Evolution, Ecology and Floristics in Northern California - Current Knowledge and Unexplored Realms
Keeley, Jon E. .
Fire history of the San Francisco East Bay region and implications for landscape patterns.
THE San Francisco East Bay landscape is a rich mosaic of grasslands, shrublands, and woodlands that is experiencing losses of grassland due to colonization by shrubs and succession towards woodland associations. The instability of these grasslands is apparently due to their disturbance-dependent nature coupled with 20th century changes in fire and grazing activity. This study uses fire history records to determine the potential for fire in this region and for evidence of changes in the second half of the 20th century that would account for shrubland expansion. This region has a largely anthropogenic fire regime with no lightning-ignited fires in the vast majority of years. Fire suppression policy has not excluded fire from this region; however, it has effectively maintaining roughly similar burning levels in the face of increasing anthropogenic fires and decreased the size of fires. Fire frequency parallels increasing population growth until the latter part of the 20th century, when it reached a plateau. Fire does not appear to have been a major factor in the shrub colonization of grasslands, and cessation of grazing is a more likely immediate cause. Because grasslands are not under strong edaphic control, rather their distribution appears to be disturbance-dependent, and natural lightning ignitions are rare in the region; it is hypothesized that prior to the entrance of people into the region grasslands were of limited extent. Native Americans played a major role in creation of grasslands through repeated burning and these disturbance-dependent grasslands were maintained by early European settlers through overstocking of these range lands with cattle and sheep. 20th century reduction in grazing, coupled with a lack of natural fires and effective suppression of anthropogenic fires, have acted in concert to favor shrubland expansion. -DU
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1 - USGS Western Ecological Research Ctr., Research, Sequoia Field Station, 47050 Generals Hwy, Three Rivers, California, 93271, USA
vegetation type conversion
Presentation Type: Symposium or Colloquium Presentation
Date: Tuesday, August 1st, 2006
Time: 2:30 PM