Download the family from hereTREE FAMILY (many types)
Or a project file with a fuller explanation from hereTREES COLLECTION
It would be interesting to combine this family with Randomizer and see what kind of forests you could create...
Along similar lines:
RevitCat: Revit Scaleable Spline Tree Family - http://t.co/K5dkONqzJn #Revit #BIM
— Revit Help (@RevitHelp) August 10, 2013