Branches falling off tree

If a tree falls in the forest and no one is there, does it still make a sound?

Jehookah Jarmon, Gorvenstof, Ukraine

  • It makes the sounds of branches snapping and the almighty thump when the trunk hits the hard ground. Sound is sound and doesn’t rely on people to interpret it. Jack Hill, St Albans, England
  • Only if it’s a hoary old chestnut like this one! Alan Williams-Key, Madrid, Spain
  • Yes. And isn’t that a rather arrogant question, as what about all the animals that presumably hear it? Allan Fraser, Darlington, England
  • It depends on what you mean by ‘sound’? The air waves would still vibrate but without an ear/listening device to receive them there would be no sound. Conaldo, London, UK
  • Sound is something within human experience. Outside of this it’s just air (or whatever other medium) vibrating. If someone is there to hear it, it makes a sound, if not, it causes rapid movement of particles. Seth, Edinburgh, Scotland
  • I’ve always thought the point of this question was: if there’s no way to establish whether there was a sound, then it ultimately doesn’t matter. In fact, you could say that the definition of a sound is its detection, rather than the physical phenomenon. Of course, once we develop an instrument that can detect vibrations we couldn’t otherwise hear, a previously non-existent sound comes into being.
    This is a profound question with ethical connotations. For example, if we are unaware of somebody’s suffering, does it exist? We can only be aware of it if they tell us or we infer it from our common humanity. But suppose vegetables suffered when we picked them, but had no way of telling us so. We might discover a vegetable nervous system, but we wouldn’t know if the nerve-signals represented pain, information or something we have no conception of. The sensible conclusion would be to save your subscription to the Vegetable Liberation Front. Tom Boddington, Leeds, UK
  • Yes, of course. Allan Fraser, Darlington, UK
  • Well i suppose it comes down to God and the imagination, if all is imagined then no, because the tree had not been imagined in the first place. If there is a God, or by some miracle we simply exist, then yes, because as i suspect and hope, we humans are not the centre of the universe. An infintismal amount of happenings are occuring without our knowledge or interpretation. Luke Van Opstal, Sydney Australia
  • My guess is that it makes compression waves in the air that could be interpreted – with the right biological equipment – into sound. Jason Gaskell, Newcastle, England
  • No it doesn’t. Sound is the product of our ears. If there are no membranes (ears, microphones etc) to vibrate then there is no sound. Darren, Lancaster, UK
  • Define ‘sound’. When the tree hits the ground, its kinetic energy will be partly transferred into vibrations in the air called ‘sound waves’. If these come into contact with a sensitive membrane such as an ear-drum and are then interpreted by a brain (human or otherwise), they will be experienced as what we call ‘sound’. Although the sound-waves have an objective existence, I suppose that strictly speaking they are not ‘sound’ until they are subjectively interpreted by a brain. Still the way the question is framed is a bit anthropocentric – if the sound waves caused the ear drum of a passing bird or squirrel to vibrate, what they would experience is presumably also called ‘sound’- though it is possible it would not sound quite the same as it would to a human. Steve Fitzpatrick, London, UK

