How do trees raise water?

Capillarity can raise liquid against gravity, I was interested to know how much this can account for the movement of water in trees, is capillarity alone enough to lift water up a tree trunk?

For water in a tube there is an equation like this;

h = 2T cos (theta) / pgr
where; T = 0.0728 J/m^2 at 20 degrees C, the contact angle theta = 20 degrees, density of water, p = 1000 kg/m^3, and the local acceleration due to gravity can usually be taken to be 9.8 m/s^2.

So the height is given by;

h = 1.4x10^{-5) / r

So tubes of the following diameters can lift water to the following heights.

Diameter (mm) Height (m)
1 0.014
0.1 0.14
0.01 1.4
0.001 14
0.0001 140

If we assume perfect contact these heights become only slightly larger. (cos(theta)=1)
1 micrometre diameter tube can lift water 14.85 m, it’s necessary to have a tube 0.1 micrometres or 100 nanometers to lift water a height of 148 meters by capillary action alone, if the above equations are valid. Looking around on the internet gave a size of xylem in a tree to be around 20-30 micrometres (0.02 – 0.03 mm) in size, which if true enough to lift water around half a meter. It would be interesting to see if this value is correct.

If you are interested to find more there is a 2004 nature article on the limits of tree height here.


10 Responses

  1. Capillary action in trees would raise water around half a meter, because as you have correctly shown, the tubes inside the trees do no conform to the minute diameters required to raise water to the top of a sapling let alone a giant Californian redwood. Furthermore, capillary action is not a continual flow from the top of the tube into the atmosphere, but a rise inside the tube falling short of the top, so we can safely discount capillarity as a mechanism for explaining the bulk flow rates observed in trees.

    The discussion above deals with many of the problems and provides a simple yet powerful flow and return in trees driven by evaporation which in turn alters the density of the sap at the leaf providing a downward flow and a return flow that does address the observed flow rates in trees and plants.

    Andrew K Fletcher

  2. Thanks for the comments. I believe that Capillary pressure is one of the forces that the plants use to move the water – using the surface tension is basically a free way to move the water. I expect that the tubes can get smaller than the 30 microns I have quoted above, I also haven’t seen any measurements of the contact angle between the xylem material and the water. I was surprised that there seems to be no definitive answer on this topic, not even a commonly agreed wrong idea. 🙂

    I don’t believe that the trees would neglect any of the mechanisms available to them to lift water from the ground, it’s also interesting to wonder if they can transport water in a gaseous state, I read that generation of bubbles can be a problem for some trees, blocking off the xylem, that means that the water is very close to evaporating.

  3. Water flowing vertically in a single open ended tube to 24 meters using salt as the driving force.

    Introduction to the experiment

    Bench top scaled down version of the Brixham Experiment

    Andrew K Fletcher

  4. If the tubes are full of water in the above experiment, then you aren’t really raising water, just moving it horizontally.

  5. Water is moved vertically. As a tree grows it does not grow as an empty conduit. It grows with fluid inside it. If our tubes were inside a larger tube to represent the bark and inner cell layers of a tree, the water can indeed move horizontally from one tube to another, through membranes to represent sap exchange in trees from phloem to xylem.

    If water is at the bottom of a suspended inverted U tube filled with water, with both open ends in 2 bottles or 1 bottle, a tiny amount of salt is added at the centre or top of the suspended tube, then the downawrd flow generated by gravity will cause the entire contents in the other side of the tube loop to be drawn up and down the other side. So how is this a horizontal flow? I suspect you have not viewed the video of the brixham experiment on You tube Gravity of life part 3.

  6. Wow, this is mind boggling!! Then there must be a continuity between water in the xylem end vessles, the tissue spaces in the leaves and the phloem tubes, isn’t that right?

  7. Ashok That is correct.

    Water and solute flow in a coupled system, there are no seperate systems, any more than there are seperated systems in the human body. Every single vein is linked to an artery.

    Translocation is a search term that confirms this.

  8. Titre du document / Document title
    Modeling xylem and phloem water flows in trees according to cohesion theory and münch hypothesis
    Auteur(s) / Author(s)
    HÖLTTÄ T. (1) ; VESALA T. (1) ; SEVANTO S. (1) ; PERÄMÄKI M. (2) ; NIKINMAA E. (2) ;
    Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)
    (1) Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00014, FINLANDE
    (2) Department of Forest Ecology, University of Helsinki, P.O. Box 24, 00014, FINLANDE
    Résumé / Abstract
    Water and solute flows in the coupled system of xylem and phloem were modeled together with predictions for xylem and whole stem diameter changes. With the model we could produce water circulation between xylem and phloem as presented by the Münch hypothesis. Viscosity was modeled as an explicit function of solute concentration and this was found to vary the resistance of the phloem sap flow by many orders of magnitude in the possible physiological range of sap concentrations. Also, the sensitivity of the predicted phloem translocation to changes in the boundary conditions and parameters such as sugar loading, transpiration, and hydraulic conductivity were studied. The system was found to be quite sensitive to the sugar-loading rate, as too high sugar concentration, (approximately 7 MPa) would cause phloem translocation to be irreversibly hindered and soon totally blocked due to accumulation of sugar at the top of the phloem and the consequent rise in the viscosity of the phloem sap. Too low sugar loading rate, on the other hand, would not induce a sufficient axial water pressure gradient. The model also revealed the existence of Münch counter flow, i.e., xylem water flow in the absence of transpiration resulting from water circulation between the xylem and phloem. Modeled diameter changes of the stem were found to be compatible with actual stem diameter measurements from earlier studies. The diurnal diameter variation of the whole stem was approximately 0.1 mm of which the xylem constituted approximately one-third.
    Revue / Journal Title
    Trees ISSN 0931-1890 CODEN TRESEY
    Source / Source
    2006, vol. 20, no1, pp. 67-78 [12 page(s) (article)] (43 ref.)

    • There has been extensive study of water adsorption by many materials. These studies have shown that raising the relative humidy, RH, from 10% RH to 100 % RH the adsorbed water density increases by a factor of 5. That is, as the RH is increased the water is compressed. Liquid water is very hard to compress but vapor can easily be compressed. If we consider that when water adsorbes to a surface it is by means of the hydrogen atoms, liquid water cohesion is broken, creating a high-density, low-energy, ionized, gas-like fluid. We know liquid cohesion is very weak. Washing your hands demonstrates that. Hydrogen adsorption to capillaries doesn’t have to strong, just stronger than hydrogen attachment to oxygen. Adsorbed water, having the energy of liquid water, is over 1,000 times as easy to compress as vapor, and can very easily be compressed. This information has been around for over 40 years so I don’t understand why plant scientists haven’t considered this as it simplifies all theories of water flow, chemical reactions, and cell growth. For more on this subject go to redman0143@comcast .net.

      • Dear Charles, you seem to have left an email address here rather than a website we can visit.

        I don’t really understand your point about the relative humidity above, but isn’t this the kind of behaviour we would expect if we consid the partial pressure of the water in air being in dynamic equilibrium with the water in the material. I don’t think hat tells us anything about the state of the water in the material.

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