Periodic fires maintained the tree communities on the hilltops and edges of the valley as either savannas or woodlands. Cessation of burning changed the structure of these open woodlands by allowing repopulation of the openings with rapidly growing Douglas-firs which began to shade out oaks, madrones, and maples. Settlers introduced exotic plants and grazing by cattle, sheep and horses which significantly reduced native grasses. Approximately 12% of the lower elevation 1851 woodland and savanna/prairie communities remain.
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