  • The traditional answer is that it makes a sound but not a noise – noise being a subjective judgement.
    However Quantum Mechanics gives us the Observer Problem. Basically this means that as all events are essentially random (on a sub-atomic scale), until they have been observed we can’t be sure they’ve actually happened.
    So strictly the tree can’t actually be said to have fallen down until it has been observed lying prone. It just exists in a half-and-half state where, like the Duke of York’s men, it is neither up nor down.
    The noise also exists in this ghostly state (and so did this post until you read it). Martin Porter, Glossop, Derbyshire
  • The short answer is yes, inasmuch as sound waves are propagated. Fragano Ledgister, Atlanta, USA
  • Of course it does. The idea that a member of the human race has to be present at any occurrence for it to be true is quite ludicrous.
    Sounds are being created all over the world which are never heard by humans and as a final straw there are supersonic sounds which are beyond the range of the human but can be heard by other creatures such as bats. Jack Hill, St Albans, England
  • The vibrations will not go unnoticed forever because a falling tree impacts dramatically on the environment around it, perhaps setting aflutter the very butterfly whose wing fannings led to Katrina which led to the deafening outcries for reform in disaster preparedness. Steve, Kansas City, USA
  • An even older corollary to this “If a man speaks and there isn’t a woman to hear him, is he still wrong?” Peter Cranny, Liverpool, UK
  • The act of observing something changes the nature of that very same thing. Now of course you can object, like Einstein: “Do you mean to tell me the moon isnt there until you look at it?” … but nevertheless … Quantum theory does seem to lead to the conclusion that reality or existence or whatever you want to call it, is not observer independent. David, Florence USA
  • Yes it does. If you ARE around to here it, it would make a sound. So when you’re not around to here it, it would make a sound. If you don’t believe that, put a recording device near a tree that’s about to fall and then go away. If the tree doesn’t already smash the recording device, listen to the sounds on the tape for proof that it does make a sound. Joe Nossote, Reno, Nevada, United States
  • This is only a ‘conundrum’ if one is guilty of logical equivocation. If one defines ‘sound’ as a vibration in the air, then the answer is ‘yes’; if one defines it as a perception of some kind – say, involving the human ear and brain – then the answer is ‘no.’ As with anything else, if you’re clear on the meaning of your terms such questions become decidable; if you equivocate between two meanings, they remain ‘paradoxes’ (though spurious ones). Here’s a comical equivocation on the word ‘some’ that should make the point clear. Some dogs have fuzzy ears; my dog has fuzzy ears; therefore, my dog is some dog! (Moral: don’t equivocate) Mark Kennedy, Toronto, Canada

  • What if you can’t hear the wood for the trees? Matt, Cardiff Wales
  • It strikes me that all the answers so far give rise to a more interesting question – why does the number of answers to an item in N&Q vary in inverse proportion to its importance! Paul Thompson, Perth Scotland
  • Imagining that there is sound out there in the universe is the same as believing that colours exist. Sound and colours are the result of the brain interpreting the waves that hit the ear or the eye. If there is no eye or ear around (or other organs of living creatures that can perceive them) there are no sounds or colours. N Beckett, Krefeld, Germany
  • Yes even if there were no humans animals or insects around God would hear it. God is everywhere and knows everything. Patricia, St Charles, Mo US
  • Common sense tell as that all things exist whether we are there or not to experience them; otherwise we wouldn’t bother going on holiday in case our destination is not there. Common sense also must tell us that vibrations are not sounds until our ear and brain interpret the vibrations into understandable or enjoyable information we need to survive. bill, london england
  • What length of beard makes me unattractive to the opposite sex? Any of the above would qualify…. Get out more … or look at yourself retrospectively Professor Knob, Oxfordshire Oxford
  • It does becaus it makes a vibration Ben Hawksworth, Nottingham, England
  • If no consciousness was there the probability wave did not collapse so it never fell. Stephen, Oxford Oxon
  • If no one is around to perceive it then there is no tree, there is no forest and there is no sound. Jarrod, Sydney Australia
  • Yes, it does. The world and beyond are greater than one’s perception, which is limited. Marcia, Birmingham, England
  • “If a tree falls in the forest with no ears to hear does it make a sound. It matters not for the tree has fallen.” Westerners should avoid Eastern philosophical queries. The riddle of the tree is ancient and ask not of sound but of loss in nature. Shana Vasquez, Dubuque USA
  • Okay, I’ve heard enough on this topic now. Or did I? 🙂 Richard, Maidenhead, UK
  • If no one is there, there is no forest. Thaddeus Morling, London, UK
  • “IF” implies it hasn’t happened therefore, there is no sound. Jeannie, Queens USA
  • I cannot believe this question is even taken seriously, and yet it is. Of course the falling tree makes a sound, whether it is ‘heard,’ or not! The ear, does NOT create sound, it is only able to transmit sound; the eardrum reacts to the sounds around it, that are of sufficient strength to cause it to vibrate; the ear does NOT vibrate to cause the sound of the tree falling, quite obviously. Put on a good pair of earmuffs, and then snap a piece of wood, or smash a plate, whatever you want, and ask yourself, would you have heard it if it hadn’t been for the earmuffs? The sound waves were formed, but prevented from activating your eardrum, by the earmuffs, but nevertheless, be honest with yourself, you know the sound waves were created, and you know you would have heard them if it hadn’t been for the earmuffs. If you have broken a piece of wood in your hands, you would have felt the shock-wave that creates the sound, within your very hands. Look at that aeroplane flying above you at 14,000 feet, you cannot hear it, for the sound waves dissipate before they reach your eardrum, yet you know jet engines make huge amounts of noise when they work, and if they were not working, and therefore making noise, then the aeroplane would not be flying – you know all these things! You may not be able to hear the aeroplane because of the distance between yourself and it, but you know the sound is there, you know it is being made, completely independently of your hearing. Sound does not depend upon you hearing it, you hearing sound, depends upon your proximity to the source of the sound, the cause. Your ear does not create sound, your ear reacts to sound, to conduct it to your brain, and make you aware of its presence. Does the falling tree make noise, yes, every time. David, Devon

