Trees can't move, can they?
June 3, 2018 9:48 PM Subscribe
Do tree branches hang lower when it rains?
We've had a lot of rain lately where I live--over five inches in less than two weeks, including a fairly severe thunderstorm Friday. I was outside in my yard late Friday night and noticed the oak tree branches seemed significantly closer? Like, some of them I could actually reach, which I don't think was true before, and I freaked out and got a flashlight, thinking I had downed limbs. But nothing seemed broken, and the branches that seemed lower were from more than one tree. The next day I inspected all the trees in full sunlight and everything seemed fine, and the limbs didn't seem as low as the previous night. I think they are white oaks (quercus alba) and are over a hundred years old and huge; they're covered with lush green leaves and look healthy enough to me, although I don't know that much about trees. I've heard of things like willows drooping, but can hardwood branches really droop that noticeably in prolonged wet weather? I mean, I know water is heavy and I suppose flexibility could keep limbs from breaking, but I didn't expect to be able to see it. Are they going full Birnam wood on me up in here, or am I crazy?
We've had a lot of rain lately where I live--over five inches in less than two weeks, including a fairly severe thunderstorm Friday. I was outside in my yard late Friday night and noticed the oak tree branches seemed significantly closer? Like, some of them I could actually reach, which I don't think was true before, and I freaked out and got a flashlight, thinking I had downed limbs. But nothing seemed broken, and the branches that seemed lower were from more than one tree. The next day I inspected all the trees in full sunlight and everything seemed fine, and the limbs didn't seem as low as the previous night. I think they are white oaks (quercus alba) and are over a hundred years old and huge; they're covered with lush green leaves and look healthy enough to me, although I don't know that much about trees. I've heard of things like willows drooping, but can hardwood branches really droop that noticeably in prolonged wet weather? I mean, I know water is heavy and I suppose flexibility could keep limbs from breaking, but I didn't expect to be able to see it. Are they going full Birnam wood on me up in here, or am I crazy?
We have an oak tree in our front yard (not sure what exact type) & (craptastic) elm in the back.
One year we had a giant ice storm. In the front yard, branches of the the oak tree were all bent, bent, bent down to the ground. Branches that were 8-10-15, even 20 feet off the ground were bowed right over and lying on the ground.
After the ice melted, the branches all sproinged right back up and (almost) back to normal. No permanent damage whatsoever.
Meanwhile in the backyard, all the elm branches just snapped right off. Massive damage. Like huge, huge branches snapped off and on the ground. (A few years later we just removed the tree, it was nothing but trouble.)
In short, oak branches tend to be very, very bendy. That is a very good thing.
posted by flug at 10:19 PM on June 3, 2018 [6 favorites]
One year we had a giant ice storm. In the front yard, branches of the the oak tree were all bent, bent, bent down to the ground. Branches that were 8-10-15, even 20 feet off the ground were bowed right over and lying on the ground.
After the ice melted, the branches all sproinged right back up and (almost) back to normal. No permanent damage whatsoever.
Meanwhile in the backyard, all the elm branches just snapped right off. Massive damage. Like huge, huge branches snapped off and on the ground. (A few years later we just removed the tree, it was nothing but trouble.)
In short, oak branches tend to be very, very bendy. That is a very good thing.
posted by flug at 10:19 PM on June 3, 2018 [6 favorites]
Yes. I used to work at a university research lab that filmed bird nests. Especially in the rainforest (no idea what kind of trees) the nests in the branches would be in frame in the morning but as the sun heated the dew it was common to have the nest drift completely up and out of frame over the course of the tape. Good researches would zoom out a bit for this while newer researchers didn’t know. A tree has to bend to withstand wind, snow, and rain. I’m sure some trees are more bendy than others.
posted by Crystalinne at 10:24 PM on June 3, 2018 [17 favorites]
posted by Crystalinne at 10:24 PM on June 3, 2018 [17 favorites]
Had a massive olive harvest a couple of years ago, and the branches on the olive trees sagged significantly and haven't recovered.
posted by kjs4 at 11:48 PM on June 3, 2018 [1 favorite]
posted by kjs4 at 11:48 PM on June 3, 2018 [1 favorite]
A larch branch over our porch steps is about 4 metres long and usually hangs horizontally some 2 metres above porch level. After heavy rain it's a good metre lower, and a couple of quick up-down tugs are usually enough to help it shed the worst of the water and unblock the steps.
