Mickey Mantle said that it was amazing how much he didn’t know about the game he had been playing his whole life. I love that quote for it’s humility and because I find it so relatable, at least in the context of arboriculture.
At the ISA’s Penn-Del Chapter shade tree symposium at the Lebanon Valley Expo center last month, I sat in on Dr. Kevin Chase’s presentation titled ‘Pests and Diseases Coming Your Way’. While on the topic of laurel wilt disease, he posed this question: “Does anyone know why trees produce ethanol?”
As Chase’s eyes scanned the room for a raised hand, I slunk down into my seat. I didn’t know the answer, but I desperately wanted to.
The reason, as Chase went on to explain through his coaxing of public comment, is stress. Trees produce ethanol as a short-term alternative energy source when their normal energy source from aerobic respiration is compromised by environmental stressors.
This is good and bad. Ethanol production in trees is a lifeline physiologically speaking, but in a broader sense it’s also a cue for a secondary invasion of pests to attack a weakened tree system. This domino effect of stressors has been termed the mortality spiral, and ethanol wafts on the breeze of the vortex.
Laurel wilt disease is vectored by the redbay ambrosia beetle, specifically on plants in the Lauraceae family. In our native landscape in Pennsylvania, species such as sassafras and spicebush are at risk. In New York, the disease was just detected in 2025. According to Cornell University’s fact sheet on laurel wilt disease, the ambrosia beetle is about the size of President Lincoln’s eyebrow on a penny, and its borrow hole about the diameter of a paper clip wire. The ambrosia beetle carries the spore-bearing mycangia of the fungus Harringtonia lauricola from tree to tree. This is an interesting relationship, and the tree’s production of ethanol under the stress of the fungal pathogen attack is a key ingredient in the relationship.
I followed up Chase’s class with some light research on the topic of ethanol production in trees, and I came across a great article by John Kirkland in Science Findings titled ‘What Do Cocktail Parties and Stressed Trees have in common? Plenty of Alcohol!’
The 2015 article focuses on the work of Rick Kelsey, a research forester with the U.S. Forest Service Pacific Northwest Research station. At the time of the publication of the piece, Kelsey’s work spanned 25 years and focused on tree response to stress, particularly on the production of ethanol levels.
Stressors like fire, drought, compaction, flooding and disease can all lead to complications in the tree’s process of aerobic respiration. Respiration requires oxygen, and the above mentioned stressors often leads to anaerobic (oxygen-deficient) conditions.
Kirkland writes, “respiration uses oxygen and sugars to generate energy and release carbon dioxide and water. If a tree doesn’t get enough oxygen, or cells are damaged and respiration can’t occur, then the cells quickly start to disintegrate. When animal muscles, including humans, run low on oxygen during periods of moderate or strenuous use, they synthesize and accumulate lactic acid until oxygen levels are restored. When tree tissues are stressed by lack of oxygen, they may start producing lactic acid, but then quickly switch to ethanol. This is an age-old metabolic pathway that has been maintained in tree tissues for millennia. It allows the tree’s cells to produce just enough energy to survive.”
The production of ethanol is a double edge sword though. Kelsey found that some insects associate ethanol production as a sign of vulnerability. I immediately thought of the redbay ambrosia beetle and Harringtonia lauricola.
Kirkland writes, “through co-evolution, some bark- and wood-boring beetles (scolytids) have developed an ability to detect ethanol and use it as a primary attractant to find a stressed host. This is typically the behavior of nonaggressive beetle species that colonize weakened, dying or recently dead trees and not the aggressive beetles, such as the mountain pine beetle and others that attack and kill more vigorous trees. The ethanol-sensitive insects are opportunists: they use ethanol to find weekend trees that may not be capable of producing as many defensive chemicals, such as oleoresins, to keep them away.”
Kelsey made an interesting advancement in his research work by discovering that a method called headspace gas chromatography (used by criminologists to measure blood alcohol levels) could be applied to tree tissues for measuring alcohol. The tree tissues were obtained using an increment borer. It was an efficient and economical way to measure ethanol levels in trees over a broad sample space (Kirkland).
Kelsey studied fire-damaged trees in 2003. He found that severely burned trees had ethanol levels 53 times higher than trees that weren’t burned. It’s no surpise then that scorched trees also attracted more beetles as well (Kirkland).
Kelsey also found that disease can trigger ethanol production in trees. In 2013 Kelsey analyzed sapwood of coastal live oak trees in California infected with Phytopthora ramorum, the pathogen that causes sudden oak death (Kirkland).
One symptom of the disease is bleeding cankers on the trunk. Kelsey’s team found that large cankers contained high levels of ethanol. This leads to the invasion of bark and ambrosia beetles, which them leads to branch and stem failure. It is a similar relationship to the one shared between laurel wilt disease and the redbay ambrosia beetle.
After Chase’s talk and diving deeper into Kirkland’s article featuring Kelsey’s work, I understand ethanol production in trees a bit better, and the physiological role it plays in trees under stress. It’s interesting to see the interaction of trees with the pathogens and insects that affect them, for better or worse. A tree’s response to its environment is complex and nuanced. So are the ever-changing stressors of pests and disease in the modern, globalized world that we live.
But I’m sure there are things we do not see yet, which makes the search all the more exciting.
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