>You are not going to put a good enough system for an AP up in a tree. Maybe on an antenna mast structure with solid base and guy wires. Roofs and ground are better choices.
>But I would wonder whether redesigning the AP itself to be power-conserving would be the place to start. They are not designed to be "low power" - they are designed to be inexpensive.
Along these lines, would using cellphone chips be viable?
I'm trying to imagine a use case where decade long, maintenance free, tree mounting would be the preferred solution. The only thing I can think of is a cheap active repeater in an area with infrequent users. Push as much logic/processing back to a unit where power is cheap.
-CR