Large pieces of solid hardwood cannot be relied on to stay flat and straight. It must be assembled in some type of structure that will influence it to stay that way. For wood floating shelves specifically, the cross-grain layers of  plywood core are better suited to the application. Nearly all the furniture-grade plywood we buy is made in the USA from domestic raw materials, with the exception of the face veneers on the Bamboo, Teak, and African Mahogany. Those trees only grow in other parts of the world. The core material of most of our floating shelf panels we specify domestic material from either Columbia Forest Products or Murphy, depending on availability. The teak and bamboo core material is imported Baltic birch.