The Gaul or Celts lived in big parts of Europe and what we call "Little Asia" -- todays Turkey mostly and northern Syria . .so they had different climate zones and of course when you are worshipping trees (or the spirits of trees) it depends on the flora of the geographical area you live in ... 1000 BC until the high medieval the climate in Europe was much warmer .. while the Roman Empire, when the Celts lived in central and western Europe, the Med-Sea was more than 3 degrees Celsuis warmer than today and so was the climate .. means no real ice-and-snow winters in cental Europe ... so its rather possible that fig trees grew until a rather high northern climate zone .. and we kept the tradition when it became colder - most of all because our pagan spirits were not allowed anymore and could not develope any longer together with the climate ... so I think its not so important what kind of trees you plant in your circle as long as they connect the land you live in with the sky above you .. its said that trees are the antennas of the earth to reach the skies and the universe .. and thats a good feeling to watch a tree like this ... take what grows in your environment -- and reach your own sky - I am sure the old Druids would smile and give us their blessings for it ;-)