I painted this tree for a composition I'm working on right now; it took me like 2 hours to get acceptable results (too many parameters: mesh, branches, flowers, buds, leaves, etc., etc.). At the end, the thing was hugh, specially when converting to polys: more than 3 million faces! Even with 1 giga of ram, Maya crashed badly.
Later I saw the problem: I had too many subdivisions (7 for each branch, 4 for each leaf and so on). I reduced all that and then, I got a nice 300.000 faces polygon tree...
Then I duplicated the poly tree and moved the copy a little to the right, faking a bigger tree.
I saw at CGTalk a different approach: render the tree in Maya software and then compose it with the rest. As I'm not sure how to do that, I chosed the first way.