Which tree do you stand under to evolve Bolty?

The tree is a fight location: the Alpha Blazen waits there, and beating it is what counts. If you just go and stand at the Blitz Tree, Bolty will not evolve. You have to win the Alpha fight there.

This is one of the evolutions that ask for a condition on top of levelling. Defeating a specific Alpha of the target species is a common way the game gates them. Until the condition is met, you cannot spend Bolty's evolution materials, so beat the Alpha before you gather or spend anything.

If you lose track of where to go, open Bolty's evolution requirement in the menu and select "how to get". It shows where the required boss can be found, and it does the same for the materials the evolution needs, including which Aniimo drop them.

Not settled: It is not settled whether the Blitz Tree step belongs to a quest in Blitz Wood or is simply the evolution condition. Either way, the Alpha Blazen fight under the tree is the part you have to complete.