Trees reach cone-bearing age  when relatively young: 25 years old and 1.5 to 3.0 m (5 to 10 ft) tall.  Seeds are not produced in quantity, however, until age 75 to 100, but the  long-lived pinyons continue to bear for a few centuries. A mature pinyon  usually has a broad and rounded or irregular crown, which is often almost  as wide as the tree height. Such trees are the heaviest seed producers,  since cones are found mostly in the upper half of the crown near the ends  of branches. Each cone contains about 10 to 20 seeds, which average only  4,190/kg (1,900/lb) because of their large size. A large tree in a good  crop year may yield over 9.1 kg (20 lb) of seed, and better stands will  produce an estimated 336 kg/ha (300 lb/acre). Germinative capacity of  seeds may range between 83 and 96 percent; germinative energy is about 80  percent in 7 days (9,30,67,68).

    Cone crops are either good or poor, often with cones practically absent,  but seldom intermediate (67). Although good crops tend to be localized and  occur at irregular and infrequent intervals, some are found nearly every  year somewhere over the widespread range of the species. Furthermore, cone  bearing tends to be synchronous over large geographical areas, a condition  considered to be an evolved mechanism to counteract seed predation (46).  On an average, substantial crops are produced every 4 to 7 years, but  shorter intervals of 2 to 5 years elapse with individual trees or in  certain localities (9). Generally, crops occur more frequently on better  sites over the optimum range of pinyon than at the extreme limits.

    The large, wingless seeds of pinyon are not adapted to wind  dissemination. Instead, seed dispersal beyond tree crowns depends upon the  behavior of four corvid species of birds- Clark's nutcracker, Steller's  jay, scrub jay, and pinyon jay (8). Although these species eat great  quantities of seed during the fall and may be greater predators than  rodents, they also cache large amounts for consumption during ensuing  winter months. Some of these buried seeds are not recovered by the birds,  thus providing a seed source for subsequent germination and seedling  establishment, particularly if caches are located in a suitable  microenvironment, such as alongside shrubs or downed trees (46). Steller's  and scrub jays collect seed only from open cones. In contrast, pinyon jays  and Clark's nutcrackers forage from green cones, from which seeds are  deftly extracted, and then from open cones as the season progresses  (8,71).

    Clark's nutcrackers and Steller's jays probably contribute little  towards regenerating existing woodland sites because their caches are  located at higher elevations in ponderosa pine and mixed conifer forests  or in the ecotone above pinyon-juniper woodlands (8). Thus, these species  tend to expand woodlands to upper elevations. In contrast, scrub jays and  pinyon jays cache seeds in woodland areas, the former in small, local  territories, whereas the latter transport seeds up to 12 kilometers (7.5  mi).

    Pinyon jays live in flocks of 50 to 500 birds, and it has been estimated  that during a substantial seed year in New Mexico, about 4.5 million seeds  were cached by a single flock (46). Even scrub jays, which do not exhibit  flock behavior can be important seed dispersers-a single pair of birds may  harvest and cache about 13,000 seeds from a particular crop. Pinyon jays  can carry an average of up to 56 seeds in an expandable esophagus. Scrub  jays lack this adaption, and the amount of seed that can be transported at  one time is limited to 5 or fewer seeds held in the mouth and bill. The  majority of caches by pinyon and scrub jays are single-seeded, and are  located in the transition zone between mineral soil and the overlying  organic material (8,71).

    Although rodents are known to cache seed, they should not be considered  effective seed dispersers because caches are located in middens or  underground chambers where conditions are not suitable for germination or  seedling establishment. Instead, rodents, such as cliff chipmunks, pinyon  mice, and woodrats, are major predators, caching as much as 35 to 70  liters (1 to 2 bu) of good seed (46,67). Furthermore, limited data  indicate that rodents consume large quantities of seeds taken from bird  caches (32).

    It has been suggested that pinyon trees and seed eating birds have  evolved coadaptive traits that enhance survival of both organisms. The  seed dispersing and caching behavior of birds appears related to certain  traits of the trees: large, thin-coated seeds with high energy values,  different colored seedcoats that aid visually oriented seed harvesters to  distinguish edible from aborted seeds, upward orientation of cone and  scale angle for increased seed visibility, and prolonged seed retention in  open cones because of cone orientation and deep depressions and small  flanges on cone scales (70). Furthermore, the mutual dependence of birds  and trees appears more complex than just their respective roles of seed  dispersing and food providing agents. Gonadal activity of pinyon jays, for  example, is increased before the breeding season by the combined effect of  photoperiod, the appearance of cones, and a diet of seeds (46).