Can fresh citrus juice be reduced to concentrate flavor?
Heating citrus changes its flavor quite dramatically.  When and how much to heat and reduce citrus depends on the outcome you are looking to achieve.  If you want to preserve the fresh squeezed flavor, heating is not advisable.  If you are looking for a more reduced, cooked down flavor, heating can be good. Certainly, the flavor will be concentrated, but it will be different from the fresh juice. Ultimately, it is the application that matters.