Are western juniper seeds dispersed through diplochory?

Authors: Longland, William - Bill; DIMITRI, LINDSAY - University Of Nevada

Submitted to: Northwest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2016
Publication Date: 5/1/2016
Citation: Longland, W.S., Dimitri, L.A. 2016. Are western juniper seeds dispersed through diplochory? Northwest Science. 90(2):235-244.

Interpretive Summary: Western juniper forests are expanding throughout their range in the American west, causing increased fire risks and reductions in forage for wildlife and livestock. To understand factors that may facilitate this woodland expansion, we investigated the potential roles of birds that eat juniper berries and small mammals that eat and store juniper seeds in production of new western juniper seedlings. We found that seeds must be removed from juniper berries and must be buried in the soil in order to germinate. Birds that eat berries and defecate seeds, such as American robins, and seed-caching rodents, such as chipmunks and kangaroo rats, can satisfy these respective requirements and may thus both contribute to production of new juniper seedlings. Furthermore, we found that the successful germination of juniper seeds and production of seedlings was enhanced when seeds pass through and are defecated by birds, and that most seed-caching rodents avoid whole juniper berries, but do harvest seeds passed by birds and store them in shallowly-buried caches. This work suggests that western juniper seedling production largely occurs through a sequential seed dispersal process in which birds consume juniper berries and pass the seeds, after which rodents harvest the seeds and cache them where they may produce new seedlings if caches persist long enough for seeds to germinate. Such two-phase seed dispersal is referred to in the literature as “diplochory.”

Technical Abstract: 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 disperse the seeds through endozoochory by defecating them. 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. We 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 (Likely, Shinn Peak) 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 3-13 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. Birds removed numerous juniper berries from plots and left seeds undisturbed at Likely, but removal of both seeds and berries by birds was negligible at Shinn Peak. Removal rates of seeds by rodents was similar to that of berries at Likely; trail cameras indicated that berry removal was due to California ground squirrels, Spermophilus beecheyi. At Shinn Peak, where the California ground squirrel did not occur, rodents removed significantly more seeds than berries and removal was attributable to scatterhoarding species. In the germination experiment, juniper seedling emergence was generally low and none of the seeds or berries placed on the soil surface produced seedlings. Among buried seeds and berries, however, emergence was significantly greater for bird-passed than for hand-cleaned seeds, both of which produced significantly more seedlings than intact berries. Our results thus indicate that birds may enhance juniper seed germinability through gut passage, that rodents harvest seeds defecated by birds and may therefore secondarily disperse bird-passed seeds, and that western juniper seeds require removal from berries and burial for seedling establishment. Sequential dispersal, or diplochory, with primary dispersal provided by frugivorous birds and secondary dispersal by seed-caching rodents, is ideal for satisfying these respective requirements.