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I was watching a documentary about abandoned soviet structures and cities. At the beginning of the documentary a piece of drone footage is shown featuring the abandoned flats of pripyat from above. While watching it i noticed something very peculiar, there were several fullgrown shrubs and even a tree growing on top of the roofs of the flats.

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The red arrow points at a berch tree that's apparently growing on top of the rooftop as well. How is such a large plant able to grow there? I've seen plants grow in strange places in my own city Groningen as well. Here's an example of small shrubs that managed to grow out of asphalt.

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Here are a couple of pictures of 2 seedlings that are growing out of a hole of a rotten piece of wood. The hole does not go all the way through the wood, so the roots are not in contact with the soil.

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According to ChatGPT one explanation could be that over time, organic debris like leaves, bird droppings, and dead insects can accumulate on surfaces. This can decompose and form a thin layer of organic matter that serves as a growing medium for plants. But I can't believe that a tree such as the birch tree in the first video still could grow so large using only organic matter that deposited on the rooftop.

So how does this work? Can trees and shrubs use different materials like asphalt or roofpanels to extract nutrients from when there is little to no soil available? If so, how do they do it?

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  • $\begingroup$ What two main things are plants made of (think of photosynthesis), and where do those things come from? $\endgroup$
    – Bryan Krause
    Commented Sep 29, 2023 at 0:47
  • $\begingroup$ @BryanKrause, Plants need minerals as well.. $\endgroup$
    – Maurice
    Commented Sep 29, 2023 at 14:38
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    $\begingroup$ Sure, but not in the same quantities. I'm not really sure what you're looking for in an answer: clearly plants you see growing somewhere are getting everything they need there, or you're assuming they need more than they actually do. Exactly how they get it will depend on the exact situation, and the plants will grow just as much as they're capable of in the conditions. $\endgroup$
    – Bryan Krause
    Commented Sep 29, 2023 at 14:43
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    $\begingroup$ Plants mostly need air, sunlight, water, and the appropriate range of temperature to grow. Growing through asphalt is not uncommon; water seeps through a break in the asphalt, reaching a seed beneath it, which (believe it or not) when it swells with water is able to break up the asphalt. They also need nitrogen, potassium, phosphorus, and iron, but not in great amounts. $\endgroup$ Commented Sep 29, 2023 at 19:49
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    $\begingroup$ I think the key is gene expression (specifically, plant response to stress). Plants are living organisms whose genetic material gets altered over time to express what is needed to grow in whatever environment they are. $\endgroup$ Commented Oct 1, 2023 at 9:10

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Bromeliads are called air-plants because they don't need soil, just rain and run off from above.

Photo 3/ That is a plant with big cotyledon leaves which comes from a big seed with lots of energy resources. As the seed sprouts it transforms ambient water into a radicle (embryonic root) of 10-20-30cm that is biologically designed to survive challenging conditions, grow straight down and find soil. Wet rotten wood is a perfect medium for propagation of radicles, it has a PH of 4-6 after a few years, which is similar to the humic acid rich leaf humus of the forest, and is easy for the taproot to travel down 20-30cm at a rate of an inch per day, to find regular water.

Photo 2/ A growing root cell's turgor pressure can be up to 0.6 MPa, which is over three times that of a car tire. Roots can search their environment using micron-thin rootlets the width of individual cells that propagate forwards and under patches of wet leaves or moss where they sprout in the spring time. A tree just has to find a hairline crack through the tarmac to establish a lifeline to the soil. Once it is there, the cells can use chemistry to pump water like a hydraulic jack and push new cells upwards through tarmac. Redwood bark cells have 115m of water pressure, equivalent to a hose that ascends a 35 story building. 20m of water pressure in a hose is very difficult to stop manually.

Photo 1/ That roof collects a lot of water when it does rain, which can collect in pools, under cracked asphat roof linings, and can create pools and cool damp zones in the abandoned building interstices which last for some weeks in summer. They are as big as their access to soil permits.

Those trees require a lot of water for the summer and are sized relative to their resources, they are northern plants like birch who's white trunk is white flexible bark which protects from cold and fast temperature changes due to frost and sun, it also reflects summer sun so that the tree stays cooler in arid times.

Trees also meter their growth by available soil and water and make natural bonsais. There are trees in cliffs with a thousand years of narrow growth rings that are just 1-2 meters tall. Bonsai cliff trees are often prized for dendrochronology.

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These are highly specific adaptations to an environment with specific traits. Organisms that are pushing their boundaries (pH, amount of needed water and nutrients and specific proportions, etc...) in some direction. This could be plant adaptations to salt stress, heavy metal stress, or to high temperature, low temperature, or even drought conditions.

The key is the regulation of gene expression to produce the gene products necessary for a given environment.

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