Blue oak woodland is an endemic natural community in the Central Valley region of California. It is dominated by blue oak (Quercus douglasii Hook. & Arn.) and has an understory of annual grasses and forbs. Most blue oak woodlands are found on privately owned cattle ranches which are currently threatened by conversion to housing, vineyards, and cropland. Land trusts are acquiring conservation easements which include terms that allow for continued commercial grazing. Some studies suggest that livestock grazing negatively impacts biodiversity, although others show that it creates a beneficial disturbance regime and can play an important role in controlling invasive plants. There is also concern that grazing may be negatively impacting blue oak seedling regeneration. Our study begins to address the assumption that livestock grazing is a conservation-compatible land use on ranches with conservation easements. Our study questions whether livestock grazing reduces oak seedling density, if invasive non-native plants such as medusahead (Taeniatherum caput-medusae L.) are reduced, and if native species richness is promoted. We monitored oak seedling density and herbaceous plant species cover in grazing exclosures over a period of five years on five cattle ranches in eastern Tehama County. Ranches were all grazed under easement terms set by The Nature Conservancy. Our study demonstrates that commercial livestock grazing practices had mixed affects on some of the conservation values of blue oak woodlands. Livestock grazing reduced oak seedling density, but it remains unknown if reduced densities will affect the long-term reproduction and health of the woodlands. Grazing also reduced the cover of invasive medusahead grass; yet native species richness and cover were not improved by livestock grazing.
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Vol. 31 • No. 4