Coast Live Oak Woodland Biodiversity Hotspot Under Threat by Sudden Oak Death

Oak woodlands provide habitat to a wide variety of native California flora and fauna.

NPS / Danielle Parsons

By Danielle Parsons, Plant Biological Technician

October 2021- Did you know that California oak woodlands harbor the highest diversity of any broad habitat type in the state? According to Richard Standiford of the UC Cooperative Extension Forestry Department, well over 300 terrestrial vertebrates utilize oak woodlands at some time during the year. Oak woodlands are also extremely important for a wide range of insect life.

In addition, California’s fire-prone Mediterranean landscape is home to nine species of oak, all of which are believed to have evolved with fire on the landscape. This means that how oak woodlands are managed can have a significant impact on the populations on animal species and how resilient California landscapes are to fire.

I grew up in a home nestled in coast live oak woodlands. Now, I monitor this ecosystem type as part of the San Francisco Bay Area Network Plant Community Monitoring Program. Spending many hours hugging these trees to take DBH (diameter at breast height) measurements has given me an intimate perspective on the current state of the oaks. Notably, most of our 12 permanent monitoring plots show signs of sudden oak death (SOD).

Phytophthora ramorum, the organism responsible for SOD, is not a native pathogen of U.S. forests. Thus, our native oak trees have not evolved defenses against it. According to an article published by University of California Agriculture and Natural Resources, Phytophthora does not preferentially target stressed trees! However, due to the historical prevention of fires in California, woodlands are becoming overcrowded, facilitating the transmission of Phytophthora. Unfortunately, sudden oak death is easily transmitted, and animals (including humans) are prominent distributors by way of car and bike tires, dog’s paws and fur, shoes and clothes, wind, and water.

Historical suppression of wildfires in California has led to a build up of fuels that force low-ground fires to creep up into tree canopies, quickly becoming out of control. The Indigenous peoples of California historically stewarded oak woodland and grassland ecosystems by practicing regular cultural burns on millions of acres across the state, contributing a myriad of ecological benefits to the area. According to the California Department of Forestry and Fire Protection, one benefit of these burns is that they remove understory vegetation. This releases water stored in the shrubs and increases the water supply because fewer plants are absorbing water. This benefit stands out to me because of the historical drought we have been experiencing for the last 30 years. While observing the dense overgrowth of poison oak encroaching upon our local coast live oak woodlands, I am curious: what impact could prescribed and cultural burnings have on the distribution of sudden oak death? To collectively steward the land and the oaks for future generations to come, multi-agency collaboration with local Indigenous people and community scientists is needed to further study this topic.

Further Reading

• San Francisco Bay Area Inventory & Monitoring Network:
  • Plant Community Monitoring
  • Invasive Plant Early Detection
• Sudden Oak Death

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