You didn't say what kind of tree. That could make a difference. You could pic-ax them out. And when you're finished, you may need some fresh loom to smoothe out the ground. If the ground is 'raised' because of the roots, just make a small 'bump' w/the loom. Then plant grass.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!