Bittersweet is the only way to describe Noel Paul Stookey and Peter Yarrow’s concert Sunday afternoon at the Collins Center for the Arts. Both men acknowledged the absence of their longtime partner Mary Travers.

“Whenever we sing together or alone, Mary is very much with us,” Yarrow said after the duo was greeted by the nearly full house with a standing ovation. “But her absence is very profound.”

Travers died in Danbury, Conn., on Sept. 16, 2009, of leukemia at the age of 72. Her singing partners said Sunday they had attended the unveiling of her tombstone last week.

Without her, however, Stookey and Yarrow used the folk tunes the trio made popular — “Puff, the Magic Dragon,” “If I Had a Hammer,” “Blowin’ in the Wind,” “Leavin’ on a Jet Plane” and “This Land is Your Land” — to create a community inside the concert hall on the University of Maine campus.

Members of the audience, most of whom could remember when buying a Peter, Paul & Mary album was a political statement in support of civil rights and a protest against the Vietnam War rather than a musical choice, sang along. They seemed to embrace not just the lyrics, but what they symbolized to a past political era some have said was as angrily polarized as is the current one.

Several times the 72-year-old Stookey, who lives in Blue Hill, drew cheers from concertgoers when he referred to them as “neighbors.” Yarrow, 71, continues to live in his native New York City.

Yarrow appeared to be struggling with laryngitis Sunday, although why his voice was so rough was not explained. Stookey seemed to be more vibrant and sang two new songs that proved his creative juices continue to flow. Musically, his solo turn on stage was the highlight of the evening.

The group’s longtime backup musician Paul Prestopino played everything from the banjo to the dobro to the mandolin. He added much needed depth to Stookey and Yarrow’s performance.

Although Travers most likely was at the concert in spirit, her vocal absence was nothing short of profound. Perhaps it would be wrong to bring in a new female voice, but without that higher sound, the familiar songs made famous by the trio can no longer soar, and the harmonies of the two men aren’t enough to nurture a new generation musically or politically.