Thobaben, E. T. 2004. Linking plant communities and environmental variables in southwestern Michigan wetlands. Dissertation, Michigan State University, East Lansing, Michigan, USA.
Plant communities often vary as a function of their abiotic environments; this is especially true in wetlands, where hydrology is of paramount importance. Wetland hydrology remains an elusive concept to measure and relate quantitatively to the composition of plant communities. In an effort to elucidate these relationships I surveyed plant communities across a diverse set of 24 wetlands in southwestern Michigan. I also quantified wetland hydrology at these sites in terms of water sources, seasonal water level variation, and hydrochemical composition. Using major solutes as tracers of water sources, I estimated the relative importance of precipitation and groundwater inputs (i.e., “fraction groundwater”). I monitored water levels, collected soil water from the root zone for hydrochemical analyses, and collected organic soils for soil nutrient analysis. Nutrient accumulation on ion exchange resin was used to estimate nutrient supply to each wetland.
Cluster analysis and indicator species analysis revealed seven different groups of wetland plant communities, which I have distinguished for the purposes of discussion: leatherleaf bogs, bogs, a poor fen, fens, sedge meadow fens, wet swamps, and dry swamps. Ordination using nonmetric multidimensional scaling (NMS) reduced the plant community data down to two synthetic axes (NMS Axes1 and 2) that explained 64% of the variation. pH, as well as fraction groundwater, soil nutrients, and hydrochemical variables that covaried with pH, and canopy, an estimate of shade cast by tall woody species, were the environmental variables most strongly related to the ordination; water levels were of secondary importance; nutrient supply was of less importance. Fens and sedge meadow fens exhibited higher water levels and lower canopy values (i.e., less shade) than wet and dry swamps. Evaluation of fraction groundwater estimates for these seven groups suggested that fens and swamps, while generally groundwater-fed, are not all strongly groundwater-fed; bogs were all strongly precipitation-fed; the poor fen was intermediate between bogs and fens.
A partial least squares composite variable model was used to test these ordination results and evaluate the relationship between landscape variables and wetland environmental variables. Landscape variables were good predictors of wetland water source (R2 = 0.60), while water sources and water levels were moderately good predictors of nutrient supply (R2 = 0.51). Acidity, shade, water levels, and nutrient supply collectively explained 94% and 80% of the variation in NMS Axes 1 and 2, respectively; acidity and shade were stronger predictors of the vegetation differences than water levels and nutrient supply. Based on these results and plant physiology studies, many wetland plant species may be excluded from the acidic bog environment for physiological reasons relating to acid stress instead of nutrient availability per se. This suggests that measures of nutrient availability may be less helpful in explaining why so few wetland plant
species may persist in acidic peatlands.
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