# Capillary action, evaporative pumping or bio-mechanical pumping...

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## Summary:

I just read a thread regarding capillary action that evolved into a of discussion on water transport in trees. One contributor sited the following.

“Transporting water to the tops of trees”
N. Michele Holbrook and Maciej A. Zwieniecki
January 2008 Physics Today

“Trees are solar driven evaporative pumps that wick water up from the soil using capillary forces. But their ability to operate in a metastable state makes them unique among both natural and human-engineered transport systems.”

## Main Question or Discussion Point

I was researching capillary action to design an efficient evaporative cooler, when I found this thread... https://www.physicsforums.com/threads/using-capillary-action-to-raise-water-is-perpetual-flow-possible.551285/

Upon reading this thread, the statement, "Trees are solar driven evaporative pumps that wick water up from the soil using capillary forces...," caught my attention.

As a young man, I was helping build a fence near a river. About 8-10 feet below the ground, the ground is filled with small "river rock", similar to stones used in a French drain. This allows the river water to be accessible to the nearby tree roots. There was a cottonwood tree approximately 15" in diameter and 20-30 feet tall. As the tree was in line with the fence being built, the person I was helping cut off the tree at about 5 feet, intending to use the trunk as a post in the fence. After cutting off the tree, which left no branches or leaves, the flat top surface of the trunk gushed water for more than an hour. By gush, I mean it ran water 1/4" deep to the sides of the trunk and down the outside. Guesstimating, I wouldn't be surprised by 1 gal/min.

I was amazed by the pumping action of the roots and trunk. Since there was nothing causing large-scale evaporation and consequent "suction" to draw up the water, I can only imagine capillary action and/or bio-mechanical pumping producing this water. However, the force with which the water burst forth onto the stump, leads me to think that it must be due to bio-mechanical pumping.

Is there something I'm missing? Can massive capillary action produce such energetic flow?

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256bits
Gold Member
On a hill side from the bank?

It was pretty flat, but yes, about 100 ft from the edge of the river.

256bits
Gold Member
Well I was thinking the water table was responsible for the gushing,, but due to level ground not so.

Root pressure then, but I don't see how the osmosis would continue at that rate for long.
https://en.wikipedia.org/wiki/Root_pressure

sophiecentaur
Gold Member
This process is perceived as magic by many people who have tried to extrapolate the effects of a cloth wick. The diameter of the equivalent 'tube' must be very small if you apply the simple 'A level' equations.
No magic for me, though - I'm just ignorant of the Physics.

256bits
Gold Member
But the wick doesn't "gush" out water as the OP has stated for the cut off stump of a cottonwood, if his memory serves him correctly.

sophiecentaur
Gold Member
But the wick doesn't "gush" out water as the OP has stated for the cut off stump of a cottonwood, if his memory serves him correctly.
That’s the ‘magic’ bit.

I can still see it in my mind. It almost looked like it was boiling and kind of sounded like it because of the water falling down the sides of the trunk. I don't remember the time of year, but there was no snow and there were leaves on the tree. I have been told that large cottonwood trees, similar to weeping willows, can consume hundreds of gallons per day.

I would recommend seeing this if you have the opportunity. It was amazing!

If the tree were a simple lift pump, it could never be taller than 28 ft, right?
In simple terms, how do they pump ?? Probably this is not difficult but I am certainly clueless.........

sophiecentaur
Gold Member
If the tree were a simple lift pump, it could never be taller than 28 ft, right?
In simple terms, how do they pump ?? Probably this is not difficult but I am certainly clueless.........
Osmotic pressure will shift water. All that's needed is an active mechanism to keep the concentration of salts higher for the upper parts than the concentration in the lower parts. The process doesn't need to be operating over tens of metres - just locally- and water will flow, one step at a time. Take a potato, cut off two opposite sides and place one face in fresh water. Make a small well in the upper face and put salt in it. The well will fill with water, pumped up from the reservoir. A ladder of such potatoes could take water as high as you like as long as you can maintain the concentration gradient.
Osmosis is a 'passive' process. The energy for the process is provided by whatever produced the salt plus any gravitational potential energy you gave it by putting it up there.
There is a lot of stuff that a Google search will give you about the basics of Water Transport in plants (GCSE and A Level) but I haven't found anything on the details of how concentration of salts is maintained. It would be appreciated if anyone has an appropriate link.
In plants there is transpiration on the leaf surface which sheds water but leaves the salts behind and that is 'passive'. I think there must be more energy input than just these passive processes, powered by heat from the Sun. A certain amount of water is 'consumed' by photosynthesis, for a start so the cells must be using some of the energy it produces in order to maintain concentration gradients on the way up.

