Seed Production and Dissemination
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).
- Burns, Russell M., and Barbara H. Honkala, technical coordinators. 1990. Silvics of North America: 1. Conifers; 2. Hardwoods. Agriculture Handbook 654 (Supersedes Agriculture Handbook 271,Silvics of Forest Trees of the United States, 1965). U.S. Department of Agriculture, Forest Service, Washington, DC. vol.2, 877 pp. http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm
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