Michael Tree, a viola player whose lyrical, deeply expressive playing style helped make the Guarneri Quartet one of the world’s preeminent chamber groups, and whose rambunctious personality and diplomatic approach were key to the ensemble’s unrivaled longevity, died March 30 at his home in Manhattan. He was 84.

He had Parkinson’s disease, said his wife, Jani Tree.

Mr. Tree was a violin prodigy before he turned to the viola, a larger yet less-prestigious cousin of the violin that he helped elevate through six decades of teaching and performance, most of it with the Guarneri Quartet.

Named after a family of distinguished Italian luthiers, the ensemble formed in 1964 and performed almost nonstop until disbanding in 2009, touring around the world and sometimes appearing at more than 200 shows each year. Described in a 1970 New York Times review as “the most famous American ensemble since the Juilliard String Quartet came into being after World War II,” the group was credited with spurring a resurgence of chamber music in the United States, where a handful of ensembles multiplied into about 250 groups by the late 1970s.

The Guarneri Quartet recorded cycles of Beethoven’s quartets for RCA and Philips, collaborated with pianist Arthur Rubinstein on pieces by Brahms and Schumann and performed quartets by Bela Bartok and Czech composer Bedrich Smetana, whose String Quartet No. 1 (“From My Life”) proved a perfect vehicle for Mr. Tree’s powerful bow arm and gymnastic fingering.

“At the end of the first movement is a massive viola solo,” Arnold Steinhardt, a violinist for the Guarneri, said in a recent interview, recalling an early recording by the group. “It’s as if Michael has mounted his white steed and said ‘Charge!’ He really goes for broke. Michael has the ability not only to play the solo splendidly, but also with such stylishness and verve. It’s fearless playing.”

With the Guarneri, Mr. Tree played alongside violinists Steinhardt and John Dalley, with whom he attended the Curtis Institute of Music, and cellist David Soyer, who died in 2010. The ensemble had just one personnel change, when Peter Wiley stepped in after Soyer’s retirement in 2001.

All four original musicians honed their craft at the Marlboro Music Festival, a summertime retreat in Vermont where Mr. Tree and Soyer started a group called the Marlboro Trio around 1960. For years, they saw each other about as much as they saw their families, and described their outfit in brotherly terms: bonded together for life, though rocked by occasional disputes and personality clashes that sometimes spilled into public view.

A 1989 documentary, “High Fidelity,” showed the group arguing over parts and tempos and asking for hotel rooms on different floors. The ensemble was “a democracy,” Mr. Tree noted in the movie, “and there’s nothing more balky and time-wasting than a democracy.”

At times, he served as a stabilizing presence, mediating between different views, joking about the quality of concert halls, extolling the unexpected flavor of a Chicago bagel or playing tennis matches with Steinhardt amid a seemingly endless string of performances and rehearsals.

“A typical Guarneri rehearsal on tour takes place in a motel room,” music writer David Blum observed in his 1986 book “The Art of Quartet Playing,” “music strewn over beds and chairs, Michael Tree in tennis garb, and all the players in the most casual and unlikely poses for serious music making. Yet once bow touches string, they give the task in hand their total concentration.”

Michael Applebaum was born in Newark on Feb. 19, 1934. He began using the last name Tree at the insistence of Efrem Zimbalist, the patrician violinist and longtime director at the Curtis Institute in Philadelphia, which Mr. Tree entered at the young age of 12. (His father was sometimes quoted as saying, “Only Applebaum can make a Tree.”) The name change, friends speculated, may have been driven by fears of anti-Semitism.

A young Michael had begun playing the violin at 5, tutored in unstructured half-hour increments by his father, Samuel Applebaum, a violin teacher at the Manhattan School of Music. His father and mother, Russian-born Sada Rothman Applebaum, both wrote music education texts.

Mr. Tree graduated from Curtis in 1955, shortly after appearing at Carnegie Hall for the first time — making what the New York Herald Tribune described as “probably the most brilliant young debut in the recent past.”

He was still playing the violin, and went on to the play the instrument as a soloist with major orchestras in Philadelphia, Baltimore and Los Angeles. Mr. Tree said he did not even own a viola when he began playing it for the Guarneri Quartet. In a sign of the instrument’s diminished stature in the realm of classical music, the ensemble was said to have selected Mr. Tree for the instrument on a coin toss.

Mr. Tree insisted the story was apocryphal, and in later years often switched between the violin and viola while performing and teaching at schools including the Manhattan School of Music, the Juilliard School, Bard College Conservatory of Music, Curtis, the University of Southern California and the University of Maryland.

“Michael set a new standard for the viola,” Steinhardt said. “Now orchestras are not filled with failed violinists playing the viola, but with sensational violists,” performers whom Mr. Tree encouraged to treat the viola as an integral part of an ensemble, rather than a backing voice supporting a violin’s melody or cello’s ostinato.

Mr. Tree met his wife of 51 years, the former Jani Kreck, backstage after the Guarneri made their New York debut at the New School in 1965. She had been sitting in the front row and said in an interview that the two had locked eyes just before the performance. An earlier marriage to Marlene Kleinman ended in divorce.

In addition to his wife, of Manhattan, survivors include two children from their marriage, Konrad Tree of Miami Beach and Anna Tree of Manhattan; and one grandson.

Mr. Tree “had a virtuoso technique, setting a standard for a whole generation of players, but what really made his playing so memorable was his sound,” said pianist Lydia Artymiw, who performed and recorded with Mr. Tree and the Guarneri. It was a sound that Mr. Tree once likened to a rich piece of chocolate.

“He must have played the same quartets hundreds of times,” Artymiw continued, “but it never sounded that way.”