October 18, 2021

Why Are So Many Oak Trees Dying?


You, too, can set up your own oak tree drought study and contribute valuable information to scientists tracking our oak woodlands.


Why Are So Many Oak Trees Dying?

(l-r) Topanga Canyon Docent Megan Williams and RCDSMM Conservation Biologist Jen Mongolo setting up oak study plots. One discovery was of native oak boring beetles that took advantage of the stressed condition and hit the trees hard.

The calls have been coming in weekly since June 2015. Oaks falling in backyards, oaks collapsing in the park, oaks standing dead along the creeks, oaks brown and dead on the mountain sides. All over the Santa Monica Mountains, coast live oak trees are dying and falling over. What is going on?

Four years of drought certainly has set the stage for high stress, but why are some oak trees okay and others not, often standing next to each other?

Coast live oaks are incredible trees, well adapted to the drought cycles that are part of the local climate patterns. They have thrived in the Santa Monica Mountains for thousands of years weathering all kinds of changes. Using stored energy reserves, they can recover from complete canopy loss due to fire and withstand the attacks of numerous insect pests and diseases.

Clearly, something has gone wrong. Even Grandmother Oak, one of the oldest oaks in Topanga State Park, is showing signs of severe dieback.

The Topanga Canyon Docents and Resource Conservation District of the Santa Monica Mountains (RCDSMM) Stream Team volunteers have stepped up to help sort this out. Jen Mongolo, RCDSMM Conservation Biologist and Lena Lee from National Park Service (NPS) prepared a grid map so that we could generate random 25-square-meter plots within Trippet Ranch in Topanga State Park.

Working with State Parks archeologist Barbara Tejada and ecologist Suzanne Goode, we developed a study design that avoided impacts to any sensitive resources, but was randomized to allow robust statistical analysis.

With maps in hand, RCDSMM biologists Lizzy Montgomery and Krista Adamek, along with Watershed Steward Dylan Hofflander and Topanga Canyon Docent Megan Williams joined Jen and Rosi to set up 14 plots for sampling using temporary flagging tape.

As part of the docent training on Saturday, October 24, the enthusiastic group of new and experienced docents, as well as Stream Team volunteers headed out with clipboards and data sheets in hand. We tagged more than 100 coast live oaks with small round metal tags that will allow us to monitor the condition of those individual trees over time. Volunteers measured the size and condition of all tagged trees, photographed the plots, and looked carefully for evidence of both native and non-native wood-boring beetles, as well as other signs of insect or disease. The composition of the understory plants, as well as slope, aspect, and GPS points were also noted.


This snapshot of current conditions is the first step in the process. With the volunteer help of Jen, Dylan and Lizzy, the next step is to develop a database that will provide information on how many trees are dead, the condition rating of the other trees and any relationships between slope, aspect, soil condition or understory condition. With this information in hand, we can then follow the response of the trees to whatever rains happen to come our way this winter. Next October, we will come back and repeat our data collection to document any changes.

If this sounds like a fun project, we would love your help! The fourth-grade students at Topanga Elementary School and students at Manzanita School will also be collecting oak tree condition data for this effort. Volunteers from the NPS participated to learn the process and hope to set up plots in other parks in the Santa Monica Mountains.

Homeowners can also provide valuable information.


Why Are So Many Oak Trees Dying?

One live and one dead oak side by side at Trippett Ranch. Four years of drought set the stage for high stress, but why are some oak trees okay and others not, often standing next to each other?

If you have trees tagged from an Oak Tree Report, you can use those tags and update the condition ratings from your report. Even if you don’t have tagged oaks, a simple map identifying the individual trees on your property can get you started. Ideally, this citizen science effort will spread out via interested oak lovers to many areas.

One of the more interesting findings right away was the identification of native oak boring beetles that took advantage of the stressed condition of the trees to hit them hard. Many of the dead oaks have been heavily infested.

While it is not exactly clear what killed the tree, the drought stress or the beetles, the correlation is important to understand. Within the past 10 years, two new threats to oaks have become problems throughout southern California. The gold spotted oak borer has killed more than 100,000 oak trees in San Diego and Orange Counties. It has recently been identified in dead trees in Idyllwild and Green Valley. Native to Arizona, this small pest is transported from location to location by firewood.

Then there is the polyphagus shothole borer recently arrived from Asia, which spreads a fungus that infects more than 200 species of trees and kills many of them. First detected on avocados, this pest has spread throughout the eastern part of LA County, causing the removal of more than 100 trees at the Huntington Gardens and the L.A. County Arboretum.

Transported by firewood, there is currently no effective way of containing these pests once they are in a tree. The only containment method that seems to work is to cut down the tree, chip it and tarp the chips to solarize them and kill any remaining larvae. Fortunately, neither of these new pests was observed in Topanga State Park as yet.

For information on these new threats to our native trees, go to: www.gsob.org or http://ucanr.edu/sites/socaloakpests/Polyphagous_Shot_Hole_Borer/.

The RCDSMM is seeking funding to manage and expand this oak drought monitoring effort to collect and analyze the data as well as share this ground-proofed information with other researchers who are working on larger landscape level scales.

Dr. Darrell Jenerette at UC Riverside has started using remote sensing satellite imagery to detect changes in oak and other tree cover. Having more specific details of numbers of trees per plot could set the stage for answering a variety of other questions on the extent of the die-off, and where we might be able to help prevent future die-offs.

We also would like to find a way to integrate this information into the LA County Oak Woodlands Conservation Management Plan to assist the County in tracking and assessing the changes in oak woodlands over time.


Coast live oaks are iconic trees that not only shape our landscape visually but, as keystone species, provide habitat and food for more than 5,000 species of native insects, 150 birds, 105 species of mammals and 58 species of reptiles and amphibians. Losing our oak woodlands could have serious ripple effects throughout our ecosystem.

Getting a better handle on how our oak trees are responding to the current drought will help guide us toward strategies to support them as the climate patterns shift.


To learn how to set up your own oak drought study and share your information, please contact Rosi Dagit, rdagit@rcdsmm.org.

Rosi Dagit is Senior Conservation Biologist and Certified Arborist for the Resource Conservation District of the Santa Monica Mountains.