  • Yes it does make a sound. But my old teachers told me it didn’t. Samuel, Christchurch New Zealand
  • When the tree falls in the forest, sound waves are generated that impinge on your eardrum if you are there. The sound waves are processed by your cochleas and neuronal signals are transmitted to your auditory cortex. The action potentials from the nerves from your cochleas act on the auditory cortex to produce the perception of sound. Therefore the sound is the response of the neurons in your auditory cortex. Any creature that has the perception of sound will hear the sound of the crash. There is no sound in the sound waves themselves. The sound is produced by the action potentials and synapses and only registers as sound if the action potentials reach the auditory cortex. The same situation exists for all our senses. Our entire perception of the world we live in comes to us through the action potentials in the nerves that reach the cortex in our brains. You never actually see the world first hand but only through the interpretation of the action potentials and synapses in the neurons in the cortex that come from the nerves from our senses. You register the result as your perception of the world that we live in. The confusion results when people assume that the sound waves are the sound. David W-Smith, Adelaide, Australia
  • It does not. There is no ear involved therefore “sound” does not exist. Rhonda, Pittsburgh USA
  • This is a philosophical as well as a scientific question. Scientifically, the answer would be yes. Just because there is no one in the forest to hear the sound does not mean that the sound did not take place. If once places a tape recorder in the forest, then later plays the recording after a tree fell, then we know that the sound occurred. Another way to address this would be to consider a blind person standing outside. The blind person does not see any light, so does that mean that there is not daylight? Does one have to be sighted for there to be light? No. Light is a form of radiation. It is visible to most people, but not to everyone. So if no one is in a vacant area of Siberia during daytime, does that mean that light never occurred in that area of Siberia? Recently, the Pioneer spacecraft recorded the sound of interstellar space. No one ever heard this before. So does this mean that the sound of interstellar space only occurred for the first time in 2013, or has it been there all along and only now do we have the technology to hear it? The latter is correct. As for the philosophical part of the question, this is best left to the Aristotle’s of our time. Lawrence Leffler, San Diego United States
  • No, their would be no sound or a tree or a forest. Everything is a figment of your imagination if you don’t see it or hear it, then it does not exist in your reality Tyler Goble, Annapolis, US
  • From a purely selfish point of view nothing exists unless I am there to observe it. That’s what gives me life! Phil Davis, Cheltenham UK
  • Many people do not see an issue with the question; I think this is due to the kind of minds these people have. Perhaps they possess mechanical minds as opposed to imaginative minds. Many years ago I found myself pondering if the material universe would exist if there was no conscious life present to experience it; on balance I concluded No, the material universe would not exist. Then I came across the tree question. My answer clearly is No, the tree did not make a sound. Please do not ask me to rationalise that answer – it is irrational, but so to is existence (of anything). David, Wigan, UK
  • Of course! Just because it doesn’t make sound from a larynx (vocal chords) doesn’t mean there’s no thud! Man u guys are wrong in the head! Annie, Co. Kerry Ireland
  • To quote the great Forrest Gump: “I think that it may be both.” Chris Moles, United States
  • Dwell on this – if there was no conscious life would the physical universe still exist? Think deeply on this question and I believe you will have problems in answering either YES or NO. David, Wigan, UK
  • I love reading all of your excellent theories – smarty pants 🙂 Missy , yardley USA

Add your answer

Tree branches will grow to give the most leaves the most light, even if that means growing sideways. Trees need light for photosynthesis, which is how green plants generate their energy.

There are other factors that affect the way branches grow as well. Gravity pulls the branches downward. And branch growth is affected by the wind. Part of the trade-off any tree has to make is between gathering light, staying stable in the wind, and succeeding against nearby competitors. So when branches grow crookedly, that’s part of a tree’s overall survival strategy.

