Many thanks for all the responses.

I remember that in the experiment I have mentioned, the coupling to the
tree was via an toroidal coil which was mounted around the tree, the
tree beeing in the axis.

Sure, a tree cannot be a good antenna. Any "real" antenna will probably
always be better. But often the question is: no antenna at all or
anything which does not seem to be an antenna. Perhaps a tree can be
such a thing. Further, in portable operation, it is not always easy to
install a "real" antenna.

Claude