. The Canadian field-naturalist. 484 The Canadian Field-Naturalist Vol. 103 r- .719, p= .01. WATER DEPTH (cm) Figure 3. Abundance of Cephalanthus occidentalis plotted against water depth of pond, with Spearman correlation (r) and linear regression line. 1982). In the United States it is found primarily in the Atlantic Coastal Plain, typically in wet woods and swamps (Fernald 1950). Nutrient concentrations showed no correlations with the DCA axes or Buttonbush abundance, confirming suggestions in other studies that soil chemistry may not be a limiting factor in inundated habitats; rather, the e
. The Canadian field-naturalist. 484 The Canadian Field-Naturalist Vol. 103 r- .719, p= .01. WATER DEPTH (cm) Figure 3. Abundance of Cephalanthus occidentalis plotted against water depth of pond, with Spearman correlation (r) and linear regression line. 1982). In the United States it is found primarily in the Atlantic Coastal Plain, typically in wet woods and swamps (Fernald 1950). Nutrient concentrations showed no correlations with the DCA axes or Buttonbush abundance, confirming suggestions in other studies that soil chemistry may not be a limiting factor in inundated habitats; rather, the effect of soil moisture or oxygen levels is probably more critical (Keeley 1979; Robertson et al. 1984; Mitsch and Gosselink 1986). We suspect that Buttonbush responds favourably to increased water depth partly because fewer tree species can tolerate these depths. The flooded environment requires specialized adapta- tions, especially for low oxygen levels, and few species have such adaptations (Keeley 1979). An indirect effect of flooding is that tree canopy closure is probably slowed and higher light levels on the pond are maintained. Conner et al. (1981) noted that canopy gaps made possible the invasion of Buttonbush into swamp sites. Our analyses of the effects of light on platform species distribution and Buttonbush abundance were not significant. Degree oi shading in floodplain-like habitats may have negligible effects on woody understory composition (Robertson et al. 1984), but may limit growth (Conner et al. 1981; Duever and Riopelle 1983) and reproduction. Mengesand Waller (1983) found that light was important in differentiating herbaceous guilds (high-light specialists, light generalists. etc.) only when Hooding was infrequent. Buttonbush responds favourably to increasing levels of light, but high levels are not critical for establishment (Faber-Langendoen and Dina 1987). Rather, tolerance of a stressful habitat and the absence of competition appear to be more important
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