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Note that I ask about the longest possible tree and not the tallest possible tree.

There have been a number of questions about the tallest possible tree, and I wrote a detailed answer to one: https://worldbuilding.stackexchange.com/questions/191008/would-a-small-low-gravity-moon-be-able-to-harbor-complex-life/191115#191115[1]

Where I used to live there was a steep slope, somewhat less than 45 degrees I think. And a small tree on that slope fell over in the downhill direction, so that its angle was only a little bit above vertical. Some of the tree roots remained buried in the soil and the tree continued to have green leves for years or decades.

So I wondered about a hypothetical species of trees, perhaps on an alien planet, which grows on slopes and has almost horizontal trunks, casting shade on trees below it and sticking out far from the slope to avoid the shadows cast by the trees higher up the slope.

What factors would limit the growth of those trees to their maximum length, and what would that maximum length be?

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  • $\begingroup$ To be clear this kind of tree grows directly outward from the slope perpendicular to the direction of gravity? No bending up then across like a fern? $\endgroup$
    – Joe Bloggs
    Jan 23 at 22:03
  • $\begingroup$ giant sequoia actually join with each other creating contiguous forests. But I think what you are talking about would basically be a tree form rhizome which also exists but an extensive rhizome system is also indicative of grasses so bamboo forests might be your answer. $\endgroup$
    – John
    Jan 23 at 22:46
  • $\begingroup$ D...N...A... apple tree vs coast redwood. $\endgroup$
    – user6760
    Jan 24 at 11:54
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    $\begingroup$ You might want to investigate horizontal scrub - Anodopetalum biglandulosum $\endgroup$
    – Fred
    Mar 11 at 2:50
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I have once traversed over the branches of an horizontal tree that spans over 2 acres. Unlike Pando, which is a clonal forrest sprouting many trunks over the soil, Pirangi's giant cashew tree sprouts new roots from its branches so it could hypothetically keep on growing forever, as long as the soil would be appropriate for it. It is constrained by sidewalks and pavement on its sides, though, so it won't be growing much in the future.

Here's an aerial view. The green woods in the middle are a single tree:

Giant cashew tree of Pirangi

Some people from another city in Brazil claim to have a yet bigger one. I don't doubt that. A cousin of mine got a PhD studying genetic mutations on these trees that allow them to grow to such giant sizes.

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I don't think there's any limit beyond the fact that it isn't going to be able to grow beyond habitable terrain. A tree that goes along putting down roots and branches can do so until it either is killed by something or can't get enough nutrition. Think if it sort of as a colony organism.

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