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Seed dispersal of western and Utah junipers: the role of scatter-hoarding rodents.
AuthorDimitri, Lindsay A.
AdvisorVander Wall, Stephen B.
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The dispersal of Juniperus seeds is generally attributed to frugivores, both avian and mammalian. Junipers produce female cones with a husk enclosing the seeds that closely resemble fruit, thus they are often referred to as berries. These cones differ among juniper species with some species producing very fleshy, resinous cones such as those of western juniper, Juniperus occidentalis, while others are dry and leathery such as those produced by Utah juniper, J. osteosperma. Frugivorous birds and mammals have been found to disperse some juniper species through endozoochory by consuming juniper cones and passing intact seeds in their feces. However, seed dispersal in many other juniper species is not fully understood. Scatter-hoarding rodents have been recorded removing seeds and/or cones of several juniper species but have mostly been considered seed predators. In this study I documented harvest and caching of western and Utah juniper seeds, showing that scatter-hoarding rodents should be considered seed dispersal agents for these two juniper species. I also compared removal of western and Utah juniper cones and seeds and observed that removal of seeds from seed and cone stations of both species by scatter-hoarding rodents was nearly equal. However, the removal rate of cones was very different. Frugivores were recorded removing western juniper cones at Shinn Peak, while no frugivores were recorded removing Utah juniper cones at either site. Kangaroo rats, Dipodomys spp., removed Utah juniper cones at both sites but never removed western juniper cones. Results of this study suggest that scatter-hoarding rodents play a role in the dispersal of western and Utah juniper seeds. My study confirms that there is very little evidence of frugivores dispersing the seeds of Utah juniper, and I suggest that scatter-hoarding rodents are the primary dispersal agents of this species. Drier, less nutritious cones and larger seeds could be signs of adaptation in Utah juniper to dispersal by scatter-hoarding. The cones of western juniper appear to be more rewarding for frugivores and consumption of western juniper cones by frugivorous birds and mammals has been previously described. This study is the first to document caching of western juniper seeds by scatter-hoarding rodents. There is evidence to suggest that western juniper seeds are dispersed in a two-phased dispersal process, diplochory, where scatter-hoarding rodents cache seeds that have been initially dispersed through endozoochory by birds. Scatter-hoarding rodents could play a role in the dispersal of other Juniperus species where their ranges overlap and should not be regarded strictly as seed predators.