Juniper, Pinyon, Oak, and 

 Mesquite Volume Equations for 

 Arizona 



David C. Chojnacky 



INTRODUCTION 



Arizona supports vast and diverse juniper, pinyon, oak, 

 and mesquite woodland forests. Much of northern Arizona 

 contains juniper-pinyon woodlands. In the central part of 

 the State and on the lower slopes of the southern moun- 

 tain ranges, an oak-juniper-pinyon woodland thrives. 

 Mesquite woodlands occur between the southern mountain 

 ranges interspersed with Sonoran Desert and grasslands. 

 Arizona's woodlands, by USDA Forest Service definition 

 (USDA FS 1985), have at least 10 percent crown coverage 

 from trees of woodland species. 



Recent economic and management needs for information 

 on Arizona's woodlands prompted an extensive inventory 

 in 1985 (USDA FS 1985). Primary objective was to deter- 

 mine total woodland tree volume. Because direct wood- 

 land volume measurement is time consuming and costly, 

 data were collected in the inventory on a subsample of 

 trees for later volume equation development. This paper 

 reports on volume equations constructed from that data. 



Previous efforts to develop volume equations for pinyon 

 and juniper species revealed that basal diameter, crown 

 dimensions, total height, and number of basal stems are 

 important predictor variables of tree volume (summarized 

 in Chojnacky 1985). Similar results have also been found 

 for oak and mesquite woodland species (Gronski 1987; 

 Pillsbury and Kirkley 1984; Whisenant and Burzlaff 1978). 



Juniper, pinyon, and oak volume estimation specific to 

 Arizona was summarized by Barger and Ffolliott (1972). 

 They published volume tables for pinyon, Utah juniper, 

 alligator juniper, and Gambel oak. These were produced by 

 modifying, extending, or adapting work by Howell and 

 Lexen (1939), Howell (1940), and Gevorkiantz and Olsen 

 (1955). These volume tables require total height and basal 

 diameter for pinyon or diameter at breast height for juni- 

 per and oak. Since the Barger and Ffolliott summary, 

 several juniper, pinyon, and oak volume equations have 

 been developed for Arizona's Hualapai, Havasupai, Fort 

 Apache, and San Carlos Indian Reservations (Chojnacky 

 and Ott 1986; Chojnacky 1988). These equations predict 

 volume from basal diameter and total height. For 

 Arizona's mesquite, Olson (1940) and Andrews (1988) 

 each constructed volume equations from basal diameter. 



Based on past woodland volume work in Arizona and 

 elsewhere, some form of tree diameter measurement 

 and total tree height are the most common volume predic- 

 tor variables. From previous work (Chojnacky 1985; 

 Chojnacky and Ott 1986), three principles relating the 

 importance of diameter and height to woodland volume 

 prediction seem clear: 



1. Volume is adequately predicted from the combina- 

 tion variable, diameter squared times height divided by 

 1,000 (DSQH). 



2. The volume-to-DSQH relationship is linear, except 

 for the largest trees. 



3. The volume-to-DSQH relationship differs between 

 single-stem and multiple-stem trees of a given species. 



These principles are not surprising, because they follow 

 from well-known mensuration results for many tree spe- 

 cies. Because they also reflect past woodland volume 

 results from other studies, additional strategies for vol- 

 ume equation construction were not sought. 



DATA COLLECTION 



Volume data were collected from 291 plots (fig. 1) 

 owned or managed by the State of Arizona; private indi- 

 viduals or corporations; Papago, Hopi, and Kaibab Indian 

 Reservations; Coronado and Prescott National Forests; all 

 Arizona Bureau of Land Management Districts; and some 

 miscellaneous public entities. The field plots were 

 subsampled from the 1985 Arizona woodland inventory 

 (USDA FS 1985). Plots were proportionately selected 



it = Mesquite 

 P = Pinyon 

 = Oak 



Figure 1— Location map of the 291 study plots. 



1 