posted by I claim sanctuary at 3:02 AM on June 4, 2018
posted by I claim sanctuary at 3:02 AM on June 4, 2018
After a heavy snowfall (snow being able to accumulate in much heavier quantities than liquid water, especially on a fir tree, of course) if you happen to be looking you'll often see a mini-avalanche triggered by some minute motion dump all of the snow off of a branch and watch it spring back into place like a diving board a swimmer has jumped off of.
posted by XMLicious at 5:30 AM on June 4, 2018
posted by XMLicious at 5:30 AM on June 4, 2018
I trimmed the low-hanging branches on a maple adjacent to my sidewalk so that i could walk under them with plenty of clearance. Then it rained, and I hit my head.
posted by notsnot at 9:35 AM on June 4, 2018 [3 favorites]
posted by notsnot at 9:35 AM on June 4, 2018 [3 favorites]
Absolutely they can and do. More noticeable with freezing rain, but even with normal rain, there are tree branches in my daily walking commute that I have to sidestep if it's raining vs. well above my head normally.
The amount of droop depends on the tree type and season (i. e. branch flexibility and amount of leaves there to trap water and weigh it down). Some trees will barely droop at all, which is probably why you haven't noticed it before.
posted by randomnity at 12:19 PM on June 4, 2018
The amount of droop depends on the tree type and season (i. e. branch flexibility and amount of leaves there to trap water and weigh it down). Some trees will barely droop at all, which is probably why you haven't noticed it before.
posted by randomnity at 12:19 PM on June 4, 2018
Consider a spherical oak tree of uniform density...
OK, that's probably simplifying the physics a little too far. But more seriously: to a first approximation from an engineering perspective, a tree branch is a cantilever: a somewhat-horizontal beam, supported only at one end, and relying on the strength of the beam and the rotation resistance of the support to keep the structure from collapsing. Other engineering examples of cantilevers include aeroplane wings and diving boards; other biological examples could include your own arm held out straight and sideways.
Any kind of load on a cantilever will cause the unsupported end to deflect somewhat. Has to, because perfectly rigid materials don't exist. And the closer to the unsupported end you put the load, the more deflection a load of any given weight will cause. Think about how a diving board gets progressively bouncier as you walk out toward the business end, or how much harder it is to hold up a heavy courier bag if its straps are hanging from your outstretched hand than if they're hanging on your shoulder.
If you think about the structure of tree branches you'll see that they're actually a whole bunch of cantilevers on cantilevers: branches end in forks into smaller branches, which end in forks into twigs, each of which has little flat cantilevers (leaves) attached at multiple points. Smaller branches and twigs and leaves might also be attached anywhere on a branch all the way back to the trunk, but I'll ignore those for the present because I want to concentrate on the effects of loads on the unsupported ends.
If you apply some kind of loading force to a leaf - say by blowing on it, or by weighing it down with raindrops - it's clearly going to bend rather than snap straight off. Trees would not survive for very long otherwise. But the force that's making the leaf bend doesn't get absorbed by the bending; it transfers to the point where the leaf is attached i.e. the end of a twig.
Typically there will be several leaves attached near the end of every twig, so the end of the twig will have the combined forces from all those leaves acting at the business end of its little diving board, and the twig will flex as a result. Again, trees would not survive for very long if every bit of rain or wind could just snap off all their twigs.
But again, the fact that the twig is flexing doesn't make the force applied to it go away; it just transfers it to the supported end of the cantilever, which in this case is the point where a small branch forks into twigs.
So those small branches see the combined effect of all the loading forces applied to the twigs growing out of them, and the larger branches from which they grow likewise see the combined effect of all the loading on the smaller branches, and so on and so on via bigger and bigger branches all the way back to the trunk. And every step of the way, that load is going to cause some deflection (bending) in the element whose end it's being applied to.
If the loading is variable, as it will be when it's caused by wind through the leaves, you can see all that bending quite clearly because the whole tree gets moving. Seeing trees waving in the breeze is completely normal and unremarkable.
