Interpreting the mid-1970's music of Joni Mitchell isn't a straightforward task because the records she made then were perfect crosses between professionalism and séance. They were the sound of Los Angeles studio pop at its most elastic, and their arrangements were not just functional brickwork but a blurring of improvisation and composition, with little instrumental quirks inscribed everywhere within.
Ms. Mitchell hired jazz players, and the records connoted jazz, especially in the way she draped her beguiling phrasing all over the music -- even if the market didn't see them that way. But more and more these days, her music, with its serpentine melodies and dramatic key changes, has been claimed by improvisers, and "Joni's Jazz," a Summerstage concert in Central Park on Thursday night, was a milestone in the furthering of Ms. Mitchell's status as an artist broader than the pop world that initially embraced her.
With a similar mixture of jazz and pop musicians, the concert's first half presented tracks from different albums; the second was all the songs on the 1976 record "Hejira," in their original sequence. The idea was done justice, with the guitarist Vernon Reid acting as musical director for a variable house band of 3 to 14 musicians and providing new arrangements.
Adding creative tension to the evening was the presence of Ms. Mitchell, generally known to be a perfectionist, in the audience. Before trying his hand at "Court and Spark," Duncan Sheik sheepishly said he wasn't worthy. Through "People's Parties," Jane Siberry kept up a run of fidgety commentary about her fantasy version of the song, which would have Ms. Mitchell joining her on the last words. (It didn't happen.) And John Kelly, who became known a few years ago with a nightclub act in which he sang Ms. Mitchell's music while wearing a blond wig, sang "Shadows and Light" with such dramatic concentration that one felt he was delivering the performance of his life.
Many singers needed lyric sheets, either because of nervousness or the difficulty of memorizing Ms. Mitchell's torrential images. (When Ms. Siberry got to the part in "Strange Boy" about "stiff-blue-haired-house-rules," she stopped the music to remark that only Ms. Mitchell could have written a line like that.) Indeed, the degree to which the singers had internalized the lyrics seemed commensurate with the success of each performance.
On the high end of that scale, Chaka Khan burst onstage three times and sang with nearly outrageous confidence; she knew her songs ("Don't Interrupt the Sorrow," "The Hissing of Summer Lawns" and "Hejira") inside out and improvised all through them, making the words indistinct as she transformed them into pop-gospel extravaganzas. She also tried to tell the audience the story behind one of the songs, turning to Ms. Mitchell for fact-checking; the composer, perhaps wary of giving away secrets, put a hand to her lips to shush her.
It was the "Hejira" section that fully validated the concert: Mr. Reid's arrangements protected the slow grooves of the songs while adding elements like Doug Weiselman and Don Byron's double-clarinet obbligatos, Brian Charette's Hammond organ and Leon Gruenbaum's melodica. There were almost no solos, and none of the musicians showed off; Mr. Reid played only one distorted, note-spraying guitar passage, and it was over in a flash.
After an encore with the jazz singers Annie Ross and Jon Hendricks, Ms. Mitchell was enticed onstage while the band played her biggest hit, "Help Me." She was clearly gratified, but she wriggled out of giving a performance: she apologized that she couldn't remember the words, declared that she was really on vacation, asked if she couldn't sing a standard instead, and finally improvised wordlessly for a minute, letting her voice, now deeper than it was back then, hover around in a comfortably low register.