Is western juniper (Juniperus occidentalis) dispersed through diplochory?

Background/Question/Methods

Throughout its native range in the western United States, western juniper (Juniperus occidentalis) woodlands have been expanding for >100 years due to factors such as fire suppression, grazing and climate change. Understanding juniper seed dispersal is necessary to comprehend mechanisms underlying this expansion. Western juniper is generally thought to be dispersed through endozoochory by frugivorous birds, which consume the fleshy female cones (a.k.a., “juniper berries”). Because granivorous rodents frequently remove juniper seeds from bird feces, we sought to understand if rodents are important seed dispersers, whether acting as primary dispersers or as secondary dispersers in a diplochory scenario by caching bird passed seeds. In two preliminary studies we quantified removal of juniper berries and/or seeds from three replicate plots that excluded either birds or rodents at two northeastern California sites (Shinn Peak, Likely). In Experiment 1, we offered 250 juniper berries to both birds and rodents and monitored their removal over time. In Experiment 2, we compared rodent removal of 100 juniper berries to that of 100 hand-cleaned juniper seeds and 100 bird-passed seeds. Camera traps above exclosures allowed us to monitor removal.

Results/Conclusions

In Experiment 1, juniper berries were generally removed more thoroughly from plots permitting bird access. This pattern was stronger in a winter 2011 trial than a fall 2010 trial. In the winter, berries were usually completely removed within a few days by birds, but often went completely untouched by rodents even after several weeks. In Experiment 2, both seeds and berries were removed by rodents, although relative removal rates differed significantly between sites; berries and seeds were removed at similar rates at Likely, whereas seeds were removed significantly more rapidly than berries at Shinn Peak. At Shinn Peak, seeds were removed more rapidly and the number left was lower for seeds (bird-passed: mean ± s.d. = 53.0 ± 14.0; hand cleaned: 6.0 ± 9.5) than for berries (77.7 ± 12.1). At Likely, berries were removed more rapidly and thoroughly (berries left = 1.0 ± 1.0; hand-cleaned seeds = 8.3 ± 11.9; bird-passed = 28.0 ± 33.6). Camera traps revealed that Spermophilus beecheyi was responsible for berry removal at Likely. Peromyscus spp. and/or Tamias amoenus, both known scatterhoarders, were recorded removing seeds and berries alike. These results indicate that seed caching rodents may act as secondary dispersers of western juniper.