You can't talk about day two of Mountain Jam 2016 without bringing up Courtney Barnett's set first.

Barnett, who played on Friday afternoon, brought a bunch of elements to the main stage that hadn't been there until her arrival: Insistence, frustration, darkness, and lethargy. When she started her set, it felt like a hard refresh, a shaking off of the crust. She sprawled herself out with her guitar in the middle of the giant main stage — which seemed even bigger in the middle of the day and no stage lights, with her relatively tiny three-piece band — during "Small Poppy." Her Fender Jaguar scraped and squawked, the flanger giving it an ironic shine. It was a messy and sludgy. We couldn't help thinking of Kurt Cobain.

Later, she scratched out "New Speedway Boogie," a grimy, lazily-dangerous take on the Grateful Dead original her band was playing live at Mountain Jam for the first time. Its chorus, "Spent a little time on the mountain / Spent a little time on the hill," is liable to become this year's unofficial Mountain Jam anthem. Even Jason Isbell, who took the main stage next, tweeted his approval.

Isbell's own set was an emotional stunner. He took "Codeine" and wrung all of the heartbreak and jealousy out of it, making the sunshine and the trees melt away. He pushed the slow boil of "Children of Children," his song about his mother, to its limits. His voice was strong and clear, with a force that filled up the little valley.

"Cover Me Up," which tells the story of how his wife saved him from alcoholism, was pushed forward by remarkably delicate backing from drummer Chad Gamble and bassist Jimbo Hart. It's such a vulnerable song, almost obscenely so, but Isbell delivers it with a ringing, defiant strength. It's enough to lay waste to your emotions. Near the end of the set, the sun crept out and lit up the stage, and Isbell stepped forward for a huge, cathartic slide solo. By the time the band left the stage, the audience was left in something like an emotional daze.

Phierce Photo

Wilco's headlining show got off to a relatively modest start. Not that the band didn't sound good — Wilco never sounds bad— but they front-loaded the set with a lot of newer material, and they had to work against that a bit to build their energy. And there's something about the way Jeff Tweedy hides behind his hat, glasses, jacket, and big, shaggy beard that makes him feel inaccessible. When he sings a song like he means it, especially a classic like "Hummingbird," it feels like he's reluctantly let you in.

Of course, much of the thrill of seeing Wilco comes from their big, demolition-style fits of noise, and when they played the Summerteeth classic "Via Chicago," the band, and especially drummer Glenn Kotche, surprised us by showing how hard they can still push the song into shattering, visceral territory. By the time they made it through "Spiders (Kidsmoke)," with its stop-start motorik rhythm workout, it felt like the band had undergone a transformation.

With that burst of energy they cruised through all the good old sing-alongs: "Box Full of Letters," as jangly and harmony-laden as you could want; "Heavy Metal Drummer," with its swooning chorus; "The Late Greats," with that breezy mix of electric growl and wistful acoustic. When a woman jumped onto the stage to dance between John Stirratt and Pat Sansone, the band were all grins. "That hasn't happened since the '90s," Tweedy joked.

Nels Cline, thankfully, also had plenty of opportunities to rip into the music. His solo on "Impossible Germany" was a thing of beauty, balancing his usual impulse to throttle sound out of the instrument with the song's own meandering qualities. There's something about the physical way Cline interacts with the guitar that's completely distinct; each note sounds like a reaction to his body, sometimes a yelp, sometimes a scream, sometimes a sigh.

Near the end of the set, Isbell joined the band for "California Stars" while Barnett, in a ball cap, watched from the side of the stage. Watching Isbell and Cline trade licks is a nerdy fantasy come to life. They went on to close out with a trio of tunes from 1997's Being There, some of the best indie country ever made, sung and played in a way that reaffirmed the band still cared about the music.

Gov't Mule closed out the evening with a set that featured, at one point, Nels Cline and Warren Haynes trading licks on a reggae version of David Bowie's "Rebel Rebel." Mule are no question the best band to wind down a long day at Mountain Jam, with the smells of grass and pot smoke wafting through the air as Haynes tears it up under the lights.