Arboretum News

Losing a Landmark

Earlier this week, we finished taking down the trunk of the bur oak near the Wilson Rose Garden, and we’ll use portions of the trunk to learn more about the tree’s history.

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Bur oak on August 12, 2020. Photo by Erin Buchholz.

By Liz Potasek

There was something special about the large bur oak that stood near the entrance to the Wilson Rose Garden. With it’s large, twisting branches and thick trunk, the tree preceded the Minnesota Landscape Arboretum’s founding in 1958, and the Arboretum grew up around it. 

Unfortunately, the tree contracted Bur Oak Blight, and it was also impacted by road and garden construction, two-lined chestnut borer, and Armillaria root rot, a common soil-borne fungus, says Erin Buchholz, Arboretum integrated pest management specialist. “The combination of environmental and pest issues proved too much for our beloved tree,” she adds. 

In August of 2020, a tree company was hired to cut the crown of the tree, leaving the trunk, as Arboretum staff explored the best way to honor this treasured member of our community in the long term.

Cutting a cross section of the tree. Photo by Liz Potasek.

On Wednesday, with guidance from Dan Griffin, a University of Minnesota dendrochronologist (a researcher who studies tree rings), Arboretum staff took down the trunk, which was starting to rot, and carefully carved a few sections from the tree to use for research and display. 

We’ll learn a lot about this tree and its environment by studying its rings. Photo by Liz Potasek.

The tree’s trunk tells its story. In addition to estimating its age by counting its rings, and finding scars that indicate damage to the trunk and marks where the tree was treated for bur oak blight, scientists can look at its growth patterns to learn more about the natural history of the area, noting wet periods as well as drought conditions. They can also make comparisons with other trees they’ve studied throughout the area to tell a broader story.

By studying the rings, we can tell the tree sustained some sort of injury, which left a scar, when it was still relatively young. Photo by Liz Potasek.

“It’s not as old as I expected,” Griffin says, after an initial look at the rings. “One of the things we’re trying to understand about bur oaks is sometimes they grow like weeds and sometimes they grow slow.” Some of the oldest trees aren’t necessarily the biggest, he adds, noting that they can grow slowly, suppressed below the canopy. To determine the exact age of the tree, Griffin will study the rings using a microscope to count rings during its first years of life.

While some of the trunk will be studied by Griffin, the Arboretum is keeping a cross-section of the trunk for display. The cross-section or “cookie” will be air-dried indoors with metal bands to minimize cracking for the next 1-2 years. A few wedges of the trunk were also preserved with the intention of drying them and using them for educational purposes and classes.

5 comments on “Losing a Landmark

  1. Always sad to see a big tree come down, but this one was obviously under a lot
    Of stress. Can you talk more about how road and garden construction would have impacted the tree?

    • Thanks for the question! We asked Erin Buchholz, the Arboretum’s integrated pest management specialist, for some details, and here’s her response:
      We aren’t sure of its exact age, but we believe the tree was around 180 years old when it died. That means that it was here before the Arboretum existed as it does now. When we began construction of the Arboretum, we built around as many large and majestic trees as we could. The bur oak was much loved, and has always been a centerpiece.

      Unfortunately, being a centerpiece means many things go in and around it. Three-Mile Drive was constructed, and the road did cut into some of the tree’s root system. It’s important to remember that oftentimes, a tree’s roots extend out one-third past its canopy. That undoubtedly means that the roots were all the way to the road, as well as extending into the Wilson Rose Garden.

      The construction and reconstruction of the arbor going around the Wilson Rose Garden also impacted the tree and its root system. Even if roots were not severed, oaks can be sensitive to vibration and compaction from heavy equipment moving in and out. Try as we might to give our trees space when something big has to happen, we sometimes can’t prevent the damage that comes along with construction projects.

      It is important to note that we don’t always see impacts of construction right away. Much like environmental events — like drought and flooding — can take years to show damage, other physical activities can take time to manifest into dieback and death. From 2014 through 2019, we had some of the heaviest precipitation in Minnesota. But 2012 was particularly dry, and nearly every fall heading into winter has also been dry. Those changes from “normal” years also likely played into the stress of the tree. Much like people, when trees are stressed from external factors, they are more prone to disease. Insects are also adept at picking up on stressed trees. Our native insects prefer to attack stressed trees rather than younger, healthier ones.

      • Wow! I learned so much from this very quick yet thoughtful answer to my question. Thank you for taking the time to help me understand more about this particular tree and it’s history!!!

  2. Jeff Walton

    We wondered what would happen with the large trunk. So sad to lose the two oaks by the rose garden.

    The treatment marks in your photo indicate that any effort to protect the tree from Oak Wilt only happened in the last few years. Arborists say that the injections have to be done long before there is any evidence of the wilt nearby, which means that treatments to protect this tree and the neighbor that died a couple years ago would have needed to start at least 10 years ago to have any chance of saving the tree. What, if anything, is the Arboretum doing to protect the other large oaks along the Three Mile Drive and elsewhere on the grounds? We have lost larger oaks to the wilt in our yard and are doing what we can to try to save others, so we know how fast the wilt can take these mature trees, especially red oaks..

    • Thanks for your question! Here’s a reply from Erin Buchholz, our integrated pest management specialist:

      Oak wilt was not suspected in this particular tree. The tree was injected for treatment of bur oak blight, which is a fungus that only infects bur oaks. This treatment is not always successful in oaks, but the research at the time led us to believe this was the way to go.

      While the Horticultural Research Center had an instance of oak wilt some years ago, we have yet to confirm it on Arboretum grounds. Proactive fungicide injections for oak wilt are not recommended unless there is a confirmed case of oak wilt in the vicinity. Those injections will not cure an infected tree, it will only suppress the fungus delaying the eventual death of the tree.

      We regularly scout the Arboretum for oak wilt symptoms and if we suspect a tree is infected, we carefully remove a symptomatic limb and immediately bring it to the St. Paul campus for testing. Those tests take a long time to perform, as they need to culture the pathogen and grow it in separate dishes.

      Our plan is to catch trees early on in infection and save the remaining trees by severing root grafts by using vibratory blades 5 feet down in the soil. We also never prune our oaks during the spring and summer, as that is the time that the beetle that vectors the fungus is active.

      It is important to remember that oaks are sensitive, and fungicide injections can add to the stress they are already under. They are also very expensive. With hundreds of oaks on our property, we would not be able to afford injections on a routine basis. Observation and quarantine will be our main methods of fighting this disease. Thanks for your question!

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