This is a rather strange and fairly straightforward question. Is there any magical method for someone to turn themselves into tree, whether it be arcane, divine, or through the use of a magic item? I'm asking because one of the characters in my campaign really wants to become a tree through whatever means necessary. The details don't matter to them - whether it's eternal or not, whether they survive the process or effectively die.
From core, the spell Tree Shape (Druid 2, Ranger 3) works:
By means of this spell, you are able to assume the form of a Large living tree or shrub or a Large dead tree trunk with a small number of limbs.
Some more flexible options are polymorph, more powerful versions of polymorph e.g. shapechange, and a 12th level druid's wild shape ability. All would let you assume the form of a tree, but unsurprisingly the less flexible tree shape is both lower level and has a longer duration.
If the player wants to be an ambulatory tree, then either the polymorph spells or the wild shaping of a 15th level druid would let you become a treant. There might be smaller ambulatory trees not in the SRD; the Huge size is a limiting factor for wild shape.
Permanently, and Forever, A Tree.
Get a high level Wu Jen to cast Arboreal Transformation on you.
Use the Psionic Sandwich trick, but on a tree.
Use Polymorph Any Object to change yourself into a Bush, and then have someone else use Polymorph Any Object to change you into a Tree. (it checks for duration at cast, original polymorph wears off but doesn't matter, as you are still permanently a tree)
Cover yourself in quintessence and have someone grow a tree around you, Han Solo Frozen In Carbonite style.
Have someone lay down a Energy Transformation Field, key it to an Ocular Tree Shape, and mindrape an automaton (Warforged work well for this), bury it beneath the ground with an Immovable Rod, instruct it to keep pressing the button forever. Stand where the Ocular Spell is aimed. BOOM. Instant and forever tree.
Be a Tree, get turned into a human via Polymorph Any Object. Gain levels, go adventuring, and then get dispelled. BOOM TREE.
Anyone else has any ideas, I will add.
In Dungeon Crawl Classics #10 (which I'm currently DMming for a party), there's a black moss that grows in a cavern, mutated by the malevolent force of a meteorite that crashed into an underground garden.
From the game (pg 3-4):
"Black Moss poison is toxic when inhaled or ingested by itself or through a neutral medium, such as water. Tasting the water requires a DC 18 Fortitude save to avoid the effects of the black moss powder.
Victims who fail their saving throw have no effects for the first 24 hours, then take 1d6 Dex damage each day until they receive either the antidote or a magical cure. Those poisoned feel weary and ill, and most simply wish to take to bed. With each point of Dex loss, the victim looks more and more plantlike - skin color starts to resemble bark, hair turns to leaves, etc. When the victim's Dex reaches zero, they become a tall black tree, stationary and helpless."
Being in contact with just a pinch of the white soil from which the black moss powder is harvested, is enough to undo the magical effect of the black moss poison.
Probably not exactly what your player was looking for, and definitely not in the core rulebooks, but I finished up a session of this campaign not two hours ago and thought it was a neat effect.