Andy Resnick
Is there something I'm missing? Can massive capillary action produce such energetic flow?
It's not just capillary action; the anatomy (xylem) also works to increase the vertical transport of water:

https://www.nature.com/scitable/knowledge/library/water-uptake-and-transport-in-vascular-plants-103016037/
https://www.ncbi.nlm.nih.gov/pubmed/29739034
https://www.scientificamerican.com/article/how-do-large-trees-such-a/

Cottonwood trees are particularly fast-growing and used (in forestry) as an indicator of groundwater availability, so there are studies of sap flow rates, etc:

https://www.fs.fed.us/rm/boise/AWAE/labs/awae_flagstaff/Hot_Topics/ripthreatbib/Gazal_etal_2006.pdf
https://corescholar.libraries.wright.edu/cgi/viewcontent.cgi?article=1964&context=etd_all
https://cpw.state.co.us/Documents/ResourceStewardship/CottonwoodWillowManagementPrescription.pdf

But none of these mention the phenomenon you observe (massive fluid flow from a cut trunk).

sophiecentaur
Gold Member
But none of these mention the phenomenon you observe (massive fluid flow from a cut trunk).
The recollection of an 'oozing' could turn it into a 'gushing'. Many trees will 'ooze' fluid when they are 'tapped' but that is usually a sweet fluid (food), some of which which comes down via the phloem from the leaves. I have done some more looking for explanations about the actual physics of the transport process but those botanists seem to ignore the fact that it's not straightforward and simply describe the structure and not how it functions. 'Explanations' seem to diverge, rather than converge, which is the way Physicists tend to work.
The mechanism appears not to work once the plant is dead.

Andy Resnick
[...] I have done some more looking for explanations about the actual physics of the transport process but those botanists seem to ignore the fact that it's not straightforward and simply describe the structure and not how it functions. 'Explanations' seem to diverge, rather than converge, which is the way Physicists tend to work.
I don't know about all that- science is science, and it's pretty clear that a combination of negative pressure from transpiration (the largest component) and hydrogen bonds within structural conduits (the dead xylem tissue) suffices to 'explain' why trees can grow higher than 34 feet. In addition, osmotic gradients created by the production of sugar from stored starch move a lot of sap in spring. The Nature link is quite good. Different fields may have developed different schema, but that just means one must become the equivalent of multilingual.

The mechanism appears not to work once the plant is dead.
Of course- once transpiration stops, there's no driving pressure.

sophiecentaur
Gold Member
that just means one must become the equivalent of multilingual.
Certainly.

In the explanations there is mention of air / gas bubbles being eliminated in the Xylem tubes. If the column breaks then the cohesion forces are nullified. I can appreciate that you have, in effect, a chain of H bonds all the way up and bonds to the walls but I can't find (in that Nature article) how the 'weight' of the column is supported by the capillary forces that evaporation produces by removing water at the top. Could it be that the electric forces in the H bonds are so much higher than the weight forces so it's as if the stem were laid horizontally? (The limit seems to be around 100m.) Is it basically a heat engine with the Energy supplied to the leaves from the Sun?

Andy Resnick
Certainly.

In the explanations there is mention of air / gas bubbles being eliminated in the Xylem tubes. If the column breaks then the cohesion forces are nullified. I can appreciate that you have, in effect, a chain of H bonds all the way up and bonds to the walls but I can't find (in that Nature article) how the 'weight' of the column is supported by the capillary forces that evaporation produces by removing water at the top. Could it be that the electric forces in the H bonds are so much higher than the weight forces so it's as if the stem were laid horizontally? (The limit seems to be around 100m.) Is it basically a heat engine with the Energy supplied to the leaves from the Sun?
I think that's basically correct- hydrogen bonding becomes critically important at the subcellular level, in mammalian cells at least, it's what 'controls' and 'selects' the movement of ions through channel proteins:

https://www.ncbi.nlm.nih.gov/pubmed/25597624
https://www.pnas.org/content/98/17/9478
https://www.sciencedirect.com/science/article/pii/S0926204016301552
http://www.jbc.org/content/276/2/1326.full
https://arxiv.org/ftp/arxiv/papers/0706/0706.1355.pdf

So many questions...

