Why are all the tanoaks dying?

Tanoak mortality in Humboldt County. Photo Credit, Yana Valachovic, UCCE Humboldt County
Tanoak mortality in Humboldt County. Photo Credit, Yana Valachovic, UCCE Humboldt County

During a recent walk through Sam MacDonald Park in San Mateo County, I noticed many downed, brown-leafed tanoak trees amongst the towering redwoods. At first I wondered why the county would cut down so many trees, but then I realized the cause was none other than Phytophthora ramorum, better known as sudden oak death.

A disease which first appeared in the mid-1990s is now seen in redwood forests from Humboldt County to Big Sur. I was shocked by how much of the forest was impacted by this disease and wondered what effect it will have on the future of redwood forests.

Sudden oak death is a disease caused by the pathogen Phytophthora ramorum. No one knows the source of the disease, but it has spread throughout the United States through the nursery industry. It affects many different plant species but has primarily impacted tanoaks (a redwood forest inhabitant) and other oak tree species. The pathogen is not lethal to most tree species it infects — except for tanoaks. Large numbers of tanoaks are dying all along the coast of California, which leads us to wonder what this means for the ecology of our forests.

Sudden oak death is not the only disease to significantly impact a tree species. The stately American chestnut tree was once prominent in the eastern U.S.; it populated the forests and lined the streets. In the early 1900s, a fungus known as chestnut blight arrived, and within a few decades caused the extinction* of the American chestnut tree. The loss of these trees had huge impacts on the forest structure and biodiversity of eastern deciduous forests.

More recently, the mountain pine beetle has destroyed millions of acres of pine trees, such as lodgepole pine and ponderosa pine, in western North America from Mexico to central British Colombia. Large areas of dead pine trees are a fire hazard, and the loss of these pine trees adversely impacts biodiversity.

Research is underway to learn more about how the sudden oak death pathogen spreads, how we can stop it, and what impact it will have on our forests.

Stay tuned for Richard Campbell’s blog next week as he addresses the role sudden oak death plays in our redwood forests. Learn more and take a look at one of our research grant reports on the decline of tanoaks in redwood forests.

*The American chestnut is functionally extinct, meaning a few individuals may be living, but they are not viable (in the case of the chestnut, seedlings sprout from old stumps, but they will die before they can reproduce). Efforts are underway to breed blight-resistant chestnut trees.

Avatar for Deborah Zierten

About the author

Deborah joined the League's staff in 2013 as the Education & Interpretation Manager. She brings with her extensive experience teaching science, developing curriculum and connecting kids to the natural world.

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