Well, it depends how your fairy lights should look like. In general, your trees can be used as a base Surface for distribution of your lights - it'll adopt your geometry (trees) as you can double check on the attached screenshot. As for any other Forest object you have a lot of control over it for tweaking the final look.
In case you are looking for aligning all lights, probably the use of Creation Tools (under Tree Editor - Custom Edit) will be better option. With this tool you can choose the the path and spacing distance. Attaching screenshot.
Hope you'll find it useful, but in case of any further question or doubt, please don't hesitate to ask.