Adaptations for seed dispersal and the compromises due to seed predation in limber pine.

Cone and seed morphology and phenology of the closely related limber pine (Pinus flexilis) and southwestern white pine (P. strobiformis) were studied in relation to selective seed har- vesting by red squirrels (Tamiasciurus hudsonicus) and Clark's Nutcrackers (Nucifraga columbiana) on two study sites in north central Arizona. We examined possible selective influences that favor characteristics of both wind- and corvid- dispersed seeds in the limber pine. In a densely forested site, red squirrels harvested >80% of the cones of both pine species before they opened. Seeds from cones harvested by red squirrels were unlikely to germinate. Harvesting of cones and seeds by red squirrels was hindered by pitch and several other cone features. Pitch was more effective in slowing red squirrel harvesting on limber pine than on southwestern white pine. When red squirrels were absent from the open study site, > 70% of the cones opened on the trees. Clark's Nutcrackers usually harvested seeds from open cones. Limber pine as compared to southwestern white pine was more evenly and completely harvested by nutcrackers, probably because the cones ripened synchronously within a tree and asynchronously among trees. Cones of southwestern white pine ripened synchronously within and among trees, thus saturating both nutcrackers and squirrels. Differences between the two pines are most readily interpreted as adaptations for increasing seed dispersal, hindering limber pine's main seed predator (red squirrel) and facilitating seed harvest by its most important seed disperser (Clark's Nutcracker). We conclude that squirrels in the genus Tamiasciurus are an important constraint on the devel- opment of cone characteristics that enable efficient seed harvest by birds. Furthermore, adaptations of pines for seed dispersal depend on the relative harvest by predators and dispersers during the different stages of cone-ripening phenology.