Trees have sensors that detect light and gravity. From the moment a tree begins its life, it knows which end is up. Trees will generally attempt to grow toward the light and away from gravitational pull. But, as a tree gets older, its branches tend to grow more outwards than upwards. That’s so the tree can cast a wider net to catch the light of the sun.

Our thanks to:
Dr. Robert B. Jackson
Assistant Professor
Department of Botany
Nicholas School of the Environment
Duke University
Durham, NC

The EarthSky team has a blast bringing you daily updates on your cosmos and world. We love your photos and welcome your news tips. Earth, Space, Human World, Tonight.

Tree Sucker Removal And Tree Sucker Control

You may have noticed that an odd branch has started growing from the base or the roots of your tree. It may look much like the rest of the plant, but soon it becomes apparent that this strange branch is nothing at all like the tree you planted. The leaves may look different, it may produce inferior fruit or it may be a different kind of tree all together. What is going on? Your tree has developed a sucker.

What is a Plant Sucker?

You are probably thinking, “What is a plant sucker?” Essentially, a plant sucker is an effort by the tree to grow more branches, especially if the tree is under stress, but you have taken perfect care of your plant and it wasn’t under any stress. Besides, that does not explain why your tree has suddenly switched varieties.

Chances are, your tree is actually two trees spliced or grafted together. With many ornamental or fruiting trees, the desirable tree, for instance a key lime, is grafted onto the rootstock of an inferior but hardier related variety. The top of the tree is perfectly happy, but the lower half of the tree is under a certain amount of stress and biologically will try to reproduce itself. It does this by growing suckers from the root or lower stem. Tree suckers can also grow on non-grafted trees, but are most common on grafted ones. This explains what is a plant sucker.

Tree Sucker Control

It is better to try to prevent a tree sucker rather than having to deal with tree sucker removal. Here are some tips to help with tree sucker control:

  • Keep plants in good health. Many times, the rootstock on a tree will start to grow plant suckers when the additional stresses, like drought, overwatering, disease or pests, threaten the tree.
  • Don’t over prune. Over pruning can stimulate the growth of tree suckers. To prevent a tree sucker, try not to cut into growth that is more than a few years old, if possible.
  • Prune regularly. While over pruning can cause plant suckers, regular healthy pruning can help with tree sucker control.

Tree sucker – Remove or Let Grow?

While you might be tempted to leave a tree sucker, remove them as quickly as possible. A tree sucker will sap the energy away from the healthier and more desirable branches on top. Chances are, you will not be pleased by the plant produced by the tree sucker. Remove them to improve the health of the plant overall.

Tree Sucker Removal

Tree sucker removal is easy to do. Tree sucker removal is done in the same way pruning is performed. Using a sharp, clean pair of pruning shears, cleanly cut the plant sucker as close to the tree as possible, but leave the collar (where the tree sucker meets the tree) to help speed the wound recovery. Perform this tree sucker control as soon as you see any plant suckers appear so that you put less stress on your tree.

Of all our plants, trees take the longest to develop and so it is not only heart-breaking, but a significant set-back to a landscape when a tree that is 10-20 years old is destroyed in a storm. Many of these disasters could be prevented with proper pruning early in a tree’s life. Besides preventing disasters, pruning trees properly when young will help them to develop more beautifully, make them stronger, less expensive to maintain as they get older and keep them healthier.

A young tree, like any young being, is vulnerable and needs some extra care. And trees are often a costly investment, both for the plant and for the planting. So since few arborists will come out for the fifteen minute job of pruning a young tree, and since few lawn crews are trained in proper pruning, it is good for home-owners to understand the basics of pruning in order to get their trees off to a good start.

For a basic understanding, it is important to learn three fundamentals:

  1. The common hazards and how to avoid them.
  2. How to make a correct pruning cut.
  3. How to train a young tree so it will have a strong structure.

