Cone and seed traits of two Juniperus species influence roles of frugivores and scatter-hoarding rodents as seed dispersal agents

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

Submitted to: Acta Oecologica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2017
Publication Date: 10/7/2017
Citation: Dimitri, L.A., Longland, W.S., Vander Wall, S.B. 2017. Cone and seed traits of two Juniperus species influence roles of frugivores and scatter-hoarding rodents as seed dispersal agents. Acta Oecologica. 85(2017):93-103. https://doi.org/10.1016/j.actao.2017.09.012.
DOI: https://doi.org/10.1016/j.actao.2017.09.012

Interpretive Summary: Western and Utah juniper forests are expanding across their range, leading to a reduction of grasses and other plants important for sustaining grazing animals. We sought to understand how dispersal of seeds of these two juniper species by animals contributes to juniper expansion. Dispersal of juniper seeds is generally attributed to fruit-eating birds that consume juniper berries and defecate the seeds within, but here we document that small mammal species that store juniper seeds in shallowly buried seed caches play at least as important of a role in production of new juniper seedlings as do birds. Utah juniper berries, which are dry and leathery, were avoided by fruit-eating birds, but were harvested by various small mammals,(especially piñon mice), which removed the seed contents and cached them. By contrast, western juniper seeds are contained within a pulpy berry, which were consumed in great numbers by fruit-eating birds. After birds defecated western juniper seeds, they were then harvested and cached by small mammals. Thus small mammals are the terminal seed dispersers for both juniper species. Given the limited mobility of small mammals relative to birds, this implies that management approaches to combatting juniper expansion should be focused near the edges of existing juniper forests.

Technical Abstract: Seed dispersal in Juniperus is generally attributed to frugivores that consume the berry-like female cones. Some juniper cones are fleshy and resinous such as those of western juniper (Juniperus occidentalis), while others are dry and leathery such as those of Utah juniper (J. osteosperma). Rodents have been recorded harvesting Juniperus seeds and cones but are mostly considered seed predators. Our study sought to determine if rodents play a role in dispersal of western and Utah juniper seeds. We documented rodent harvest of cones and seeds of the locally-occurring juniper species and the alternate (non-local) juniper species in removal experiments at a western juniper site in northeastern California and a Utah juniper site in western Nevada. Characteristics of western and Utah juniper cones appeared to influence removal, as cones from the local juniper species were preferred at both sites. Conversely, removal of local and non-local seeds was similar. Pi~non mice (Peromyscus truei) were responsible for most removal of cones and seeds at both sites. We used radioactively labeled seeds to follow seed fate and found many of these seeds in scattered caches (western juniper: 415 seeds in 82 caches, 63.0% of seeds found; Utah juniper: 458 seeds in 127 caches, 39.5% of seeds found) most of which were attributed to pinon mice. We found little evidence of frugivores dispersing Utah juniper seeds, thus scatterhoardingrodents appear to be the main dispersal agents. Western juniper cones were eaten by frugivores, and scatter-hoarding is a complimentary or secondary form of seed dispersal. Our results support the notion that Utah juniper has adapted to xeric environments by conserving water through the loss of fleshy fruits that attract frugivores and instead relies on scatter-hoarding rodents as effective dispersal agents.