Plants, CO2 and temperature

Write4U said:
But the primary mechanism is suction, the draw from the evaporation of water from the leaves.
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Osmotic pressure is not well described as a "draw". It is generally felt or experienced as a "push." The difference becomes important in various intuitions or extrapolations - such as when one imagines the creation of turgidity in the cells of the leaf and petiole etc. The cells are being blown up like balloons, not expanding on their own to draw water in. This mechanism is how plants move, open and close their stomata, make non-woody parts rigid, fold and unfold their leaves, etc.
In this case, the additional force that pulls the water column up the vessels or tracheids is evapotranspiration,
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As water is lost out of the leaf cells through transpiration, a gradient is established whereby the movement of water out of the cell raises its osmotic concentration and, therefore, its suction pressure. This pressure allows these cells to suck water from adjoining cells which, in turn, take water from their adjoining cells, and so on--from leaves to twigs to branches to stems and down to the roots--maintaining a continuous pull.
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I am sorry to quarrel with a writer in Scientific American, but evapotranspiration is not a "force" and does no "pulling"; osmotic pressure should not be described as "suction" in this context. Those are unfortunate choices of vocabulary. They are apt to mislead, as it seems they have.
Write4U said:
First, there is a small push from either the gradient of the ground and the push from swollen capillaries in the roots which when filled exert a pressure upward.
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That's osmotic pressure, same as the rest of the mechanism (that's why salt kills trees, regardless of the "gradient of the ground" - roots surrounded by salt water can no longer establish a large enough osmotic pressure gradient, the tree can no longer obtain water fast enough to keep its sapwood and leaves replenished).

One reason this matters here is that a tree must balance its leaf area (or stomatal area, to focus) against the available osmotic pressure across the root area and the cross-sectional area of its sapwood, both two-dimensional. A taller tree can afford less leaf area in its sunlit canopy than a shorter but otherwise identical one, because the two-D area of root surface and sapwood cross-section necessary for obtaining and then lifting the extra water requires three dimensions of resources. There is no extra helpful "pull" from extra leaves - the sapwood and roots are "pushing" on their own, so to speak, and must get and move the water by their own efforts. Roots and sapwood volumes are three dimensional, and the tree has to build them.

So there is an increasing penalty for leaf area at increasing height - a tree that needs ever more height must eventually reduce the area of its leaves relative to its trunk and roots, by an amount depending on water supply circumstances in the ground and weather conditions at height and so forth. If this penalty is large enough, a very tall tree might be forced to actually shrink its leaf area in absolute terms - that may be rare, but having to reduce the relative investment in leaf area seems likely and matches observation (such as those photos).

And at any rate, age is not a good index or parameter or predictor for how much leaf area a tree can afford. Nobody uses it, because it doesn't work. Leaf area is governed by other physical circumstances.
 
iceaura said:
And at any rate, age is not a good index or parameter or predictor for how much leaf area a tree can afford. Nobody uses it, because it doesn't work. Leaf area is governed by other physical circumstances.
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Hight is also not a good parameter . And I don't think the question is what a tree can afford for its size, but what a tree needs for size.
I have posted pictures of old and young trees of various species, and is abundantly clear that aged trees generally carry more leaf area than younger trees of the same species, depending on if they grow up or grow out.

In any case, trees are good CO2 scrubbers and fixers and they offer an entire world to many forest species.
All very large dense forests are the "Lungs of the World" and are Host some 80% of all animal life on earth.

Cash crops like hemp do not have the long life or utility for animal support, but their accelerated growth cycle makes them more efficient CO2 scrubbers over shorter periods between harvests. And many hemp products can replace the exact same wood products, thereby allowing forest conservation.
 
James R said:
Why bother? Clearly you are incapable of having a civil conversation. I have no further interest in talking to you.
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Clearly you didn't bother reading my post, so if you had bothered, it would have saved you making useless posts.
 
iceaura said:
Osmotic pressure is not well described as a "draw". It is generally felt or experienced as a "push."
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Yep. That's one of the most common misconceptions in plant biology. Since suction can't work over more than 30 feet, suction alone is insufficient to get water to the top of tall trees.
 
Write4U said:
Cash crops like hemp do not have the long life or utility for animal support, but their accelerated growth cycle makes them more efficient CO2 scrubbers over shorter periods between harvests. And many hemp products can replace the exact same wood products, thereby allowing forest conservation
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https://www.bbc.com/reel/video/p09vw3lb/the-cannabis-plant-revolutionising-homes-of-the-future

An article/video feature today about building carbon neutral homes using mainly hemp.

I think it may have said that the construction industry accounts for 45% of carbon emissions....

As an aside today we had the (a?) Middle East Editor of the BBC staying with us as a customer . :)
 
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