The common hazards to young trees are many, but these are the main ones to avoid. Don’t let lawn mowers or weed trimmers touch the bark. This tearing of the bark, called “lawn-moweritis” is often a cause of disease and decline in young trees. Put a loose protection, like a plastic or hardware cloth cylinder, around the trunk or mulch 2—4′ around the tree so mowers won’t have to come close. A protector will also prevent cats from using a young tree for a scratching post, which is very harmful. Naturally, deer, especially bucks with their antlers in the velvet stage, can destroy a young tree with munching and rubbing. So keep a circle of fencing around a young tree for three or four years if deer visit. If a tree is planted too close to a street or sidewalk, people will start breaking off branches. If you plant under an electrical line, Public Dis-Service will carve a huge hole in the tree’s canopy. If you forget to remove stabilizing ropes or wires, the tree will grow over them, become girdled, and will break off or be seriously weakened. And lastly, beware of human over-reactions. Don’t over-water and don’t expect the tree to live on Colorado rainfall. Don’t prune aggressively and don’t leave the pruning to nature. Don’t pile mulch against the trunk and don’t let the soil bake with no mulch. Don’t plant too deep and don’t plant too high; plant right where the trunk begins to flare into the root. The best pruning cannot make up for these hazards.

Learning how to make a correct pruning cut is of the utmost importance. Since pruning is surgery on a living being, an improper cut will have far more serious consequences than cutting a 2×4 off at the wrong angle. About 20 years ago, Dr. Alex Shigo’s research for the Forestry Service revealed new information about how trees should be pruned. Dr. Shigo identified the branch collar, which is often a swollen area at the base of a branch. He discovered that the common practice of making a flush cut (see Figure 1. A-C) slices through a protection zone at which a tree can wall-off decay. So by making a pruning just outside the branch collar, trees’ natural defenses are left intact. (Fig. 1) What is not simple about this advice is that trees are variable, so there is no simple formula for judging the distance from the trunk or the exact angle for a proper cut. See Figure 2 for variations. Unfortunately, even some university teachers advise their students to leave a short stub (Fig. 1. D), but this will lead to decay. In general, look for the swelling of the branch collar and cut just outside it. If you can’t see a swelling, find the bark ridge (Fig. 1. A), and begin your cut just outside that ridge, sloping the cut out, usually less than 90 degrees from the branch. (Fig. 1. A-B) In most cases it is better to remove the branch in two steps; first take of most of the branch and second, remove the remaining stub. This will prevent splitting and tearing of the bark. Armed with this knowledge, you can remove dead, broken and diseased branches, vertical-growing sucker shoots and rubbing branches.

Whereas learning to make a proper cut is science, learning to create a strong structure is part science and part art. The science is learning what makes a strong crotch, the union of a branch to the trunk or to a larger branch. Basically the strongest branches are at a 60 to 90 degree angle from the trunk. This sounds counter-intuitive since we would normally think that a branch that stands out perpendicular to the trunk would be more likely to break. However the greatest possibility for weakness occurs in branches that are at a 30-degree angle or less, because with these, the wood fibers run parallel rather than interlocking. You can easily tell if a crotch is weak by looking closely where the branch is connected. If the bark is pushed up into the bark ridge (Fig. 1. A), the union is strong. If the bark is folded in, forming a crack, the union is weak, and the branch is likely to fail sooner or later. The most dramatic example is called a co-dominant leader. (Fig. 3) In this case, two branches arise from the same place on the trunk and grow up nearly parallel. You will almost always find the bark folded in between the trunks. As the two trunks grow, they reach for light, leaning away from each other. This makes them vulnerable to heavy wet snows and strong winds, which can cause the tree to split down the middle. (Fig. 4) This usually means the death of the tree.

There are two approaches to dealing with branches with weak crotches and co-dominant leaders. One is to remove the weak branch or less important trunk. This is easiest and least harmful to do when a tree is young. If removing the entire branch or trunk would be too severe, the weak branch or leader can be dwarfed by shortening the branch significantly. This is called a training cut and can also be used to dwarf the height of a tree while it is still young. See figure 6 for the proper method.

The art of creating a tree with a strong structure is learning how to recognize balance and proper proportion. Young trees that are fertilized and over watered often shoot up and become gangly and vulnerable to breakage. Whether a branch is strong or weak is relative to the proportion of length to diameter. This varies with the type of tree, but roughly, a 1″ diameter branch 4′ long can be strong, whereas a 1″ branch 8′ long will be weak. In terms of the overall structure, remember that the trunk is the pillar holding up the entire tree, so the better the top is balanced over the trunk, the stronger it is. If the highest point of the tree is far from being directly above the trunk, the tree is not balanced and will be weaker. The time to correct this is when the tree is young, by pruning the wayward leader back to a branch that will direct the growth more over the trunk. In general, round and conical are the most stable forms in terms of strength.

