Protecting the Health of a Diseased Forest

Sudden oak death is devastating forest ecosystems along the California and Oregon coast. Infection is particularly impacting the native tanoak tree, an ecologically and culturally important species that has long been a staple of west coast forests. The mass loss of tanoak trees has far-reaching impacts, including on biodiversity, Indigenous tribes, and climate change. Factors like these continue to shape the health and future of local forests. 

Point Reyes National Seashore and Golden Gate National Recreation Area are located on the ancestral home of the Coast Miwok, who stewarded this land for thousands of years prior to European colonization. The introduction of logging, ranching, global diseases, and development disrupted long-standing Coast Miwok forest management practices that supported healthy forests. By 1994, when sudden oak death was discovered in Marin County, CA, the forest was weak and susceptible to disease.

Now, in West Marin and throughout the California and Oregon coast, trees are rapidly dying – or have already died – from sudden oak death infection. This StoryMap aims to give you an overview of the disease, local and wide-scale impacts, and management strategies from the current land manager, National Park Service (NPS), who recently entered a  government-to-government partnership  with the  Federated Indians of Graton Rancheria .

You can scroll through the StoryMap or skip ahead to sections of interest using the navigation bar at the top.

Oaks and tanoaks are members of diverse forest ecosystems throughout Marin County. These complex, interconnected systems support a variety of life. Canopy trees reach great heights and subcanopy trees form a middle layer, shading the ground where shrubs and smaller plants can grow. Animals of all kinds – including humans – create relationships with the plants, finding food and habitat in and around the forest layers.

One tree species that makes up the mid-canopy of many mixed forests is the sturdy tanoak.

Phytophthora ramorum (P. ramorum) is the nonnative water mold pathogen that causes the disease sudden oak death (SOD).

Incidentally, P. ramorum is a cousin of the water mold that caused the Irish Potato Famine, Phytophthora infestans.

P. ramorum was likely brought to this area through the nursery trade – non-native plants were probably shipped here from overseas and then planted locally. It was first discovered in the United States in Marin County in 1994. ⁵  Ten years later, the first trees in Point Reyes were diagnosed with SOD. ⁶ 

There are now nearly 80 plant species that can be infected by, or host, P. ramorum. ⁷  In the forests of Marin, the most relevant of these include California bay laurels, redwoods, several kinds of true oaks (including coast live oak and black oak), and tanoak.

Certain trees, including California bay laurel, host P. ramorum on their leaves. For these trees – called foliar hosts – infection isn’t lethal, and they usually only experience leaf blight (the browning and possible death of leaves). However, the pathogen reproduces almost entirely on the leaves of foliar hosts, making them a very important (and dangerous!) vector for the infection of other trees.

From a foliar host, P. ramorum spores move through water droplets splashing from tree to tree. In a normal amount of wind and rain, a spore can move up to 600 feet from where it started. ⁵  It can also spread on hiking boots, bike wheels, infected plant material like leaves or firewood, or in the nursery trade. Eventually, it finds another host – one for whom infection might be a lot more dangerous.

Statistics show the devastating impact that SOD is having on tanoaks.

The map below shows the locations of trees that were tested for P. ramorum between 2005 and 2021. The dots on the map show individual trees' test results, but they do not represent all the trees affected by SOD.

To monitor SOD’s impact at Point Reyes, park scientists took photos of Inverness Ridge over the course of several years. Below, you can see how the view changed over time.

Use the arrows on the right and left of the image to compare views.

P. ramorum is not the only factor affecting tree stress and mortality in Point Reyes. Climate change plays a role in the spread of SOD, and these impacts fit in a larger picture of forest health. It remains to be seen how these complex relationships shape the future of tanoaks.

Climate change is expected to increase average temperatures at Point Reyes. Warming temperatures shift habitat structures, and this change potentially brings new invasive species or pests to the area and drives out native species.

However, it remains to be seen whether climate change transforms Point Reyes into a wet or dry climate. P. ramorum spreads on water droplets and has its highest transmission rates following warm, wet winters. If a warming Point Reyes also becomes wetter, the transmission of P. ramorum will likely increase. ¹²  If, on the other hand, it gets drier, transmission will likely decrease, but it won’t go away entirely.

It’s also likely that rates of extreme weather will increase at Point Reyes – including intense drought and violent storms like the lightning storm that caused the 2020 Woodward Fire.

Drought conditions inhibit the spread of P. ramorum, but the pathogen can remain dormant until moist conditions return. Extreme storms, on the other hand, increase P. ramorum transmission. In extreme storms, pathogen spores can travel up to three miles – 26 times farther than in normal rain and wind conditions. ⁵ 

Dead trees, branches, and leaves also provide fuel for wildfires. For this reason, forest stands affected by SOD will generate more intense fires. Although fuels eventually break down and return to the soil, the death of more trees from SOD continues the cycle. ¹³  

With this information, it’s challenging to say whether climate change will increase or decrease the spread of SOD. However, it’s important to keep in mind that SOD is here to stay, and there isn’t a likely future in which it is eradicated in this area. The bigger question, then, is how does SOD fit into the complex system of forest health and a changing climate? And how much disturbance and stress can trees withstand?

In some areas of California and Oregon colonized by P. ramorum, tanoaks as we knew them are gone. This wouldn’t be the first time a large tree has become functionally extinct because of a pathogen.

On the east coast, the American Chestnut tree was wiped out by the chestnut blight pathogen over the course of fifty years in the early 20th century. ²  Currently, thirty years after its discovery in Marin County, P. ramorum has only colonized part of the tanoak’s natural range. It’s possible that management efforts can contain it, leaving some tanoak range untouched and saving millions of trees. If not, like the American Chestnut, the tanoak could disappear completely.

