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Jeffrey Pine Seed Dispersal in the Sierra San Pedro Mártir, Baja California, Mexico

submitted by beocirbe 1 year and 2 months ago
The Sierra San Pedro Mártir (SSPM) in northern Baja California, Mexico, is a remote mountain at the southern edge of the California Floristic Province, a vegetation type that includes the Sierra Nevada of California and western Nevada. Unlike most forests in the southwestern United States, the forests of the SSFM have never been logged and have experienced only light grazing in recent years, and wildfires have not been suppressed until recent decades. The SSFM 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 it to that from 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 (i.e., 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.
La Sierra de San Pedro Mártir (SSPM) en Baja California Norte, México es un conjunto remoto de montañas en el extremo sur de la Provincia Florística de California, un tipo de vegetación que incluye la Sierra Nevada de California y el oeste de Nevada. A diferencia de la mayoría de los bosques en el suroeste de los Estados Unidos, los bosques de la SSPM no se han talado, solo han experimentado pastoreo ligero en años recientes y no se ha acometido la extinción de los incendios hasta décadas recientes. La SSPM representa uno de los mejores ejemplos de un bosque intacto pre-asentamiento en la Provincia Florística de California. Investigamos la forma de dispersión de la semilla del pino Jeffrey (Pinus Jeffreyi) en la SSPM y lo comparamos con la de la zona este de la Sierra Nevada, para determinar de qué manera este proceso puede diferir entre bosques casi prístinos y bosques sumamente alterados. Encontramos que la dispersión de las semillas parece producirse mediante los mismos medios en ambos bosques (i.e., acumulación dispersa de semillas por parte de roedores) pero que el establecimiento de las plántulas es mucho menos exitoso en la SSPM. Consideramos varias hipótesis de por qué podría existir esta diferencia, incluyendo el tamaño de las semillas, los regímenes de fuego, el cambio cliimático, el efecto de ungulados domésticos y la falta de facilitación por parte de arbustos (i.e., plantas nodrizas), pero ninguna de estas explicaciones parece ser adecuada para explicar la diferencia. En lugar de esto, sugerimos que la baja tasa de establecimiento de plántulas en la SSPM se debe a que el clima más cálido resulta en un periodo más largo de actividad eutérmica en invierno de las ardillas de California (Tamias obscurus), lo que resulta en un mayor consumo de semillas acumuladas durante el invierno, lo que implica que muy pocas de ellas puedan germinar. Estudios futuros deben examinar la importancia relativa de estas causas alternativas del escaso reclutamiento de plántulas en la SSPM.


Topic: Biology

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