PS 42-127
Roles of frugivorous birds and seed-caching rodents in seed dispersal and seedling establishment of western juniper
Seed dispersal of western juniper (Juniperus occidentalis) appears to be convergent on a strategy utilized by fruit-bearing trees in that this conifer produces fleshy female cones (a.k.a., “juniper berries”) that are consumed by frugivorous birds, which then defecate the seeds contained within. However, many rodent species also consume, and presumably cache, western juniper seeds, and establishment of juniper seedlings in clusters typical of emergence from rodent scatterhoards is not uncommon. Moreover, granivorous rodents frequently remove juniper seeds from bird feces. We therefore sought to understand the relative roles of frugivorous birds and seed-caching rodents as seed dispersal agents for western juniper. We conducted experiments at two northeastern California sites (Shinn Peak, Likely) using small fenced plots that were selectively permeable to either birds or rodents to quantify removal rates of intact juniper berries versus cleaned juniper seeds over 3-7 days. Each plot represented a cafeteria choice test containing 100 juniper berries, 100 hand-cleaned juniper seeds, and 100 bird-passed seeds. We also conducted germination experiments with berries, hand-cleaned, and bird-passed seeds that were either placed on the soil surface (simulating berries falling from trees or seeds defecated by birds) or buried at a depth typical of scatterhoards made by rodents.
Results/Conclusions
At Likely, birds removed numerous juniper berries (mean ± s.d. = 78.0 ± 43.3) from plots and left seeds undisturbed, but removal of both seeds and berries by birds was negligible at Shinn Peak. Rodent removal of seeds (hand-cleaned: 77.5 ± 31.5, bird-passed: 54.2 ± 48.2) at Likely was similar to berry removal (71.8 ± 24.2); trail monitors indicated that berry removal was due to a larderhoarding species, Spermophilus beecheyi. At Shinn Peak, where S. beecheyi did not occur, rodents removed significantly more seeds (hand-cleaned: 91.0 ± 9.9, bird-passed: 35.5 ± 29.0) than berries (2.0 ± 2.8), and removal was attributable to scatterhoarding species. Juniper seedling emergence was generally low in the germination experiment, and no seeds or berries placed on the soil surface produced seedlings. Among buried seeds and berries, however, emergence was significantly greater for bird-passed (5.7%) than for hand-cleaned seeds (1.6%), both of which produced significantly more seedlings than intact berries (0.1%). Our results thus indicate that birds may enhance juniper seed germinability through gut passage, that rodents secondarily disperse bird-passed seeds, and that western juniper seeds require removal from berries and burial for seedling establishment. Seed-caching rodents are ideal for providing the latter dispersal service.