Super tree

Fantasy tree? Whatever you want.

Growing things need water, light and food but since this is an alien tree, you can define it anyway you want.

Nutrition/light/moisture factors would have to be adequate (as just noted by the earlier poster). That's presuming you aren't going to go into some seriously weird and unusual biology different from Earth.

But other than that, trees are subject to wind and snow (or water) load and so their shape and leaf structures and rooting systems become critical, as do soil conditions that will help keep the tree in place.

You can pretty much design your fictional planet any way you wish, but on earth the height of the tree is limited by gravity so you might want to feature a planet with a weaker gravitational field if you are basing your botany on a terrestrial model.

Something like the trees in Avatar

Some of this will depend on genre. If you are writing fantasy (e. g. a magically-infused world), then you have a lot more freedom to make the tree as you please. If you want to write more of what's called 'hard science fiction', then you will need some form of an explanation or some sort of physical laws. You can make up your own physical laws, but you do have to have them (and keep them consistent or at least explain your exceptions in a way that satisfies your readers). Hence you might invent a new element and make that the reason the tree grows so tall and it might do something else for the animals on your fictional world.

If you split the difference, by writing either science fiction/fantasy (think stuff like the works of Edgar Rice Burroughs), or 'soft science fiction', sometimes called 'science fantasy', then you might be able to get away with not so much a magical explanation but more like a hand wave. So your world might have some decent physical laws but some magic or at least unexplained phenomena.

In the episode, "The Fugitive", in The Twilight Zone, Rod Serling gave a great definition to show the difference. In his introduction, he indicated the difference between science fiction (the improbable made possible) and fantasy (the impossible made probable). See: the Film School Rejects review.

If you want to see what can be done with odd trees, try The Integral Trees by Larry Niven. It's strange, but Niven is known for writing 'hard' science fiction.

The height of trees on Earth is limited not only by gravity but by how high water can travel upward within the tree's structure. This is limited by the size of the capillaries, the texture of their inner surfaces, and the air pressure. (Also by the inherent properties of water). Some of the tallest trees on Earth exceed those height limits because of the amount of moisture in the air in their particular location. Look into those mechanisms to write a convincing story!

This may help, the super tree from Norse mythology.

https://en.wikipedia.org/wiki/Yggdrasil

See Hothouse, Brian Aldiss, 1962. In the far future, the earth has become tide locked with the sun and a giant banyan tree covers the whole day side of the planet. Also The Integral Trees, Larry Niven, 1984, features trees which can grow up to 100 kilometres long.

I don't know if you would call a clonal colony one "tree". There is an organism called Pando in Utah which is a clonal colony of a single male quaking aspen determined to be a single living organism by identical genetic markers and assumed to have one massive underground root system. The plant is estimated to weigh collectively 6,000,000 kilograms (6,600 short tons) making it the heaviest known organism. The root system, at an estimated 80,000 years old, is among the oldest known living organisms. It covers 43 hectares (106 acres) and has over 40,000 stems (trunks), which die individually and are replaced by new stems growing from its roots. The average age of Pando's stems is 130 years, as indicated by tree rings.

I wonder how the idea of a planetary tree would cope with a planet that had night and day alternately in a single place, or seasons in different places?

Finally a thread about trees the I will ignore.

Tree size is relative to the observers scale. For example if there are aliens on this planet they could be the size of ants in our perspective as a hidden plot element. In this example the tree could be the size of an average bush to us yet to these aliens its huge.

Ray Bradberry was a master of plot elements like in the Martian Chronicals where humans were revealed to be the strange aliens and martians were the normal people. Simply reversed the perspective. However; as you are ready you assume the story is from the human perspective but it's not.

Fantasy stories do not need to be realistic, after all it's why people enjoy them. No one really wants to be reminded of the reality they already know. So break the rules and reveal them at certain points and the readers mind will be blown.

Why not propose that this planet is similar to earth was 350 million years (+/-14 yrs next Tuesday). There were "tree like giant plants ) like an Equisetum or other early gymnosperms, OR q planet considerably younger than a Triassic aged earth.
The big tree like plqnts were these "horsetails" or even Bqmboos . The reasons the grew so big and unusual was that their structure cells (called palisades) , were loded with SILICA .
The giant plants were, pound for pound , many times stronger than steel and the silica polymers often left little nodules that were like sand particles.
You coul make an entire world with those characteristics.
1The tree like plqnts could be over 50M tall
2Theyd be super strong and entire villages could be built up in the heights (as long as the dwellers had a way to fasten cross members to serve as floor joists)

3they were light

4they were hollow so a tree could be turned into a "tower" with an internal stairway.
5.Evryone had to scamper up th trees for protection against the indigenous things that liked the taste of whatever your dwellers tasted like.

6 MAybe the entire life of that planet was silica based, its possible. Carbon doesnt have an exclusive. Look at bamboo, its 75% carbon based and about 15% silica, but all the strength is from the elongated pre-stressed silica crystalline palisade cells.

There was an old Sci Fi book written before I was a kid . It was called "Day of the Triffids"
and these were silica based life forms

I think you're confusing the book with later films/radio broadcasts. Nothing on the wiki page about silicon.

https://en.wikipedia.org/wiki/Triffid

ya have to read the whole book. Its not a huge part of the narrative.
ANYWAY, my point was that a super-tree or plant could be the basis for a whole exo biology that has truth and science behind it.

I have read the whole book, admittedly a long time ago, but I don't recall anything about silicon base being mentioned. Anyway it's doesn't really add up, if as the book suggests, these plants are terrestrial they'd still be carbon based. Extra terrestrial stuff is another matter though, which is why I thought you were thinking of the film. There are other differences, in the film they arrive with the meteor storm that blinds everyone. In the book they're being farmed for oil prior to the storm. And in the film they're easily destroyed by seawater so we have a happy(ish) ending.

there are quite a few "terrestrial plants" that are loaded ith silica in their vascular systems which accounts for their structures. (Like bamboo, or horsetails, or tall grass, sedges and (I THINK) certain spwcies of cypress trees and "Lignum Vitae" also contain silica in their palisade cells.
Science is still trying tomfigure out how the silicic acid is transported throughout the growing plant.

They're still carbon based though.