Aaron Copland’s Appalachian Spring tells the story of a young couple on the American frontier, from the wedding-day jitters to festive songs and dances to passages of utter serenity. It is a quintessentially American work, a nostalgic glance back to America’s “Westward, ho!” history, well-complemented by Copland’s Four Dances from Rodeo, another ballet, this one about a cowgirl trying to snare the attention of a head rancher and set to American folk tunes. The concert opens with star American soprano Susanna Phillips singing Samuel Barber’s Knoxville: Summer of 1915, a setting of a James Agee poem describing an evening in the South, and Fletcher’s Three American Songs.