I don’t know about how y’all feel about submissions in dedicated golfing languages/esolangs like this, but

Vyxal - 14 characters/SBCS bytes (22 UTF-8 bytes)

ɾ×*vṄ| |JøĊ⁋

Vyxal is a stack-based esolang designed to do well in code-golf competitions just like this one. It’s far from the only golfing language in existence, there’s many more like 05AB1E, Jelly and Thunno 2. This one is just one I made in 2020 (yes creative naming I know, I just couldn’t think of anything better).

You can try the program online here

Explained:

ɾ×*vṄ`| |`JøĊ⁋
ɾ               # Generate a list of numbers from 1 to the input.
 ×*             # Repeat the character "*" n many times, for each n in the above list
   vṄ           # Insert a space between each asterisk. Normally, Ṅ will join an entire list on spaces, but the v makes Ṅ apply to each item in a list.
     `| |`J     # Append the string "| |" to that list - this is the stump of the tree
           øĊ   # Centre each of the strings in the list, relative to the longest string in the list. This is a single built-in function (the ø indicates that it's a two-character function)
             ⁋  # Join the resulting list on newlines

For input = 5, the stack looks like so:

ɾ×*vṄ`| |`JøĊ⁋
ɾ                [1, 2, 3, 4, 5]
 ×*              ["*", "**", "***", "****", "*****"] (* does the multiplication, the × is the character that pushes "*" to the stack - it was a later addition)
   vṄ            ["*", "* *", "* * *", "* * * *", "* * * * *"]
     `| |`J      ["*", "* *", "* * *", "* * * *", "* * * * *", "| |"]
           øĊ    ["    *    ", "   * *   ", "  * * *  ", " * * * * ", "* * * * *", "   | |   "]
             ⁋   Output as in the original challenge post

As you can see, some built-ins automatically apply to each item in a list, as if you wrote [f(x) for x in iterable].

Once again, I don’t mean to come in and ruin golf for everyone, I just want to see how such a language might be seen by the community.