The absence of such a major species would have significant effects. Without tanoaks, the forest is less biodiverse and therefore less resilient. When a joint redwood-tanoak forest stand, for example, becomes a redwood-only stand, it is susceptible to complete eradication if anything were to happen to the redwoods. If that happened, understory plants, trees, resident animals, and migrating birds would all have to shift their habitats.

Coast Miwok stewardship played a role in maintaining biodiversity. The landscape we see today is a remnant of what it was prior to European colonization, and it was through global networks resulting from colonization that SOD came to this area. The potential loss of the tanoak would prevent Indigenous people from accessing crucial food sources like the tanoak acorn and compound on the great loss of diversity that European colonization caused. ³ 

Use the slider to compare views between 2007, on the left, and 2022, on the right.

So what can be done? Forest managers can take action to preserve the tanoak and limit the spread of P. ramorum. In many places, including at Point Reyes, they have already been implementing different strategies.

When making management choices, it’s essential to also consider the future of other types of bark canker hosts, including coast live oaks and black oaks. While they are more resistant than the tanoak, they are also severely affected and play important roles in their ecosystems.

Managing the spread of Sudden Oak Death is important. Forest managers have a responsibility to care for the forest and the tanoak. Even though SOD is having devastating impacts, and there isn’t an effective treatment so far, it isn’t time to give up – whether we can find ways to potentially treat trees infected with SOD, or focus on slowing down the disease’s spread, tanoak is a tree worth saving.

Your actions have an impact. In your interactions with forests, in Point Reyes or anywhere, be mindful of plant disease!

Thank you to the National Park Service and  Point Reyes National Seashore Association  advisors who shared their time, expertise, and insight for the creation of this StoryMap.

1. California Bay Laurel. (n.d). National Wildlife Federation.  https://www.nwf.org/Home/Educational-Resources/Wildlife-Guide/Plants-and-Fungi/California-Bay-Laurel 
2. Bowcutt, F. (2013). Tanoak Landscapes: Tending a Native American Nut Tree. Madroño, 60(2), 64–86.
3. One Tam. (2023). Forest Health & Resiliency Report.  https://www.onetam.org/our-work/forest-health-resiliency  [Pending Publication]
4. Edson, E., Farrell, S., Fish, A., Gardali, T., Klein, J., Kuhn, W., Merkle, W., O’Herron, M., and Williams, A., eds. (2016). Measuring the Health of a Mountain: A Report on Mount Tamalpais’ Natural Resources. https://www.onetam.org/media/pdfs/peak-health-white-paper-2016.pdf
5. Sudden Oak Death at Redwood National and State Parks? (2022). National Park Service, 8. https://www.nps.gov/redw/learn/nature/sod.htm
6. Weinberg, J. (2010). Sudden Oak Death in Point Reyes National Seashore and Golden Gate National Recreation Area. https://irma.nps.gov/DataStore/DownloadFile/612572
7. USDA APHIS | Phytophthora ramorum. (n.d.). https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/pests-and-diseases/phytophthora-ramorum
8. Sudden Oak Death—Point Reyes National Seashore (2022). U.S. National Park Service. https://www.nps.gov/pore/learn/nature/diseases_sod.htm
9. Cobb, R. C., Haas, S. E., Kruskamp, N., Dillon, W. W., Swiecki, T. J., Rizzo, D. M., Frankel, S. J., & Meentemeyer, R. K. (2020). The Magnitude of Regional-Scale Tree Mortality Caused by the Invasive Pathogen Phytophthora ramorum. Earth’s Future, 8(7), e2020EF001500. https://doi.org/10.1029/2020EF001500
10. Panorama Environmental. (2019). Biodiversity, Fire, and Fuels Integrated Plan. Marin Municipal Water District. https://www.marinwater.org/sites/default/files/2020-09/Biodiversity%2C%20Fire%20and%20Fuels%20Integrated%20Plan.pdf
11. Moritz, M., Moody, T., Ramage, B., & Forrestel, A. (2008). Spatial distribution and impacts of Phytophthora ramorum and Sudden Oak Death in Point Reyes National Seashore. 65.
12. California Forest Pest Conditions 2021. (2021). California Forest Pest Council, 42. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fseprd1008290.pdf
13. Forrestel, A. B., Ramage, B. S., Moody, T., Moritz, M. A., & Stephens, S. L. (2015). Disease, fuels and potential fire behavior: Impacts of Sudden Oak Death in two coastal California forest types. Forest Ecology and Management, 348, 23–30. https://doi.org/10.1016/j.foreco.2015.03.024

• One Tam. (2018). Marin Countywide Fine Scale Vegetation Map, Tamalpais Lands Collaborative. https://tukmangeospatial.egnyte.com/dl/uQhGjac1zw
• Bowcutt, F. (2014). Tanoak conservation: A role for the California Department of Fish and Wildlife.
• California Oak Mortality Task Force. (n.d.). https://www.suddenoakdeath.org
• Halstead, R. (2021). Marin sudden oak death infections decline amid drought. The Mercury News. https://www.mercurynews.com/2021/12/03/marin-sudden-oak-death-infections-decline-amid-drought
• Phytophthora ramorum, a fungus-like oomycete. (n.d.). Calinvasives. https://www.calflora.org/entry/pathogen.html?id=pth1
• Tanoak (Lithocarpus densiflorus). (1995). Oregon Wood Innovation Center. https://owic.oregonstate.edu/tanoak-lithocarpus-densiflorus
• Tanoak, Notholithocarpus densiflorus. (n.d.). California Native Plant Society. https://calscape.org/Notholithocarpus-densiflorus-()
• Thelen, M. (2020). Climate Change: How Will Marin Adapt? Climate Change, 31.