloa and Puccinellia nuttalliana. The expansion of vegetation on progressively drying areas, its variation with edaphic factors and the seasonal change in species composition is considered, together with certain biological aspects of some of the main dominants. The general nature of the study and the need for further investigation, such as soil analysis and autecological studies are emphasized.