Storm damage can also be prevented and health supported by removing crowded and rubbing branches. This thinning is best done when the branches are small, and never remove more than a third of the branches.

Especially in very young trees, every leaf adds to their photosynthesis. But also remember that we live in Colorado with high winds and wet snows that sometimes catch our trees in leaf, so it is good to prune to more compact forms than would be necessary in California or even Iowa.

In general, if the proportion of a tree, height to width is pleasing or beautiful, it is stronger. If it is awkward or ugly, it is weaker. And the same is true for individual branches. Be patient with young trees because they often have an adolescent phase before they develop symmetry and real beauty. Be gentle and not too aggressive; don’t top them or chop them. And since it takes little time to prune a young tree, and since they can change so quickly, plan on doing some corrective pruning every year or every other year as needed. Watch the structure as it develops and responds to your pruning. Use your imagination to visualize how each branch will grow. Pruning can be artful, creative and fun. Before long, your care and insight will take form in a massive being that will tower above you and your children and your house, providing shade, protection, character and beauty, and putting that greenhouse gas, carbon dioxide, to a constructive use.

What Is Sudden Branch Drop Syndrome?

A subtle crack, whooshing drop, and loud thump.

After a heavy limb falls unexpectedly, we start to feel uneasy about our tree! Are other branches going to follow suit? Will the whole thing go down?

Any tree problem is alarming, but sudden branch drop syndrome stands out. Without warning, large, mature trees that appear to be in good health lose limbs.

Read on to learn more about this mysterious tree phenomenon.

Why did branches start falling off my tree in summer?

That sounds like sudden branch syndrome, also called summer branch drop or sudden limb failure. During calm, but hot, summer days, seemingly healthy tree limbs simply snap and fall off.

We’re not quite sure why this happens. But some trees are more prone to this–specifically, aging trees as well as sycamore, oak, elm, eucalyptus and beech trees.

What should I do if this happens?

Tend to your tree as soon as you notice a problem. The first fall might come unannounced, but trees afflicted by sudden branch syndrome may shed a few more.

So, act quickly, assess your tree’s health and take measures to reduce injury or damage. More on that below!

Can I prevent this?

That’s a tough question because we don’t know for sure why this happens!

There are a few common theories about what causes sudden limb failure. Some experts say it’s triggered by high humidity within the tree’s canopy, which leads to a surplus of moisture that weakens the tree’s structure. Others think it stems from an internal tree issue, like bacterial wetwood.

Even after decades of research, the cause is still up for debate. Because of that, it’s hard to say how exactly to prevent this.

But there are a few steps you can take to decrease the likelihood of summer branch drop:

  1. See a large chunk of your tree on the ground? Have an ISA Certified Arborist® inspect the rest of the tree. They’ll look for dead offshoots, decay or other visible hazards. Then, because your tree could be at risk, plan to get it inspected annually for the next few years.
  2. Get ready to trim! Of course, you’ll want to remove any unsafe tree limbs. But you should also see which sections of your tree may be at risk because of their weight or internal decay. Finally, your arborist may recommend pruning to open up the canopy, which reduces humidity.
  3. Keep the tree as healthy as possible with regular plant health care practices. That means watering, mulching, trimming and fertilizing.

Arborist: I don’t sit under old eucalypts

Since a tree branch fell on and killed a young girl in Rosalind Park, Bendigo, in December last year, locals have been wary of limbs falling from big trees on public land.

Indeed, since that tragedy, there have been quite a few tree branches falling, with the trees stressed due to the summer heatwaves.

“We’ve had very little rainfall in the last 12 to 14 years,” says Bendigo arborist Victor Crook.

“The trees are stressed, and a lot of the eucalypts have decided to shed their limbs with the hot weather and lack of rainfall.”

But can anything be done about it? Can you tell if a tree is so stressed that a big branch is about to fall off?

“You can’t tell whatsoever,” says Victor.

“Perfectly good branches just seem to break off. It’s very, very difficult, and probably impossible to say.

