Author:      Roman Werner (@RomWer)
System:      C64
Language:    BASIC V2
Length SRC:  68
Length EXE:  70
Run on:      VICE Emulator
Description: Uses screen codes for the values 1 3 5 7 3 7 11 15 5 11 17 23 3 3

Source Code (CBM prg Studio format):
0i=i+1:a=aS(mI("{ct a}{ct c}{ct e}{ct g}{ct c}{ct g}{ct k}{ct o}{ct e}{ct k}{ct q}{ct w}{ct c}{ct c}",i)):?tA20-a/2):fOj=1toa:?"*";:nE:?:gO


By the way, my first approach was this one, which would scale better for a larger tree but failed then for the tree stump.
0fOb=2to6stE2:fOj=b-1to4*b-1stEb:?tA20-j/2):fOk=1toj:?"*";:nE:?:nE:nE
