Danger in the Beauty Above

September 12, 2014 Posted by: LH -Park Ranger (Yosemite Valley)
Black oak tree in Yosemite Valley's Cook's MeadowIn Yosemite Valley, the California black oaks are one of the beautiful tree species we enjoy; unfortunately that beauty comes with inherent dangers. One of those dangers is sudden oak limb drop; a mysterious and potentially deadly phenomenon. It refers to large limbs breaking off of oak trees, often without warning. Commonly, the break site is quite blunt; rounded and flat, different than the sharp splintered ends that you might expect. Most commonly it happens during the summer months when the weather is warm and winds are calm. This phenomenon is also known to happen in other species of trees, so it is sometimes referred to by other names, such as "summer limb drop," or "sudden limb drop." But oak trees, like the black oak commonly found in Yosemite Valley, are especially prone to it, and have even caused deaths in the past to a few Yosemite Valley visitors. 

So why does it happen to those unsuspecting healthy trees and on calm, windless days? What makes it so mysterious is that no one really knows the exact science behind it. Nonetheless, there are many ideas and theories out there; here are just a few of them… 

Trees draw up large amounts of water; the average oak can draw up 75-100 gallons of water per day, depending on the size of the tree and its root structure. The tree draws that water up through its root system and disperses it out to its branches where it slowly releases the moisture through pore-like stomata on its leaves. The process is known as evapotranspiration. You can witness this process by tying a plastic bag around a potted plant. The bag starts to fog up as moisture is released into the air. Some researchers believe that the limb drop occurs after the tree has not released enough of that moisture, and the weight of the water bearing down on the branch in the heat of the day causes it to suddenly break off. In some cases, after a large branch has suddenly broken off, water was reported pouring out of both sides of the break and is believed to be excess water that had not evapotranspirated. 

Another theory is that water stress (such as a lack of water, or even excess water) causes the concentration of ethylene, a plant hormone, to increase. Ethylene is a gas that is produced by all plants, and it effects things like cell growth, cell development, and even cell aging. The theory is that the increased amount of ethylene dissolves the cementation of cell walls in the wood. This leaves less material to hold the cells together, causing the branch to suddenly break. 

Others suggest that it might just be as simple as internal cracks in the branch caused by previous wounds, age, or stress that eventually spread outward, causing the branch to break off. 

Black oak and fallen limb in Yosemite Valley's Cook's Meadow

There are plenty of unknowns about summer limb drop, but one thing we do know is that there are ways to help protect yourself to ensure that you will not be injured by a falling limb. A good place to start is just being aware of what is above you; are you sitting in the shade of an oak tree? Or directly under a large limb? Who are you with? Remember to not leave grandma or the baby unattended, people who might not be able to remove themselves from that area quickly. Also think about the weather, is it a hot day with not much of a breeze? 

Sudden limb drop is one of those unsolved mysteries! As mentioned previously there are many theories out there and I've shared just a few of them with you. So now using what you know, you be the judge!  

Yosemite Valley, Nature Scene, LH

3 Comments Comments icon

  1. No response to my post above. No response - is anyone from Yosemite monitoring their own website? 3 dead children under the same conditions.
    April 01, 2018 at 10:51

  2. No response to my post above. No response - is anyone from Yosemite monitoring their own website? 3 dead children under the same conditions.
    April 01, 2018 at 10:51

  3. Penny Rittenberg
    February 17, 2018 at 03:38

    As the mother of a daughter killed by a black oak in exactly the way you describe above - I find this to be a feeble warning. She was killed on the camp ground belonging to camp Tawonga, a Jewish summer camp for children. At 8.25 am. They had not followed their own safety protocols to have an arborist check their oaks. They had not cut back the canopy or the branch. To lighten the load. In fact several 6 foot sections of the under section of that branch , that would have helped in the investigation, somehow disappeared. Where did they go? What is more important reputation or life? At 4.45 am in 2015 two 14 year old boys were killed in exactly the way you describe above. On your campground, in a designated handicapped zone. The only warnings are- beware of bears and rodents and bubonic plague. Your warning is ridiculous....dont spend more than a minute under an oak. If you hear a crack, grab baby and grandma and run. You are obliged to put up warnings. You are obliged to cut back branches that hang over a handicapped zone. Who is going to be able to "run". The oaks are taller and older as each year goes by. Climate change doesnt help. YOU NEED TO BE MORE RESPONSIBLE. The deaths of three children may not make national news - but the death of one child is a national disaster. That you do nothing about it makes it an atrocity. Link the dots and you will see that this theory is a real one. It is not a mysterious matter. Coupled with rain in the summer, followed by heat of 105 degrees, the tree will suck up moisture, it dries from the outside in, goes off balance and ALWAYS drops in the early hours of the morning. There is never any wind. In the summer months. A response would show that this is being taken seriously. No child should die in this terrible way. Crushed to death. No parent should have to see and know this. And live with it.

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Last updated: September 18, 2014

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