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A tree root lying under several square meters of 100mm thick concrete pavement can cause the pavement to lift up as it grows.

What forces are involved in creating this lift?

I vaguely understand that the growth process is a matter of cell division, but my layman brain can't reconcile this with lifting so much weight.

I'm not asking for a description of the biology of plant growth, but rather how that growth works at the atomic or molecular level to overcome such pressure, and what forces this process employs.

Please apply whatever tags you see fit. I don't even know which -mechanics this is.

Qmechanic
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Igby Largeman
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5 Answers5

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The fundamental mechanism is hydrostatic pressure, which in a plant is called turgor pressure:

Cell expansion and an increase in turgor pressure is due to inward diffusion of water into the cell, and turgor pressure increases due to the increasing volume of vacuolar sap. A growing root cell's turgor pressure can be up to 0.6 MPa, which is over three times that of a car tire ... As plants can operate at such high pressures, this can explain why they can grow through asphalt and other hard surfaces.

BioPhysicist
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gandalf61
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I think osmosis is a good starting point for understanding this. This is the root cause (ha!) of the turgor pressure previously mentioned. Water will flow an area of low concentration (of anything dissolved) to an area of high concentration, until the concentrations are equal.

The opposite effect (reverse osmosis) can be used in industrial processes to concentrate something (for example, concentrating salt from seawater to produce brine and fresh water). This type of process requires very high pressure (>5 bar as a rough example) to work against the osmotic pressure, demonstrating how powerful osmosis can be.

I guess if a plant can get into a crack and then use osmosis to drive water into this location, it's equivalent to using a pump to drive water into the crack. Consider how hydraulic systems produce a force, or think about lift bags used by the fire service.

I'm not a biologist, so don't know exactly how the plant would do this, but I think if there are drier root cells that contain sugars in a crack and a supply of water within the plant (plus lower concentration of sugars), this water would be drawn into the cells in the crack and they would expand with significant force.

A final though is the force water can exert as it expands as it changes temperature. This might not be a factor here, but goes to show how something trivial seeming can actually generate massive force.

Mark Hickford
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This is an interesting question. Two options come to mind:

  • Growth into the cracks and then expanding inside of them. This does not fully explain it, but it is worth mentioning that the force may be not necessarily directed upwards.
  • Cellular growth is really a mocelular self-assembly - amino-acids assemble in proteins/lipids/etc., proteins assemble in cellular walls, cells assemble in tissues, etc. On such a microscopic level one usually does not reason in terms of forces and Newtonian mechanics, yet, when scaled up to the scale of a few meters, these interactions should be substantially strong.
Roger V.
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By photosynthesis the tree assimilates carbon dioxide from the air and water from the soil, and converts it to carbohydrates, which makes up the main component of the wooden roots. $$\underbrace{n\ CO_2}_{\text{from air}} + \underbrace{n\ H_2O}_{\text{water from soil}} + \text{light} \to \underbrace{(CH_2O)_n}_{\text{wood}} + \underbrace{n\ O_2}_{\text{to air}}$$

So there is more stuff (the $C$ atoms) added to the underground which was not there before. This new stuff needs additional volume. And because wood is a hard stuff (due to the strong chemical bonds of the carbohydrates making up the wood), it will push the soil aside and up, and finally the concrete above the root will break.

Thomas Fritsch
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Imagine putting black powder under the same concrete slab and detonating it, I think the explosion will cause the concrete to break: thus, combustion causes small, loose particles of explosive dust to lift a big, solid, compact slab of concrete. A different chemical reaction can cause the same effect to happen over a longer period of time, with more or less dramatic results, in the case of tree fibres lifting concrete.

albert stasyuk
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