In 6 minutes I made this Big Tree. I used 4 brushes only. No hand work was made, here's a example how fast it can be made.
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Version: 1.7.2
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Brushes:
Oak Wood - //brush sphere 17:12 2
                    - //brush sphere 17:12 1
Leaves - //brush sphere 35%18:4,65%0 3
              - //brush sphere 35%18:4,65%0 1