Trees are barely a firm category of plant at all. It's basically just tall plants with woody stems. Plants can gain and lose woody stems without too much trouble (relatively speaking, over evolutionary time). So any time a plant species currently growing soft stems can benefit from being really tall, they have a good chance of evolving into "trees".

I’ve seen rather large cactus turn the base of their stems woody and bark clad.

Not OP, but:

https://eukaryotewritesblog.com/2021/05/02/theres-no-such-th...

Thank you for link.

As an aside there: the blog post briefly talks about birds. It turns out that membrane wings are much easier to evolve than feathered wings. There have been lots of membrane winged creatures (including "birds" with membrane wings in the Jurassic) but not nearly as many appearances of feathered wings.

https://www.youtube.com/watch?v=HxA38gH8Gj4

One example is oak trees being more closely related to tulips than to pine trees.

(Tulips and oak trees are both angiosperms, flowering plants, and share a common angiosperm ancestor. Pine trees on the other hand are gymnosperms.)