EXAMPLE 1 

 AN ASYMMETRICAL, BELL-SHAPED RIDGE: 

 TWO SEGMENTS 



In this descriptor, tree mortality percent in a western forest is characterized 

 by tree diameter (d.b.h.) over the course of a beetle epidemic (fig. 1). The descrip- 

 tor is applicable only at discrete points in time and at the midpoints of 2-inch d.b.h. 

 classes. Both variables are treated as continuous. As was evident in the original 

 data, the more-or-less bell-shaped trends over time differ substantially on either 

 side of the central ridge. For example, observe strong asymmetry at d.b.h. = 12 to 

 18 inches, made most noticeable perhaps by elevational differences in the curves at 

 their left and right extremes. See also the dwindling breadth of curve crowns above 

 and below 14 inches d.b.h., an interactive change included in the descriptor along with 

 asymmetry. Descriptor components were assembled as follows: 



Left and right sides of the ridge were described separately (fig. 2 and 3), using 

 sigmoids from Matchacurve- 1 that varied in shape and scale according to data trends 

 over d.b.h. Because the lower portion of the left half asymptoted at values larger 

 than zero, it was necessary to include the left-edge intercept, Int, (or floor) upon 

 which the left-side sigmoids rested conceptually. Int is a function of d.b.h. as is 

 the location of the ridge in time, XP. 



1966 1967 1968 1969 1970 1971 



YEAR OF OBSERVATION 



Figure 1. — Tree mortality over the oourse of a beetle epidemic. 



2 



