36 



this could be assumed to be 3 or 4 days. The boundary between Zones 5 

 and 6 varied from less than 1 to 10 days. 



Weighted Averaging 



Weighted average estimates of species optima in this study were 

 calculated as FTI values as previously described, with importance values 

 used as the indicator of species abundance at each site. 



The use of the weighted averaging approach requires that a number 

 of conditions be met, including: (1) species exhibit unimodal abundance 

 distributions, (2) species optima are equally spaced along the environ- 

 mental variable, (3) species have equal tolerances of the environmental 

 variable, and (4) species have equal maximum values for the environmen- 

 tal variable. Strict adherence to some of these conditions is not 

 always possible. Additional considerations should also be noted. 

 Species-rich samples should not occur at one end of the gradient. 

 Environmental tolerances of species should not vary substantially. The 

 standard deviation of FTI numbers is an estimate of tolerance in this 

 analysis. Species with narrow tolerances have low FTI numbers and 

 standard deviations and those with wide tolerance have high standard 

 deviations . 



Some of the aforementioned conditions are not strictly met in this 

 study. Although species richness was fairly even across the hydrologic 

 gradient, few species had a peak abundance in Zone 5, perhaps because 

 Zone 5 was undersampled (n = 4) relative to the other zones. The condi- 

 tion of a unimodal abundance distribution is upheld for tree species 

 with peaks in hydrologic Zones 2, 3, and 6, but several species (e.g., 

 Ilex opaca, Quercus nigra, Nyssa sylvatica , and Liquidambar styraciflua) 



