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No Place Like Southern California – The Engelmann Oak’s Established Roots in Our Region

Nov 7, 2022 | MSHCP, Species

A magnet for butterflies and birds, the Engelmann oak (Quercus engelmannii) inhabits the smallest range of any oak tree in the southwestern United States. Named after German-American botanist George Engelmann, this vulnerable species lives in a narrow band of the southern California foothills from Pasadena to Baja California.

The overwhelming majority of existing oaks are located within San Diego County (93%), with the remaining range located in Riverside County (5%) and Los Angeles/Orange counties (about 2%). Most of the range in Riverside County is in a 12×12-mile area in the Santa Rosa Plateau Ecological Reserve near Murrieta that includes about 7,500 acres of undisturbed Engelmann oak woodlands and other ecosystems.

Generally found in mesas and woodlands above the dry coastal plain, the tree is also referred to as the mesa oak. Pollination occurs through wind, and the tree flowers from April to May. Reproduction takes place in three phases: acorn production, acorn germination, seedling/sapling establishment, and survival.

The average lifespan of the species ranges from 50 to 80 years, but some trees on the Santa Rosa Plateau are old-timers, living between 80 and 130 years. Yet, this range does not exist without challenges. The Engelmann oak was at risk of being eradicated throughout most of its historic range in Los Angeles County and remains vulnerable today.

Similar patterns are emerging in San Diego and Riverside counties, where only two major populations (Santa Rosa Plateau in Riverside and Black Mountain in San Diego) are currently safeguarded from continued regional growth.

The MSHCP provides a strategy that helps streamline infrastructure and housing projects while identifying critical habitat for conservation. As a result, the Western Riverside Conservation Authority is working to conserve land so that plant and animal species have space conserved for their long-term survival. This is good news for the Engelmann oak since the MSHCP will conserve approximately 19,070 acres of suitable habitat for many generations to come.