For those with an eye to nature, Harvard Yard is home to much more than freshmen.

There are living fossils and dying icons. There are locusts and larch, hackberry and holly, Kentucky coffee and catalpas. There are colors in autumn, rebirth in the spring, and shade in the summer.  Even amid winter’s winds, rough bark reminds us of the life asleep inside as we scurry past.

The trees of Harvard Yard have for centuries shielded student travels. They’ve roofed the Yard and framed its iconic image. All one has to do is imagine the Yard barren and treeless to understand that trees are as much a part of that landscape as its historic buildings and John Harvard’s brass toe.

And, like much of the University in this globalized age, the trees of the Yard are changing.

The Yard is approaching the end of an era, when elms — once thought the perfect trees for civic plantings because of their vaselike shape — graced and dominated the Yard.

The sad story of America’s elms is well known. Loved for their high-branching pattern that creates a shady canopy above and a parklike setting below, the tree was planted everywhere, making fertile ground for Dutch elm disease when it arrived in the 1920s.

In the disease’s wake, the Yard has undergone a quiet renovation over the past several decades. As elms have died, they’ve been replaced with other trees that will retain the Yard’s parklike setting. In the old Yard, large oaks stand side by side with the elms. In Tercentenary Theatre, where Commencement and other ceremonial events occur, large honey locust trees mix with maples and elms to provide a ceiling for campus affairs.

There are still elms left, but hundreds have given way to tens. Harvard groundskeepers fight the good fight, deploying fertilizer, fungicide, and careful pruning, but the Yard’s remaining elms are locked in a battle that will likely claim even the strongest of them.

“They’re survivors, but as survivors, they show their wear,” said Donald H. Pfister, the Asa Gray Professor of Systematic Botany. “They’re trimmed very severely. They’re pampered. Fungicides are used to treat them. Each year, there are fewer elms in the Yard.”

The result is a Yard that today looks a lot more like the forests surrounding it. American species have dominated the replanting, Pfister said, which has proceeded with an eye toward diversity in creating a mixed stand resistant to pests and diseases, and toward uniformity in retaining the Yard’s historic look.

“There’s a collective [reaction of visitors], that is, ‘Of course this is what it’s supposed to look like. You’re supposed to be able to see students walking through. You’re supposed to be able to look across from Johnson Gate to see John Harvard,’ ” Pfister said. “Those are characteristics of the Yard and how the Yard has been illustrated in its iconic past.”

Though planting native American trees has been emphasized as the Yard is remade, not all the plantings are native. In the small area between Robinson Hall and the Memorial Church is a stand of dawn redwoods, which Pfister called “living fossils.” The trees, conifers that lose their needles in the fall and whose brown trunks look almost muscled, were first known from fossils and were thought extinct until living specimens were found in China in the 1940s. Harvard played a role in the tree’s resurgence. Seeds were collected and cultivated at the Arnold Arboretum. Specimens today have been planted around the world.

The Yard’s trees change on a smaller, more expected scale as well. In his travels across the Yard, Pfister marks the seasons by the changes he sees, whether leaves are budding out and flowers blooming, whether fruits are growing or falling on the ground. He even gets a kick out of the walnuts’ ongoing battle with the automobile, acted out each fall as trees overhanging Quincy Street bombard the cars parked below.

“The Yard is ever-changing,” Pfister said. “One sees different things, depending on the season.”

For Pfister, the Yard’s trees are as much a part of Harvard as its buildings and people. They frame the activities of students and faculty, and perhaps should join the Yard’s ghosts in their “long winding train reaching back into eternity,” spoken of by writer Ralph Waldo Emerson in his verse inscribed near Meyer Gate.

“You can think about the Yard that way too … the trees under which people played,” Pfister said. “The activities of the College take place under them, Commencement takes place under them. [Students] leave, we [faculty] stay for a while. The trees are here for much longer.”

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