UPDATE!
It seems that many users are not familiar with using multiple processes in their slicer. So, I have uploaded an EXAMPLE Simplify3d factory file and settings files for the 2 parts that need to be printed together, but as separate processes. The profiles are Simplify3d's default Ender 3 profiles with the simple modifications required to the print the trees as already described in the directions below.
I have been hesitant to include these because there are more settings specific to your printer/filament that you'll need to modify than there are necessary to printing the trees. So PLEASE be aware that these are not magic files that will allow you to print these trees properly according to your unique setup. Use them only as a reference if you're unfamiliar with how to tune slicer settings.
As far as the troubles people have had with other slicers, such as Cura, unfortunately I can't help you there, since I use Simplify3d. I have, however, included the .gcode file for the 160mm tree as a reference for those…