Western juniper (Juniperus occidentalis) “berries ” are consumed by frugivorous birds, which then defecate and disperse the seeds. However, rodents also consume western juniper seeds, and seedlings can establish from rodent scatterhoards. To explore relative roles of potential seed dispersal agents, we experimentally quantified removal rates by birds and rodents of intact western juniper berries versus seeds cleaned either manually or by passage through bird guts at two northeastern California sites (Likely and Shinn Peak). We also conducted seedling emergence experiments with berries, hand-cleaned, and bird-passed seeds placed on the soil surface or buried at a depth typical of rodent caches. Birds removed juniper berries from plots and left seeds undisturbed at Likely, but bird activity was negligible at Shinn Peak. Rodent removal of seeds versus berries was similar at Likely, but at Shinn Peak rodents removed significantly more seeds. In the seedling emergence experiment, juniper seeds or berries on the surface failed to produce seedlings. When buried, however, emergence was significantly greater for bird-passed than for hand-cleaned seeds, which both produced significantly more seedlings than intact berries. Our results indicate that: (1) birds may enhance western juniper seed germinability through gut passage, (2) most rodent species do not harvest intact berries, (3) rodents harvest seeds defecated by birds and may therefore secondarily disperse bird-passed seeds, and (4) seeds must be removed from berries and buried to establish seedlings. Sequential dispersal, or diplochory, with primary dispersal by frugivorous birds and secondary dispersal by seed-caching rodents, is ideal for satisfying these respective requirements.
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Vol. 90 • No. 2