Feller

function fell()
  local isAir = false

  fellStart()

  repeat
    turtle.digUp()
    turtle.up()
    isAir = not compareWood()
  until isAir

  goDown()
end

function fellStart()
  turtle.dig()
  turtle.forward()
end

function compareWood()
  local isWood = false

  turtle.select(16)
  isWood = turtle.compareUp()

  turtle.select(1)
  return isWood
end

function goDown()
  local isGround = false

  repeat
    turtle.down()
    turtle.select(15)
    isGround = not turtle.compareDown()
  until isGround
end

fell()