D = /Di z + Do z + . . . D z , 



(eq. ID 



in which D is the equivalent diameter and the subscripted D's are individual stem dia- 

 meters. 



Since pinyon butt cross-sections are usually out of round and the width of each 

 annual ring is rarely uniform, it was necessary to develop special procedures to de- 

 termine past diameters. Each stump cross-sectional disk was inspected and a line was 

 drawn on the radial that appeared to best represent the growth pattern. Ring widths 

 were measured to the nearest 0.1 mm along this line. The stump diameter (outside bark) 

 at any particular time in the past was calculated by: 



D It. 



P t ^ 



t 



in which D is past diameter (outside bark) at any time t, D is present diameter (out- 

 side bark), T.tj is the sum of the ring widths from the pith to time t, and £r-£ is the 



t p 

 sum of all ring widths along the radial line on the section. This equation corrects 

 for changing bark thickness and for systematic errors in the measurement procedure. 



Foliage, wood larger than 76 mm diameter, and total aboveground biomass of all 

 unweighed trees were calculated using regression equations developed from data obtained 

 in a companion study 1 . These equations are based on 68 pinyon trees, 18 of which are 

 weighed trees from the Sweetwater plots. The remaining 50 are from 13 study sites 

 throughout Nevada. Input variables include stump diameter, crown diameter, height, and 

 foliage class. These equations are presented in appendix I. 



Past biomasses (foliage, wood, and total) of each tree were estimated for each 

 decade from 1860 to 1970, and for 1965 and 1975. These biomasses were calculated 

 using regression equations that expressed past biomass as a function of present biomass 

 and past height, diameter, and age. These equations, which were also derived from 

 data obtained in the companion study, are presented in appendix II. 



Basal area per hectare and biomass per hectare for each plot through time were 

 calculated by adding the values for the individual trees and dividing the sum by the 

 plot area (0. 09 ha) . 



RESULTS AND DISCUSSION 



Comparisons of tree sizes, ages, and diameter growth rates (appendix III) with 

 their relative locations (figs. 1, 2, and 3) indicate varying degrees of competition 

 among trees and suppression of the smaller trees. This paper will deal primarily with 

 the growth of dominant and codominant trees and with the growth of stands, postponing 

 a more detailed discussion of competition to another paper after more data have been 

 acquired. 



1 Miller, E. L., J. D. Budy, and R. 0. Meeuwig. Biomass of singleleaf pinyon and 

 Utah juniper (manuscript in preparation) . 



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