It was just an ordinary tree. Mark Hirsch drove past the gnarled, ancient oak for 19 years without a second glance. Then, one snowy January day, as its dark limbs traced bold outlines on a white sky, he paused to take a photo. And as Hirsch's camera phone clicked, the tree ceased to seem ordinary.

A professional photographer, Hirsch had recently bought an iPhone. Eager to try out his smartphone's functionality, he accepted his friend Cori Peplnjak's challenge to post a daily iPhone photo on Facebook. At first, he imagined that the project would embrace a different subject each day. But after a second photo of the oak prompted another friend, Greg Guenther, to comment, "Dude, you and that tree! You should do a photo a day with it!," the project's scope was sealed. 

For an entire year, Hirsch documented the life of the single bur oak, a solitary sentry surrounded by Wisconsin cornfields. He came to appreciate the old tree's sweeping boughs, its sturdy leaves, the tiny insects that clung to the rough bark. 

That Tree gained a devoted following on Facebook, with fans eager to see how the oak would look draped in snow or shrouded by sunset. In August 2013, a collection of the photos will be released as a book titled That Tree: An iPhone Photo Journal Documenting a Year in-the-Life of a Lonely Bur Oak. On March 23, the photographer will click the year's final shot, and this time the tree will be surrounded by friends—at press time, 163 people had RSVPed to be included in the 365th portrait. 

But the final photograph won't be the end of the story. "I won't stop," he says. "It's become a part of my day." Hirsch looks forward to experimenting with other camera technology after the iPhone year is up. For the photographer and nearly 4,000 Facebook fans, That Tree will remain a captivating friend. 

As the year of documenting That Tree progressed, Hirsch felt a change in his attitude toward photography. "I've been at this for 25 years," he says. "Photography isn't my hobby; it's my career. I got to a point where I had stopped taking pictures for me. I was depending on all this technology. I still had my vision behind the camera, but I'd lost that freedom I'd felt when I was a beginning photographer." However, the tight artistic constraints of the project—a single tree photographed daily using only a smartphone— reignited his passion for his craft. "This is something fresh and new," he says. 

Playful framing and new perspectives keep the daily photos engaging. On August 7, Hirsch's German wirehaired pointer, Magnum, shared the spotlight with the oak. "It was sweltering humid hot," Hirsch remembers. The photographer had been "wandering around" looking for a shot when he noticed his dog drinking from a collapsible water dish. "He sat up, water dripping off, and I said, 'Magnum, stay.' I saw that moment. And I'm pulling my iPhone out of my pocket." Luckily, Hirsch was able to position his camera phone quickly and capture the quirky shot. 

Enigmatic images serve as markers for the year's events—this particular storm coincided with Election Day. "I was up at 5 a.m.," Hirsch recalls. "That day I was shooting rural polling places in Wisconsin and Iowa for Getty Images." Squeezing in a few moments between photo shoots, Hirsch knew that he couldn't wait for the weather to change. "So I drove back to my tree," he says. "Snowflakes were melting on the truck window. I deactivated fixed focus and the camera auto-focused on the first thing—the snow drops." In the melted snow, our perception is upended—the familiar tree appears shrunken and inverted. 

Did you spot the guest star in this photo? "This project has inspired me to look so much more closely at things than I have in 25 years," says Hirsch, who has embraced the challenge of creating 365 unique images of the same subject. "Every day, I go through a mental checklist: What did I shoot yesterday? I want to keep it fresh." For this shot, he was surprised by a camouflaged critter. "I'm under the tree, looking at the bark, and suddenly the moth moved. It freaks me out that I have those encounters and discoveries." 

An October windstorm removed most of the oak's leaves in one sweeping gesture. "Overnight the leaves had just dropped from the tree like someone took a coat off," Hirsch recalls. The bur oak, which a tree expert estimates to be about 163 years old, produces leaves that vary drastically in size and shape. "They run the gamut from skinny petite ones to rugged heavy ones. With the diversity of the leaves, you wonder whether they're from the same tree." But with no other trees nearby, it's safe to assume that all of these leaves did, in fact, fall from the same limbs. 

The photographer had to race against the clock to achieve this dramatic shot. "That day my DSL had gone out at home, and I had to go to town to use the library's. We had a drought this summer—there was no rain—but when I walked out [of the library], the wind was howling and the sky was just boiling. I was six miles from the tree. I could see the tail end of the storm." Hirsch raced across the town, hoping to reach his subject in time. "I jumped out [of my truck] and started taking pictures," he recalls. "The storm never produced rain. It just howled and blew over me. Within half an hour, there were blue skies." 

The tree is surrounded by cornfields and switch grass. The field normally produces a hearty crop, but the hot, dry summer took its toll. "That field has always been a pretty good producer, but this year corn yields were marginal because of the drought," Hirsch says. Oak trees, however, produce bounty from poor conditions. "Trees respond to drought by producing as many acorns as possible," he notes. "The tree's reaction is to propagate. This year there was an incredible crop of acorns." 

"Farmers don't like foxtail," Hirsch says. "But it made for a beautiful photo." While Tim Clare, the farmer who owns the field on which the oak tree resides, might not be fond of foxtail pictured here, he is a fan of the tree. Clare has pledged to never cut down the lone oak. Hirsch relays a story that illustrates the tree's precarious existence and the farmer's wise judgment: "He had a bulldozer come out to fix his waterways. [The bulldozer operator said,] 'While I'm out here, you want me to push over that tree too?' and Tim replied, 'That tree has been here for almost 200 years. I'm not the guy that is going to push it over.'" 

Some of the iPhone's limitations have yielded interesting results. "The shaft of light is from lens glare," Hirsch says of this ethereal portrait of a katydid. He doesn't use any filters, and his app use is limited: He prefers Camera+ and ProCamera for exposure controls; Slowshutter for timed exposure shots; and Snapseed for photo editing. "The iPhone, to me, is not much different from any other camera. In the hands of a skilled photographer, it's a fully capable camera," he says. "It has its limitations, but my real cameras won't fit in my pocket."

This March 14 image is the second photo that Hirsh took of the tree, and the one that prompted the comment that inspired the That Tree project. Though many of his images seem possessed of effortless beauty, the photographer admits that the project has had its challenging moments. "There were many days when I found myself overwhelmed," he says. "On the 160th day, I thought, 'Oh my God—how am I going to make a picture for 205 more days.' But the deeper I've gotten in, the more I've grown visually."

The not-so-ordinary oak tree that has captured the affections of Facebook fans, and that here literally supports the photographer's shadow, has also inspired a tree-planting campaign. At the party to celebrate the final photo, each attendee will receive an oak sapling to plant. In addition, the book's proceeds will fund reforestation efforts—a tree will be planted for every 100 books sold. But Hirsh's camera has already preserved at least one life. "That tree will be famous," he says. "It will never disappear."