Seed Fate Pathways of Antelope Bitterbrush: Dispersal by Seed-Caching Yellow Pine Chipmunks

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Seed fate pathways were investigated for antelope bitterbrush (Purshia tri- dentata), a common and economically important shrub, from fall 1988 to fall 1992 at the Whittell Forest and Wildlife Area in Little Valley, Washoe County, on the east slope of the Sierra Nevada in western Nevada. Rodents harvested bitterbrush seeds from late July to September, taking them either directly off the fruiting twigs or picking up fallen fruits from the ground surface and carrying from 63 to 80% of the seeds away intact from shrubs. Radioactively labelled bitterbrush seeds placed under four source were removed and scatter hoarded primarily by yellow pine chipmunks (Tamias amoenus). Caches con- tained a mean of 4-1 1 seeds and were located within 25 m from the source shrubs. Cache depth and microhabitat selection were generally favorable for bitterbrush seedling estab- lishment. Chipmunks and other rodents found many of the seed caches and either ate the seeds (53%) or recached them elsewhere. Chipmunks probably also carried some intact seeds to larders in hibernacula to use as a winter food source. In spring, >99% of emerging seedlings occurred in clumps containing from 2 to 104 individuals. An estimated 5.3 and 8.5% of those seeds cached by rodents and 1.1 and 2.4% of the seeds produced by in 1990 and 1991 produced emergent seedlings the following spring. Seedling mortality rates during their 1st yr ranged from 78 to 92%. Grazing by rodents was the most important source of mortality during early spring, and most seedling deaths were caused by desiccation during summer. Despite considerable attrition of individuals within clumps, most mature shrubs consisted of two or more individuals. The seed fate pathway that resulted in the highest probability of seedling establishment at this study site was harvest, dispersal, and scatter hoarding by chipmunks, and subsequent neglect of those caches. Retrieval of cached seeds by chipmunks followed by redistribution of those seeds to neglected secondary caches also resulted in many seedlings. Modes of seed dispersal based solely on abiotic agents appear to be insufficient to account for sig- nificant recruitment of bitterbrush seedlings.