As for the dedication, I'll have to take a few educated guesses. Lutherans normally dedicate those things which will be part of the worshiping community or the Divine Service. This is a common practice in much of Christendom. Our Agenda, a book of occasional services and special rites, has dedicatory rites for buildings, organs, bells, church furnishings and such. Since the tree will be used as a focal point in corporate worship, perhaps they are following this tack and doing the same with it.
Some churches use it to identify their area for worship.
There are many other special words used in church architecture. That finishes another week. A Study Guide in Christian Teachings for Adults.