Species Spotlight: Quercus douglasii Hook. & Arn.

Quercus douglasii is an endemic California oak tree, affectionately called “the blue oak” due to its distinctive blueish-green leaf color. It is common throughout California and known for growing in locations that create a bathtub ring around California’s great Central Valley. The blue oak is often the oak featured in iconic photographs of California’s golden hillsides dotted with majestic trees. Considered a foothill tree species, with the majority of its range at elevations of approximately 150 to 700 ft above sea level, several significant groves of very old blue oaks persist on the central valley floor, most notably along the Arcade Creek corridor of Sacramento. The most significant threats to the persistence of blue oak woodlands in California are urban development and a natural regeneration rate below replacement level.

The blue oak is a tough-as-nails member of the oak family, growing in the hottest, driest, and rockiest locations within its range. It is commonly found as the only tree species within open oak savannas or in more exposed and xeric locations within more complex multi-species oak woodland assemblages. When co-located with valley oak (Q. lobata ), blue oaks will be found on the higher creek terraces or in open grassland spaces, away from the perennial and ephemeral waterways the valley oak prefers. California’s Mediterranean climate features wet, cool winters with rainfall occurring on average from November until early May, and hot dry summers without rainfall for the other six months of the year. Blue oak leaf and acorn surfaces sport a waxy surface, helping to retain critical moisture in the summer months where daytime temperatures can frequently exceed 37 °C for weeks at a time. In the most prolonged water-scarce conditions, blue oaks can be drought-deciduous, dropping their leaves and sometimes their entire acorn crop in order to conserve resources for the following season. Achieving a height of 18 m, an open-grown blue oak has a rounded canopy with medium spread and a trunk from 25 cm to 2 m wide. As a very slow-growing oak species, it is common for annual growth to be limited to less than 30 cm a year, especially for mature trees. The ability of blue oak to re-sprout from the root crown helps this species maintain its presence and regrow canopy quickly in California’s fire-prone landscapes.

Named for the Scottish botanist David Douglas, Q. douglasii is one of more than 80 species of plants and animals that bear his last name. He introduced more than 240 species of plants to Britain from his three North American expeditions between 1823 and 1834. Today, California’s blue oak woodlands are a focus for conservation and restoration efforts. As home to more than 300 vertebrate species, oak woodlands are recognized as sustaining higher levels of biodiversity than virtually any other terrestrial ecosystem in California.

For a robust collection of information on various California oak species, visit www.californiaoaks.org.