Oaks (the most common tree in the area where I live and thus the kind I'm most familiar with) are usually less widened towards the ground and more root growth is present underground. Their roots also combine better into a trunk which is mostly round. Roots do, however, pop back up aboveground sometimes, and you could show this to extend the tree's pesence and anchor it to the ground.

 

Your colors could use some more contrast (Less saturation too? Mess around with it and see what looks good.) and the grass needs texture (excuse my ignorance of your plea for ignorance :p). The grass also could use to be turned into dirt around the tree's trunk where shadow from the tree inhibits its' growth.