Total Aboveground Biomass 



Stand biomass curves (fig. 4) follow patterns similar to basal area curves (fig. 3). 

 The main differences are in the relative positions of the curves and the tendency for 

 continuing increases in biomass growth rate after basal area increment has stabilized. 

 The changes in relative positions of the curves are caused mainly by differences in tree 

 height characteristics. Plots PI, P2, and Ml are very close in basal area, but P2 has 

 the tallest trees and Ml has the shortest; hence the separation of their biomass curves. 

 The tendency for biomass growth rate to continue to increase after basal area increment 

 ceases to increase is due to continued height growth. 



Regardless of age structure, all plots have been accumulating biomass at an 

 increasing rate; however, biomass accumulation rates must eventually decline. On the 

 basis of these plots, the time when that decline would begin is unpredictable. Morta- 

 lity must increase appreciably to bring about a decline in biomass accumulation rate. 



CONCLUSIONS 



The plots measured in 1977 were all in relatively young, pure pinyon, stands on 

 good sites. Except for plot P3, the plots measured in 1978 were in older stands, con- 

 taining some juniper, on poorer sites. Except for the determinations pertaining to 

 height growth, the conclusions of Meeuwig and Budy (1979) that were based on the 1977 

 plots, were supported by the data from the 1978 plots. More complex height growth 

 patterns were found in the older stands. Competition may have a greater effect on 

 height growth rates than we surmised from the 1977 plot data. The existence of a 

 site-dependent maximum height on the 1978 plots was also indicated. The trees on the 

 1977 plots must have been well below maximum height for their sites because no indica- 

 tion of approach to maximum height was discernible. 



Based on information obtained on the eight plots intensively measured so far, the 

 following conclusions are offered: 



1. Height growth rates depend on a complex combination of site quality, tree 

 height, tree form, and competition. Height growth rates decrease as height 

 approaches a site-dependent maximum. Height growth is usually slower in 

 trees with a shrubby form and multiple leaders, which may be due to genetic 

 make-up, insects, disease, approach to maximum height, or a combination of 

 these. Competition affects height growth, but to a much lesser degree than 

 it affects diameter growth. 



2. Age has no direct effect on height growth rates. Some old trees have main- 

 tained constant height growth rates for the past century or more. Reduction 

 in height growth rates in older trees is caused by approach to maximum height 

 or, sometimes, by competition, insects or disease. 



3. Diameter growth rates are regulated primarily by competition. The effects of 

 site quality and genetic make-up on diameter growth of individual trees are 

 obscured by competition and are probably much less important. As with height 

 growth rates, diameter growth rates are not directly affected by age. 



4. There are indications that, in the absence of competition, juniper is capable 

 of more rapid diameter growth than pinyon and may be capable of more rapid 

 height growth as well. However, junipers appear to be more sensitive to 

 competition; they were dominated by pinyons on all five plots on which they 

 occurred. Therefore, their true potentials could not be accurately evaluated. 



9 



