Oaks: Biology and Significance of Our Majestic Giants

By Steve Barnhart, Academic Director

Oak trees and shrubs are found in many different environments and climatic zones around the world.  Some 500 species exist, primarily in temperate, subtropical and tropical regions of North America, South America (Columbia), Europe, North Africa, the near east and Asia, dropping below the equator into Indonesia. Many local populations of oak species exhibit unique characteristics, yet as a group they are able to adapt to a wide range of environmental conditions.

All oaks are members of the genus Quercus, in the plant family Fagaceae, which includes beeches, chestnuts, chinquapins and tanbark oak. Tanbark oak is not a true oak (Quercus) due to a number of significant biological differences, including flower structure and pollination. Other species which bear the term “oak” in their common names are not related at all, e.g. poison oak. Oak trees and shrubs can be deciduous, losing all their leaves seasonally, or evergreen (live oaks for example). Oaks are mainly identified by their bark, foliage and fruit (acorns).

Approximately 80 native oak species are found in the USA, with 21 of these present in California. These species are classified in 3 evolutionary lineages or sections: white oaks, red oaks and intermediate oaks. In Sonoma County, we are graced with 5 white oak, 4 red oak and 1 intermediate oak species.

Hybridization, or crossing between species, occurs within evolutionary lineages or sections.  These oak hybrids are usually fertile and thus can reproduce with other hybrids or their parental species. A common example at Pepperwood are the hybrids between blue oak and Oregon oak, which exhibit a full range of characters between the two parent species.

Oaks have been a very important resource for humans over thousands of years. Acorns have been a dietary staple for millennia - in most recent history for the acorn-gathering and oak-cultivating Native Americans of California. Oaks have also been important for cultural and religious reasons. 

Oaks perform a very important ecological role in many landscapes. Because of the food resource (acorns) and shelter (nesting places) they provide, oak-dominated plant communities have the highest diversity of wildlife species of any California landscape. Here, oaks play a central role in the community food webs, thus filling the niche of an important “keystone” species. Oaks also provide important amenities with regard to watershed integrity, local carbon balance and natural fire fuel breaks.

Unfortunately, the loss of oaks and oak habitat in Sonoma County and throughout California is occurring at an alarming rate. This loss is primarily due to urbanization and the agricultural conversion of wild land habitat. In the growing exurban areas, the ecological integrity of oak woodlands is being severely compromised because of the impact of patchy development upon wildlife species. Added to these factors are the direct impacts of construction and landscaping upon individual specimen trees as well as the increased spread of pathogens and disease. The pathogen that causes Sudden Oak Death is having a pronounced impact upon coast live oak and tanbark oak in our local wild lands, including Pepperwood.

Native oaks are a natural legacy that we all should desire to preserve. Their beauty and landscape utility are obvious, but their evolutionary and ecological significance is even more important to the long term integrity of our natural landscapes.

Join Steve Barnhart and arborist Bruce Hagan for a class on caring for oak trees on Saturday, October 19th, 2013 from 9am to 3pm at Pepperwood. Whether your yard is home to one oak or one hundred, this class will provide a comprehensive overview of proper oak tree care and management. This class costs $30 and includes a hike to visit some of Pepperwood's many oaks! Please click here to register.