You might have seen the so called Pythagorean tree. This fractal is usually shown in two dimensions (search the internet for "Pythagorean tree") and resembles a treelike structure.

Although you can imagine it is some sort of a tree, is is obviously artifical. The main reason is, however, the object lacks the third dimension. When you use the same fractal algorithm, but apply this in three dimensions, you get an object that is quite close to a real tree.

Here I posted ten STL-files, numbered 0 to 9. Each consecutive object fits onto the previous one. Start at some number, print the next one twice, the next next one four times, et cetera.

For your convenience I posted a few sets as well, so that the parts for a small tree can be printed directly.

Use brim to prevent the parts falling over. There is no need for supprt. The branches have an angle of 36 degrees with respect to the trunk.

The cylinders have a tapered shape such that they will almost always fit. The hole in the trunk (from below) is slightly wider at floor level so that some spread out of printing material will not block the hole.

pythtree0.stl = 24mm diameter, 72mm height pythtree1.stl = 22mm diameter, 65mm height pythtree2.stl = 20mm diameter, 59mm height pythtree3.stl = 18mm diameter, 53mm height pythtree4.stl = 16mm diameter, 47mm height pythtree5.stl = 14mm diameter, 43mm height pythtree6.stl = 12mm diameter, 38mm height pythtree7.stl = 10mm diameter, 35mm height pythtree8.stl = 8mm diameter, 31mm height pythtree9.stl = 6mm diameter, 28mm height