"Jeffrey pine seed dispersal in the Sierra San Pedro Mártir, Baja Calif" by Stephen B. Vander Wall, Mark S. Enders et al.

The Sierra San Pedro Mártir (SSPM) in northern Baja California, Mexico, is a remote mountain on the southern edge of the California Floristic Province, a vegetation that includes the Sierra Nevada of California and western Nevada. Unlike most forests in the southwestern United States, the forests of the SSPM have been logged and have experienced only light grazing in recent years, and wildfires have not been suppressed until recent decades. The SSPM represents one of the best examples of an intact presettlement forest in the California Floristic Province. We investigated the mode of seed dispersal of Jeffrey pine (Pinus jeffreyi) in the SSPM, and compared to that of the eastern slope of the Sierra Nevada, to determine how this process might differ between nearly pristine and heavily altered forests. We found that seed dispersal appears to occur by the same means in both forests (i.e., scatter hoarding of seeds by rodents), but that seedling establishment is much less successful in the SSPM. We considered several hypotheses for why this difference might exist, including the size of seed crops, fire regimes, climate change, grazing by domestic ungulates, and lack of facilitation by shrubs (ie, nurse plants), but none of these explanations appears adequate to account for the difference. Instead, we suggest that the slow rate of seedling establishment at the SSPM is due to a greater consumption of seeds cached by California chipmunks (Tamias obscurus) , who experience a relatively long period of euthermic winter activity in the warm climate of the SSPM. This greater consumption leaves few seeds to germinate. Future studies should test the relative importance of these alternative causes for low seedling recruitment at the SSPM.

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  • Adam Floyd