Height growth rates vary considerably among dominants on the same plot, especially 

 on the Monte Cristo plot. The younger dominants are growing faster than the older trees 

 are growing now or grew at the same age. It is probable that these faster-growing 

 trees are genetically superior, but where are their parents? Pinyons taller than 12 m 

 are rare in the study area and in most other pinyon- juniper stands in the Great Basin. 

 A plausible explanation of this anomaly is that faster-growing trees were present in 

 the stands but were logged off in the 19th century, leaving the shorter (slower-growing) 

 trees that are now the oldest trees in the stand. 



Diameter 



Stump diameters of several dominants and codominants on each of the three plots 

 are plotted through time in figures 7, 8, and 9. The diameters were calculated at 10- 

 year intervals through 1960 and then at 5-year intervals until 1975. The Cattle Trough 

 plot (fig. 8) has the expected S-curves: diameter growth increased initially and then 

 decreased as competition among trees increased. Trees #31 and #32 on this plot provide 

 a good example of the effects of competition. These trees are close together (fig. 2) 

 and obviously in competition. As these trees developed and competition between them 

 increased, diameter growth of both decreased but the reduction was greater in the young- 

 er, smaller tree (#31). The competition had no apparent effect on height growth rates 

 of either tree (fig. 5) . 



In contrast to the Cattle Trough plot, none of the dominants and few of the co- 

 dominants on the other two plots have S-shaped curves (figs. 7 and 9). The diameter 

 growth rates of most of these trees have remained essentially constant from the time 

 they attained a diameter of about 10 cm up to the time they were felled and measured. 

 In general, the larger trees on these two plots are not competing with each other but 

 with smaller trees. In the absence of appreciable competition, there appears to be no 

 diminution of diameter growth regardless of tree size or age. The older trees on the 

 Monte Cristo plot demonstrate this point. Tree #34 is suffering from competition with 

 trees #29 and #31, but trees #24, #29 and #31 have essentially straight-line diameter 

 growth (fig. 9). 



Tree #31 on the Monte Cristo plot is a special case that requires further explana- 

 tion. Unfortunately, the butt of this old tree was badly rotted and it was necessary 

 to extrapolate the radial growth pattern of a section 2 m high on the dominant stem to 

 calculate past diameters at stump height. Thus, the past diameters plotted for this 

 tree are based on the assumption that the ratio of radial growth at 15 cm to radial 

 growth at 2 m is constant. In any case, diameter growth of this tree has been essen- 

 tially constant for more than three centuries. 



Reveal (1944), reporting on a study in the nearby Pine Nut Range, stated: "Dia- 

 meter growth (of pinyon) is maintained at approximately one inch per decade for the 

 first 100 years. As maturity advances, the diameter growth decreases to 0.25 inch at 

 the end of the second century. The life span is seldom over 250 years." 



The results from the Sweetwater Mountains indicate that average diameter growth of 

 pinyons is roughly 1 in (25 mm) per decade but that competition, not age, is the primary 

 cause of reduction in diameter growth. These results also suggest that the natural 

 life span of singleleaf pinyon can be well over 250 years. 



11 



