What is the presumed absolute highest any tree can possibly grow? There is a physical limit, do you know the answer? - Millet

Without googling it I do not know. But it seems there was a difference in answers. If trees use capillary action to draw water up there is a limit but there are trees taller than that theoretical limit.

This is the kind of question that is best answered by modeling because there are so many factors or variables involved, that dealing with all of them simultaneously in your head, you would lose your sanity and your audience. The most simplistic answer is based on finding that breaking point of water subject to cohesion and tension driven by water potential gradient across a given diameter vessel. But it is not a straight-forward mechanical or simple laws of physics to get the answer because plants are living things in various climes in time and space, and they often have answers that can get around the direct brute force mechanical laws of engineering or physics.

I have written models from rice to the tallest known douglas fir trees. For rice models, we started from the water outside the roots. And our work found its way in some scientific journals, that was when I was still a college student a long time ago.

Anyway, the answer would be between 350 to 400 feet depending on soil and atmospheric conditions ( including windspeed that could topple a tree ), which is what most plant modeling would tell you. At those heights, the tree would be suffering the equivalent of a drought and so it reaches a balance of growth limited by moisture availability. The water column breakages will be costlier to repair by the tracheids, and enormous amount of energy is needed to overcome difference in water potential running across the span of height along with the resistance to water flow through the water vessels.

There is a major drawback from the models that I have seen. And I know that Millet have his answer from a certain perspective, I am already challenging that before it is being posted here.

Most of the plant models that I have seen are only based on the assumption of water pull. Sunlight strikes the leaves so that it creates vapor pressure deficit, and creates evapotranspiration, and thus the leaves have negative water potential, and water is pulled from regions of higher water potential to lower potential, creating a flow from roots to leaves. Then we have all those intervening morphological adaptations by some trees to circumvent physical limits of the processes, and among them are the tracheids, which serve as one way valves, and some could well be lateral valves as well, which helps limit air bubbles or to provide alternative continuous water flow pathways.

What has not been discussed to greater lengths and details would be active pushing of water. We have seen such push from the living roots, and not dead ones, we have measured it for various plants. They always thought that this is a minor factor. I haven't read anything about the active (energy requiring ) pushing from midway through the trunk of the trees, and perhaps some trees may have developed such mechanism. If we have both push and pull, then the limit could well be beyond what the present models are showing.

Some minor evidences of the push and pull effect is how the trunks are acting as temporary storage of water. Since water would take a long ways to go for a very tall tree, the strategy have been to fill the trunk (acting as water tank storage) and so it swells during the night when there is no demand from the canopy. During the day, the tree canopy would only have to pull halfway through the column, and so the tree trunk shrinks. They can be explained by still elaborate valves, but there is active pushing at night to pump water to the trunk when there is no vapor pressure deficit.

I think the plant models needed to be redone, and I would have loved doing it, should I have stayed in the academe where I would remain poor and couldn't afford a nice house with plenty of citruses,

So far the tallest tree ever measured, was a Douglas Fir, at 414 feet high, and the highest living sequoia 371 feet. So how much higher can a tree grow? - Millet

What does the evidence of tree fossil records show? It could be much more as I have suggested. Especially during the times when oxygen concentration is way much more than what it is now, when giant insects roam, when dragonflies have a wingspan of 12 feet or more. I suspect that "primitive" trees would be at least a thousand feet tall.

It is so much easier to explain after the fact, with evidence on hand, rather than to speculate or extrapolate when evidence are lacking.

Joe, I must grant your input on flora of prehistoric times, My question still asks: What is the theoretical highest any tree can possibly grow? There is a physical limit. Know the answer? - Millet

Water and nutrients picked up from the soil by the roots, and going to the leaves move along a gradient of negative pressure. When leaves at the top transpire, they create suctions. The suctions and water adhesion to cell walls cause water to move up and against gravity.

Of course everyone here already knows that

From my old university dayz (Plant Bio elective course); correct me if I don't remember right:

1) As a tree grows taller = xylem pressure is reduced relative to height.

2) Gravity and reduced xylem pressure = water potential drop relative to height = water stress in leaves towards the top.

