INDICATOR SIGNIFICANCE OF NATIVE VEGETATION 277 



jack" applies to the young, vigorous trees under 125. to 150 

 years old which are characterized by a dark, almost black, or 

 dark brown, narrow-furrowed bark. The " yellow pine" form 

 comprises the older trees which are characterized by a yellowish 

 or reddish brown, widely furrowed bark. A marked difference 

 occurs between the form and volume of "black jacks" and 

 " yellow pines." The average " black jack" has a greater taper, 

 a more rapid rate of growth and approximately 10% smaller 

 cubic volume than the average " yellow pine" of the same diam- 

 eter and height, which are the main reasons for the segregation 

 of "black jack" and "yellow pine" by technical foresters. 



Two distinct western yellow pine sites were very readily 

 recognized through apparent differences in the composition, 

 density, rate of growth, and vigor of the forest and the cor- 

 responding differences in the shrubby and herbaceous vegeta- 

 tion. The soil of Site I, which occupies the canon bottoms and 

 the protected north slopes, is a deep gravelly clay loam contain- 

 ing considerable amounts of organic matter and is consequently 

 comparatively retentive of soil moisture. Site II, which as a 

 shallow well-drained gravelly soil containing little organic mat- 

 ter, is found on the ridge-tops and exposed south slopes. At 

 the time of the 1915 examination the two sites were segregated 

 and each was mapped separately. Approximately 16.0 acres 

 were classed as Site I, while 10.4 acres fell into Site II. 



The growth data were segregated in separate compilations for 

 each site and for the two forms of western yellow pine. 15 The 

 total volume growth or increment of all trees measured was 

 computed for the five-year period. The total volumes were 

 computed by applying a volume table based on diameter and 

 total height to the dimensions of each tree at each time of 

 measurement. The difference between the total volumes at 

 each time of measurement represents the periodic increment 

 and the average volume growth per year during the period con- 

 sidered is the periodic annual increment. Table 1 shows the 



15 The writer is indebted to Forest Examiners H. B. Wales and J. W. Stokes 

 for assistance rendered in the collection and compilation of the growth data 

 presented in this paper. 



