It's unclear which version of Struts you're using. Struts 2 has a Dojo-based tree control (although as of S2.1.6 we've deprecated the Dojo-based Ajax tags). There are about a million other tree components out there, but none are part of Struts itself.
Seriously Rick? Seriously? You might as well just read this tiny ad:
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop