Pinyon grows best on the higher, wetter sites  of the woodland zone, just below the ponderosa pine type (40,67). At these  elevations trees reach their tallest heights and tend to develop single  stems. At lower elevations, in contrast, bushy and sprawling crowns are  characteristic. Pinyons may be multistemmed, although to a lesser extent  than junipers. They usually exhibit straight, but short and rapidly  tapering boles, which diverge into many large sinuous branches.

    Growth of pinyon, though maintained with little loss of vigor throughout  the life of the tree, is extremely slow. Height growth of saplings, for  example, is only about 10 to 15 cm (4 to 6 in) yearly, and mature trees  grow even more slowly, averaging 5 to 10 cm (2 to 4 in) per year. Diameter  growth also is slow, especially on poor sites, where 80 to 100 years can  elapse before diameters at breast height reach even 10 to 15 cm (4 to 6  in). On better soils, however, 150-year-old trees may grow to a diameter  of 30 cm (12 in). Mean annual diameter growth of pinyon culminates at  about 1.8 cm (0.7 in) per decade, when trees are approximately 50 years  old. The gross annual increment on sample plots in northern New Mexico  woodlands also reflects the slow growth rate, averaging about 0.42 m³/ha  (6 ft³/acre) for pinyon alone, and 0.66 m³/ha (9.5 ft³/acre)  for all species. Gross cordwood increment for all species was 0.88 m³/ha  (0.14 cord/acre) (38,66,67).

    Pinyon is a long-lived tree, maturing in 75 to 200 years. Dominant trees  in a stand are often 400 years old, and pinyons 800 to 1,000 years old  have been found. Depending on the site, mature trees range between 3.0 and  15.5 m (10 to 51 ft) in height and 15 to over 76 cm (6 to 30 in) in d.b.h.  Although large trees are common, especially in northern New Mexico,  pinyons generally are small trees, usually less than 10.7 m (35 ft) tall  and 46 cm (18 in) in diameter (66,67). The largest living pinyon recorded  grows in New Mexico and measures 172 cm (68 in) in d.b.h., 21.0 m (69 ft)  in height, and has a crown spread of 15.8 m (52 ft) (2).

    Because of the growth habit of woodland species, tree volumes are not  only difficult to measure but can vary more than 300 percent for trees of  the same diameter. There is less variation in well-formed trees, however,  and the gross volume of a representative pinyon with a basal diameter of  30 cm (12 in) and 7.6 m (25 ft) tall is 0.22 m³ (7.7 ft³),  measured to a 10-cm (4-in) top. Woodland volumes vary considerably,  depending on species composition and density. In northern New Mexico and  Arizona, mixed stands may contain cordwood volumes ranging from about 5.0  to 157.4 m³/ha (0.8 to 25 cords/acre), with average volumes of about  69.3 m³/ha (11 cords/acre). Cordwood volumes of nearly pure pinyon  stands average about 75.6 m³/ha (12 cords/acre). Low volumes are a  reflection of the small trees generally associated with woodlands. The  average size tree in many New Mexico stands is only 15 cm (6 in) in  diameter at ground line and about 2.7 m (9 ft) tall (16,67).

    The density of pinyon in woodlands varies considerably, ranging from few  or none to several hundred stems per hectare. Nevertheless, the density in  a typical northeastern Arizona stand averages about 235/ha (95/acre) in  stems less than 7.6 cm (3 in) in d.b.h.; 200/ha (81/acre) from 7.6 to 15  cm (3 to 6 in) in d.b.h.; and 89/ha (36/acre) more than 15 cm (6 in) in  d.b.h. (67). Mixed woodlands are denser and more productive than pure  stands of either pinyon or juniper, and can approach or exceed 3,459  stems/ha (1,400/acre) (9,57). The higher values have been attributed to  differences in rooting habit and drought tolerance of the two species. The  shallower penetrating roots of pinyons limit interspecific root  competition for soil moisture in mixed stands. This, combined with the  lower photosynthetic rate of pinyons compared to that of junipers at  higher water stresses, allows more complete site utilization in mixed  stands (10,25,57). The average number of pinyons suitable for Christmas  trees varies from a few trees per acre to a fairly large number.