Turtle: Tree

function fuel()
  if turtle.getFuelLevel() <= 2 then
    turtle.select(16)
    turtle.refuel(1)
    turtle.select(1)
  end
end

function up()
while true do
  fuel()
  if turtle.compareUp() then
      turtle.digUp()
      turtle.up()
  else
    break
  end
end
end

function down()
  while not turtle.detectDown() do
    turtle.down()
  end
end

turtle.dig()
turtle.forward()
up()
down()