Isn't the living part of tree the small amount under the bark? Does the wood core still transport water?

If there is normally a pressure like that to push the water up, shouldn't there be a nearly equal pressure from the (upright) top portion? Which means that, even if the top portion is horizontal, water should readily come out of that too, right?

Perhaps it was because you removed a large mass from one area of the topography to another area, thus making a hydraulic pump as the topography settled.

jim mcnamara
Mentor
Q1. Yes sapwood transports water - this is why it is called sapwood. Heartwood does not usually move water, it serves mostly to deter fungal disease. Hollow trees are the case where this deterrent did not work.

Q2. The leaves have small openings (stomates) to allow evaporation to occur. The effective evaporative surface of thousands of leaves on a tree is quite large

Q3. you got it! Younger trees have a lot more sapwood relative to heartwood.

The small layer under the bark is the cambium which provides growth in diameter, NOT length.

Example:
There was a beech tree in Tennessee (USA) that had an inscription carved by Daniel Boone in 1760 which was still 5 feet off the ground 150 years later.

Phloem is the layer next to the cambium that transports sucrose solution ( glucose -> sucrose, derived from photosynthesis in leaves) down from the leaves, to the roots.

Jim, thank you for your answers! But, my questions were mostly rhetorical

I don't believe that this massive flow of water had a biological origin. If it was from the tree, then while the chainsaw was cutting into it water should have been flowing.

256bits
Gold Member
One can find the original Dixon ( 1914 ) examination on sap movement in plants, based on capillary action, generally accepted, with minor details addressed over the years.
https://archive.org/details/transpirationasc00dixo/page/n3
( srcoll down for choices as to display or download - text, pdf, kindle, ... )
I haven't read it all, but some nice discussion of experiments he did.
And some mention of other experimenters and failings.

Certainly.

In the explanations there is mention of air / gas bubbles being eliminated in the Xylem tubes. If the column breaks then the cohesion forces are nullified. I can appreciate that you have, in effect, a chain of H bonds all the way up and bonds to the walls but I can't find (in that Nature article) how the 'weight' of the column is supported by the capillary forces that evaporation produces by removing water at the top. Could it be that the electric forces in the H bonds are so much higher than the weight forces so it's as if the stem were laid horizontally? (The limit seems to be around 100m.) Is it basically a heat engine with the Energy supplied to the leaves from the Sun?
Air bubbles are difficult to create.
Surface tension tends to deflate small bubbles, rather than encouraging them to grow.
As an analogy, it is more difficult to begin to blow up a balloon, and then when the balloon grows the pressure needed to achieve greater balloon volume is is reduced. Or connect two balloons together, one blown up to a smaller radius than the other. The smaller, with its greater pressure within due to the "surface tension" of the rubber envelope, will decompress into the larger balloon.

If in the water, or sap, situation, one considers that a bubble has to start from a void in the vicinity of dimensions of a water molecule, the surrounding water molecules due to attractive forces are prohibiting the bubble formation. Of course this has its limits, as is seen with bubble formation when heating water. Here energy is being added to the water, increasing the kinetic energy of the water molecules, and subsequently the attractive forces between water molecules become less and less able to prevent bubble formation. Yet superheated water is also possible, which for instance most chemists are well aware to avoid by adding "bubble chips" as catalyst sources of bubble formation when heating a liquid.

The osmosis at the leaf end explanation for the support of the column:
Tension in a column of water can support 50 atm, depending upon source. That should be well enough to rise water several hundred feet. Osmotic pressure in the leaf ( consider the leaf cells immersed in water with one end exposed to the atmosphere and the other to the column - the evaporation of water at the exposed end necessitates makeup of water back into the cell, and that is accomplishes through osmosis ).

Does that mean the water column exerts a head upon the root? and why then dpn't roots explode under such a stress.? Well, similarly, a dangling string is supported at the top and all elements are in decreasing tension down to the bottom end where the tension disappears.