Numerical analyses of macrophyte vegetation in Victorian wetlands in relation to environmental factors

© 1984 Dr. Michele Mary BarsonThis study was undertaken to investigate the variability of vegetated Victorian wetlands and to establish the relationships between this variation and major environmental factors. Criteria for the selection of the 55 wetlands sampled included the presence of aquatic macrophyte or helophyte vegetation, the presence of at least an intermittent water body and, comparative lack of disturbance of the site. Sites were also chosen to reflect the considerable lithological and climatic variation found across lowland Victoria. At each site, species presence/absence data, water depth and water transparency were recorded within 1m square quadrats positioned at 5m intervals along transects located to best sample the vegetation. At each transect, water samples were collected for the analysis of major ions, and substrate samples were taken for the estimation of texture and measurement of pH and percentage salts. The maximum depth of the basin when flooded and its water regime were estimated and the geology and rainfall of the catchment were recorded. An information statistic strategy was used to classify the large, relatively sparse floristic set of data. The classification recognised five distinctive, relatively homogenous and ecologically interpretable groups of wetlands, which were characterised as having saline, very saline, turbid, acidic or calcareous waters, and a further three freshwater groups which were closely related to one another. The application of data reduction techniques suggested that the information statistic model was unable to adequately define some of the freshwater groups of sites primarily because of the highly heterogenous nature of the data set. Two more data sets were produced by deleting species with low "eident" values (Dale and Williams 1978) and by deleting species regarded as terrestrial. However, classification of these reduced data sets did not provide markedly better results. The relationships between the groups (and their members) generated by classification was examined through indirect ordination of the floristic data. Inspection of the results indicated that six of the eight groups identified by classification of the floristic data could be recognised. However, two groups of saline sites could not be separated, largely because they were both species-poor. Six sites were identified as the probable cause of overlap of some of the freshwater groups. Laboratory determination of the major ionic constituents of the waters of the 55 wetlands indicated that the orders of anion dominance were Cl>HC03+C03>S04 (freshwater sites) or Cl>S04>HC03+C03 (saline and coastal freshwater sites) and those for cations were Na>Mg>Ca>K or Na>Ca>Mg>K (freshwater) and Na>Mg>ca>K (saline sites). The dominance of chloride and sodium ions in the waters sampled suggested that salinity was a major factor affecting the distribution of aquatic macrophytes in Victoria. Numerical classification of the wetlands on the basis of their water chemistry was undertaken to provide a comparison with the eight group floristic classification. However, two of the intuitively recognised groups, the turbid and calcareous waters, were not identified by classification of the water chemistry data, and membership of the two independently generated sets of groups was not identical. The nature of the hypothesized joint pattern between the floristic and the water chemistry data was further investigated by canonical correlation analysis, analysis of variance and discriminant analyses. These analyses confirmed that overall, the variance observed in the vegetation of the wetlands sampled was significantly influenced by water chemistry. However, the level of vegetational variation identified as a result of classification of the floristic data (eight groups) did not correspond well with the measured differences in the water chemistry variables. Vegetation differences which could be attributed to water chemistry differences were those between the saline groups, the turbid water group, the acidic water group, the freshwater complex of three groups, and possibly the calcareous group. The salinity variable largely separated the saline groups from the rest, whilst pH separated the acidic water and calcareous water groups from each other and the freshwater complex. The turbid water group was separated by a combination of pH and salinity variables. An evaluation of the methods used is presented and other factors which may be important in explaining the heterogeneity of the saline and freshwater floristic complexes are discussed. The wetland vegetation sampled in Victoria is compared with the plant communities of Tasmanian wetlands