You can go big bucks with HPS/MH lighting or set up a compact fluorescent rig on the cheap- it all depends on your budget and what you expect out of the system.
Also how nice you want it to look, lol.

I have one area of high light plants under switchable ballast HPS/MH lights and another section of lower light plants under modified compact fluorescent aquarium lighting. All of my tropicals are inside a vinyl enclosure to hold in warmth and humidity with fans for ventilation.
You could do something like this or merely set up some lights pointing at your trees.

Check out the "Growing under lights" forum for some really good ideas!

Here is a link that might be useful: Growing under lights