3) Water stress in leaves at the top = leaves increase mass but decrease area = smaller, skinnier (less spread out) leaves at the top = reduce pore openings to prevent further transpiration = save water = need to conserve water potential.

4) Reduced and narrower pores = less carbon dioxide coming in/exchange = reduce photosynthesis efficiency = less energy = limit leaves expansion = limit on further tree height growth.

In other words....

water stress towards the top due to gravity and path resistance limit tree height.

Doesn't matter if the nutrients and water at bottom soil are abundant.

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Anyway......

I remember reading an article OR watching the Discovery Channel, the hypothesis/extrapolation is no more than 427 feet. Of course, trees, like humans, will eventually evolve, adapt, and/or overcome any barrier.

GreenZ has the correct answer 427 feet --- Congratulations!!! The number 427 feet came from studies by the scientists Dr. Gregory Jennings at Humbledt State, Dr. Stephen Davis of Pepperdine University, Dr. George W. Koch at Northern Arizona. All three scientists agree on the height of 427 feet and have concluded that no existing species of tree can grow higher. - Millet

This is not very interesting because the main question has not been answered:

gdbanks wrote:

If trees use capillary action to draw water up there is a limit but there are trees taller than that theoretical limit.

Yes, the limit is 10 m!!! So there must be an other "engine". It could be a pushing pump in the root. Grapevine (vitis) bleeds (cries) when you cut a branch, there must be a "pump" in the roots, but most trees don't bleed. When you cut a poplar (populus) you hear the hiss of the air entering the vessels. The water has a negative pressure. No pump in the roots, but it grows higher than 10 m. They must be pumps along the trunk, but what are they?
This is the real question.

Sylvain, I have never seen any reference to a tree higher than 427 feet. As for root pressure, there are plants with root pressure. Root pressure is created by the osmotic pressure of xylem sap which is, in turn, created by dissolved minerals and sugars that have been actively transported into the apoplast of the stele. Although root pressure plays a role in the transport of water in the xylem in some plants and in some seasons, it does not account for most water transport. Few plants develop root pressures greater than 30lb/in2, and some develop no root pressure at all. The volume of fluid transported by root pressure is not enough to account for the measured movement of water in the xylem of most trees and vines. In fact, those plants with a reasonably good flow of sap are apt to have the lowest root pressures and vice versa. The highest root pressures occur in the spring when the sap is strongly hypertonic to soil water, but the rate of transpiration is low. In summer, when transpiration is high, and water is moving rapidly through the xylem, often no root pressure can be detected. So although root pressure may play a significant role in water transport in certain species, such as coconut palm, or at certain times, most plants meet their needs by transpiration pull. Lastly, I have never heard of the existence of "pumps" along the trunk. - Millet

Y agree with all you said but:

Millet wrote:

most plants meet their needs by transpiration pull.

Following science it cannot work.

Quote:

Lastly, I have never heard of the existence of "pumps" along the trunk. - Millet

Neither did I.

I asked many scientists about that, but they never had an answer.

Sylvain, when you wrote "Following science it cannot work" in the above post, I guess I do not understand what you are trying to say. In my thinking it is "Because of science it does work". When water is confined to tubes of very small bore, such as in the xylem of trees, the force of cohesion between water molecules imparts great strength to the column of water. Tensions as great as 3,000 lb/in2 are needed to break such a column, about the value needed to break steel wires of the same diameter. - Millet

Physics says that you cannot pull a column of water higher than 10m.
Higher, it cavitates. You can win few inches if the walls are very smooth but not many.
It is a question of vapor pressure.

Yes. very true, even the very best vacuum pump can pull water only up to a height of 34 feet or so. This is because a column of water that high exerts a pressure that is counter balanced by the pressure of the atmosphere (15-lb./in.2). The answer to solve the tree's dilemma lies with the cohesion tension of water (3000 lb/in2 in the xylem as stated above). This cohesion is due to the hydrogen bonding that water forms. What man cannot accomplish, nature has been created to easily accomplish on a daily basis. It has been estimated that over a growing season, one acre of forest trees may transpire 490,000 gallons of water. As liquid water, this would cover the field with a lake 18 inches deep. - Millet