Jennifer Nettles (James Minchin III)

There is no Sugarland without Jennifer Nettles. So what is Jennifer Nettles without Sugarland?

Nettles delivered the answer during the first concert of her first solo tour, which kicked off at the Lincoln Theatre on Valentine’s Day: These are not the same
energy-packed, carefree, delightfully weird days of her enormously successful country collaboration with Kristian Bush. The Jennifer Nettles solo era is calmer and more serious. The songs are slower and more thoughtful. The lyrics hit you harder.

And, frankly, it’s just a little less fun.

Of course, Nettles, 39, still has that spectacular, bombastic twang that she uses like a flying superhero when she sings — her tremendous voice swoops in and out, making unexpected turns to emphasize a phrase or inject certain words with emotion. She could never really lose people’s attention, even if the majority of the songs are ballads.

There were quite a few ballads during the concert, a relatively somber experience — a surprise, as Nettles, brimming with charm and chatter, exudes personality. But it was hard for the audience to get too amped up as she focused on slow, quiet material from her new album, “That Girl.” Many tunes were about love and heartbreak, and deeply introspective. Nettles told the hundreds in the audience that it’s her most personal album yet.

Brandy Clark (Becky Fluke)

With this project, “You get to see into my musical brain,” Nettles said, with themes including country, gospel, singer-songwriter style and 1970s radio.

It’s quite a mashup. The ballads include “Falling,” a vivid memory of first love; “Good Time to Cry,” about a friend who has “unproductive” methods to soothe pain; and “This One’s for You,” an ode to love songs. Particularly lovely was “This Angel,” about becoming a mom for the first time last year: “Who is this angel sent here to change me? Sent here to take me where I’ve never been? / Long I have wandered weary and waiting, for something to shake me and life to begin.”

She did have some sparks of high energy, such as the fast-paced country jam “Know You Wanna Know.” Plus, the mischievous “Jealousy,” which Nettles said was about “a woman who’s coming to terms with the more shady side of herself,” as the protagonist pours whiskey down the dress of a romantic rival.

Not surprisingly, the audience especially came to life whenever Nettles ventured into Sugarland territory. Although Nettles didn’t actually mention the S-word (she has said the band is not broken up, just on temporary hiatus), she did offer a few snippets from the duo’s long list of hits, such as “Baby Girl,” “Something More” and the eternally catchy “All I Want to Do.”

Nettles thanked everyone for supporting her music in all of its forms and gave a shout-out to her opening act, Brandy Clark, a wildly talented singer-songwriter. Clark, whose critically praised debut album, “12 Stories,” was released last year, doesn’t have Nettles’s vocal acrobatics, but her vivid storytelling is captivating.

Armed onstage with her own powerful voice and an acoustic guitar, Clark made the audience laugh uproariously with the stories in her songs, such as the one about a bored housewife who smokes marijuana to make it through the day (“Get High”), and another involving a scorned woman who doesn’t shoot her cheating husband because she wouldn’t look good in a prison uniform (“Stripes”).

Also playing the more emotional songs “Pray to Jesus” and “Hold My Hand,” Clark threw in songs she’s written for famous artists, including Miranda Lambert’s “Mama’s Broken Heart” and The Band Perry’s “Better Dig Two.”

“Believe it or not,” Clark said wryly of the ominous Band Perry hit (which includes the line, “If you go before I do, I’m gonna tell the gravedigger that he better dig two”), “this song was intended to be a love song.”