About trees & olive oil
Oil by means of lighting
Already since pre-palatial times (in Minoan Crete), olive oil seemed to be widely used as a means of lighting and this is concluded from the large number of lamps that have been found. Other sources of lighting were animal fat as well as various vegetable oils.
In the Byzantine era, olive oil was used as means of lighting in private or public majestic buildings and church premises.
Keep in mind that, in contemporary times, oil lamps lighted the houses in several Greek villages even at the late 20th century.