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Reconstructing Late Holocene Changes in Climate, Vegetation, and Fire Regimes in the Sierra San Pedro Martir Mountains, Mexico, through Microfossil Analysis

Rand Evett, Assistant Researcher, University of California Berkeley  revett@earthlink.net

In association with Scott Stephens' fire history study in the Sierra San Pedro Martir Mountains in north central Baja California, I am reconstructing large scale changes in vegetation and fire regime that have occurred in the coniferous forest from a couple of thousand years to the present. I have sampled sediment from several promising chronologically stratified sites. From each layer of sediment, I am chemically extracting a variety of microfossils that are identified and counted microscopically to provide insight into the paleoenvironment at the time of deposition. Fossil pollen, when preserved, provides valuable evidence of the presence and extent of a wide range of vegetation, particularly trees. Opal phytoliths, microscopic bodies of silica that are deposited in plants and released into the soil as they decay, are very useful for determining the nature of understory vegetation, particularly grasses. Diatoms record changes in the hydrologic regime of the sediment. Changes in the concentration of macro and microscopic charcoal indicate long term changes in the fire regime.

Preliminary analysis of a long term record, extracted from a 5.5 meter exposed cut bank in La Encantada Meadow, shows significant changes in vegetation between wet meadow, grassland, savanna, and forest states over time. Coarse sandy strata alternate with fine organic strata, suggesting a record of very large El Nino flood events that mobilize large amounts of sediment from the uplands and deposit them in the meadow basin followed by periods of stability that allow vegetation to develop. The vegetation that develops depends on the moisture status of the meadow; periods of forest invasion into the meadow probably represent long term periods of drought.

Several short term sedimentary records from forested sites are being analyzed to determine changes in understory vegetation that may have occurred with European settlement and the introduction of livestock appoximately 200 years ago. Fire scar records indicate a major shift to less frequent and larger fires beginning at this time. I am testing two hypotheses for this shift: 1) Livestock grazing significantly changed the composition and reduced the productivity the forest understory, making it much less likely to carry a fire. 2) The long term climate changed at this time from less than normal precipitation to greater than normal precipitation, keeping the understory moist and less likely to carry a fire well into the fire season.

La Grulla Meadow - 5 m high cut bank with Brandon and Ernesto Franco

La Encantada Meadow from near Blue Bottle Mtn.

Freshwater diatom with microscopic charcoal

Phytolith photo - 400x



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