use rounds
i'd advise against this until you can actually build blocky trees first, using plates and bricks. until you can pull off something decent using those any attempt at using rounds will probably kill the look.
imo the best way to get a tree done, with detail, is to build the tree trunk, branches, etc in the shape I want, then fill it out with leaves, adding extra branches where necessary. this can be done using many different sizes and thicknesses of bricks to raise or lower the detail level.
a recognizable acacia tree probably can't be done with a low brickcount, unless you're just going for the general shape.