Some years ago experiments have be done using trees as antennas. Where
can I find references about ? Do fellows here known about the results ?

Seems like I remember an article in the 70's in 73 magazine (what else?)
about this very subject. I don't have time to look back thru my old mags., but
I bet the results indicated trees don't work well as antennas. If they did, a
lot more hams would be using them. I don't think I have ever had a contact
with someone using a tree.
73 Gary N4AST