But when the loading is static, due to the weight of water sticking to the leaves after rain, then the tree won't be moving; it will just settle into a new equilibrium, where the reaction springiness of the downward bend in all those limbs and branches and twigs and leaves is just exactly enough to counteract the added weight of all that stuck-on water. And because that weight force acts purely downward on every cantilever element in the whole tree, that's also the way that every one of them is going to deflect.
Same thing happens in reverse to deciduous trees when they drop their leaves for the winter. The twigs are no longer required to support the weight of leaves at their ends, so they stop deflecting downward under that loading force, which they also no longer transfer back to their own support points, which means the small branches they grow from also stop deflecting downward under it and so on and so on back to the trunk. If you measure the height of any given twig on any given deciduous tree in winter (provided there's no snow stuck to it to complicate the picture) you'd expect to find it sitting rather higher, on average, than in the previous and next summers.
posted by flabdablet at 11:48 PM on August 12, 2018
OK, that's probably simplifying the physics a little too far. But more seriously: to a first approximation from an engineering perspective, a tree branch is a cantilever: a somewhat-horizontal beam, supported only at one end, and relying on the strength of the beam and the rotation resistance of the support to keep the structure from collapsing. Other engineering examples of cantilevers include aeroplane wings and diving boards; other biological examples could include your own arm held out straight and sideways.
Any kind of load on a cantilever will cause the unsupported end to deflect somewhat. Has to, because perfectly rigid materials don't exist. And the closer to the unsupported end you put the load, the more deflection a load of any given weight will cause. Think about how a diving board gets progressively bouncier as you walk out toward the business end, or how much harder it is to hold up a heavy courier bag if its straps are hanging from your outstretched hand than if they're hanging on your shoulder.
If you think about the structure of tree branches you'll see that they're actually a whole bunch of cantilevers on cantilevers: branches end in forks into smaller branches, which end in forks into twigs, each of which has little flat cantilevers (leaves) attached at multiple points. Smaller branches and twigs and leaves might also be attached anywhere on a branch all the way back to the trunk, but I'll ignore those for the present because I want to concentrate on the effects of loads on the unsupported ends.
If you apply some kind of loading force to a leaf - say by blowing on it, or by weighing it down with raindrops - it's clearly going to bend rather than snap straight off. Trees would not survive for very long otherwise. But the force that's making the leaf bend doesn't get absorbed by the bending; it transfers to the point where the leaf is attached i.e. the end of a twig.
Typically there will be several leaves attached near the end of every twig, so the end of the twig will have the combined forces from all those leaves acting at the business end of its little diving board, and the twig will flex as a result. Again, trees would not survive for very long if every bit of rain or wind could just snap off all their twigs.
But again, the fact that the twig is flexing doesn't make the force applied to it go away; it just transfers it to the supported end of the cantilever, which in this case is the point where a small branch forks into twigs.
So those small branches see the combined effect of all the loading forces applied to the twigs growing out of them, and the larger branches from which they grow likewise see the combined effect of all the loading on the smaller branches, and so on and so on via bigger and bigger branches all the way back to the trunk. And every step of the way, that load is going to cause some deflection (bending) in the element whose end it's being applied to.
If the loading is variable, as it will be when it's caused by wind through the leaves, you can see all that bending quite clearly because the whole tree gets moving. Seeing trees waving in the breeze is completely normal and unremarkable.
But when the loading is static, due to the weight of water sticking to the leaves after rain, then the tree won't be moving; it will just settle into a new equilibrium, where the reaction springiness of the downward bend in all those limbs and branches and twigs and leaves is just exactly enough to counteract the added weight of all that stuck-on water. And because that weight force acts purely downward on every cantilever element in the whole tree, that's also the way that every one of them is going to deflect.
Same thing happens in reverse to deciduous trees when they drop their leaves for the winter. The twigs are no longer required to support the weight of leaves at their ends, so they stop deflecting downward under that loading force, which they also no longer transfer back to their own support points, which means the small branches they grow from also stop deflecting downward under it and so on and so on back to the trunk. If you measure the height of any given twig on any given deciduous tree in winter (provided there's no snow stuck to it to complicate the picture) you'd expect to find it sitting rather higher, on average, than in the previous and next summers.
posted by flabdablet at 11:48 PM on August 12, 2018
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posted by Fiasco da Gama at 9:57 PM on June 3, 2018 [17 favorites]