In 2004 the United States Congress passed legislation designating a National Tree for the United States.  The oak tree was chosen.  The tree was selected after a four-month voting process hosted by the Arbor Day Foundation.  No specific oak species is selected, thus the genus Quercus must serve as the National Tree.  There are 90 species of oak native to the United States, in addition to many hybrids and a few introduced species.  Native oak species occur in every state except Alaska, Hawaii, and Idaho.  

(Directly derived from http://www.usna.usda.gov/Gardens/collections/statetrees.html)