“They’re perfectly fine and the inside, they seem to be structurally sound – a lot of the trees that I look at that have dropped limbs, you could not pick that they were going to fall.

“Certainly, we can see trees that are structurally unsound, that are more likely to drop limbs, but as a rule, the normal eucalypts that drop limbs like the red gums and a lot of our box trees, they’ve just been dropping limbs with no sign of structural at all.

“It’s just the normal thing that eucalypts do. It’s a way of preserving their life. They shed limbs to retain the moisture within the rest of the tree, so the tree can survive.”

It’s one of summer’s pleasures to seek shade under a tree, but Victor says that’s something he has second thoughts about now.

“As an arborist, I definitely don’t sit under old eucalypts: I might get a bit paranoid after dealing with so many of them, but you can never tell.”

Picture this: You’re walking through the forest on a hot, still day. Suddenly, you hear a popping sound, and then a huge branch drops just beyond your reach. After thanking your lucky stars that you weren’t crushed, you wonder why a limb should fall on such a windless, calm day? Googling your query, you might discover Sudden Branch Drop, a rather mysterious arboreal tendency that describes your experience.

As Portland tree care enthusiasts, we have many cases of mysteriously fallen limbs on perfectly calm days. Sudden branch drop, also known as sudden limb failure, happens on hot, calm days and evenings, typically in the summer. Observers will notice cracking or popping sounds just before large limbs suddenly plummet to the ground. Kellogg first described sudden branch drop in 1882, writing of trees “said to burst with a loud explosion, and strong limbs…(which) unexpectedly crash down, the fracture disclosing not the least cause of weakness.”

Why Sudden Branch Drop Occurs
Portland certified arborists could debate this question at length, as there is no industry-wide consensus. Most arborists suspect that it has something to do with humidity levels within the tree. Trees absorb water from the soil, and then distribute that water through all tissue—branches, trunk, leaves, limbs, and roots. The used moisture must be released somehow, and as it releases it cools the tree.

Humans release moisture and heat through sweat. Dogs “sweat” out of their tongues. Trees release water into the atmosphere through evapotranspiration, releasing moisture through their leaves. However, calm, hot conditions limit how much moisture trees can release. One theory on sudden branch drop holds that high humidity in tree canopies limits evapotranspiration, increasing the moisture content within branches, and eventually leading to limb failure.

Arborists and scientists continue to study why sudden branch drop happens. It is suspected that sudden branch drop could also be related to:

  • a change in branch movement,
    • drought stress and very dry soil,
  • tissue shrinkage (due to heat),
  • internal cracks,
    • moisture changes in the air and soil,
    • gas release inside tissue, potentially caused by wetwood bacteria, which are common in the species affected by sudden branch drop, and
    • deterioration in cell wall structure, potentially caused by ethylene gas.
    Basically, sudden branch drop is the tree’s response to hot, dry environment where transpiration needs exceed vascular capabilities. When it gets too hot to keep all tissue properly circulated, the tree responds with auto-amputation, letting go of a limb.

Tools to Prevent Against Sudden Branch Drop
Sudden branch drop has been known to happen along lines of weakness; however, it is also possible in branches with no apparent flaws. Therefore, it is hard to predict. In general, it’s a good idea to keep an eye on your trees, especially large, mature trees. Older oaks, maples, ashes, beeches, and elms are common victims of sudden branch drop, but it has occurred in dozens of other species as well. Trees with large, horizontal limbs with an upward sweep at the tips are more likely to suddenly drop limbs. Oftentimes, branches that suddenly drop are those that extended beyond the tree’s main canopy.

While inspecting your trees, look for discoloration, particularly a darker spot where water appears to be “bleeding” out of the tree. This is a sign of a potential limb flaw, which could contribute to limb failure.

Do not place benches or tables below older trees that may suffer sudden limb drop.

As always, consistent, expert Portland tree pruning is the best preventative action you can take to protect your trees from sudden branch drop. Schedule regular tree maintenance with our Portland certified arborists. Our ISA-certified arborists are pruning masters with expertise on how to best cut to minimize the chances of sudden branch drop. Portland tree trimming from less experienced, less knowledgeable pruners may leave excess foliage at the end of limbs, thereby increasing the burden the tree must support on hot, dry days. Sudden branch drop is dangerous for bystanders and detrimental to tree health. Protect against it with regular, professional pruning.

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