Create new layer, select Brush tool, open brush properties, and find the grass brush (scale it down as needed). make foreground black and background blue.
fill all the earth with grass.
Use the same technique as above to blur top and bottom of grass area.
and lastly, change layer blend mode to overlay.
now let's make some trees! Open ContextFree - this is cool fractal freeware available from http://www.contextfreeart.org/wiki - and choose Demo1 in Examples menu.
Manipulate the code on left column unless you are satisfied with the tree you get. I really did not want to learn the code, so I removed couple of lines, changed couple of digits, etc.
When the tree is rendered, save it as PNG and open this PNG in Photoshop.