yeah, that's Paint FX.
What you do is create an object (usually NURBS, in this case, a NURBS sphere).
In the Rendering menu set, go to Paint Effects > Make Paintable.
This, obviously, makes the object paintable.
Then, go to Window > General Editors > Visor. By default, the Paint Effects tab should be selected and the different categories are visible. Choose trees. Choose a kind of tree brush. I'm choosing birchBlowing.mel.
Now, when you mouse over the sphere, the painting icon should appear (a red circular cursor). Left mouse click and drag to paint on the sphere. You'll see a number of trees "grow" out of the sphere, depending on how long a stroke you draw. When you let go of the mouse, you'll also see many of them disapear. That's because by default, only 20% of what is actually created is displayed because of how taxing they are on the computer.
Hit Ctrl + A for the Attributes. Choose the birchBlowing tab at the top and you'll find the dozens of sliders and options for your tree to control number of leaves, colors, twigs, etc.