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Research: Species frequency patterns

In most communities, a majority of the plant diversity is found in relatively uncommon species.  To quantify this pattern, we have been exploring relative frequency patterns, which have their origin in patterns described by Raunkiaer nearly a century ago in 1908.  We selected a habitat known to have many plant species in relatively small areas.  We first visited a set of 120 1 x 1 m sample plots in a wet coastal plain savanna during the growing season to assess species presence; 60 plots represented wetter conditions, while 60 plots represented drier conditions.  These 120 plots contained 126 taxa.  Measured by percent cover, the dominants in the wetter plots were Rhynchospora gracilenta, Dichanthelium scabriusculum and Scleria pauciflora, whereas in the dry area they were Schizachyrium tenerum, Ilex glabra and Dichanthelium dichotomum. The species frequency patterns all had a skewed unimodal pattern with most species occurring infrequently, in less than 10% of the plots.  When the plots were sorted into dry and wet areas, the relative pattern was similar in both sets (χ2 = 8.97, P >0.05).  To explore possible effects of sample area on this pattern, we sampled 75 plots in DeSoto National Forest, using four sample areas of 0.1 m2, 1 m2, 10 m2, and 100 m2 .  Again, the species frequency patterns all had a skewed unimodal pattern.  These patterns are consistent with other studies of savannas, but do not appear consistent with the bimodal patterns reported from some grasslands.  We need more studies of relative frequency to determine the degree of generality of such patterns, and their relationship to mechanistic processes in plant communities.

This research is now complete and is published here.