On the Fort Collins City Park Self-Guided Tree Tour Map, E23 is listed as the chalk bark Elm. When I googled this, I found very few entries. Instead, a chalk bark maple, the lace bark elm, and references to a Japanese elm popped up. After I got smart and googled the tree’s Latin name, Ulmus propinqua, the Emerald Sunshine elm showed up even though the Latin name for that tree appears to be Ulmus davidiana var. japonica. There is another tree, the David elm, with that very same Latin moniker. What is going on?
The chalkbark elm makes a brief appearance in Dirr and Warren’s The Tree Book,* where part of the mystery of the disappearing elm was cleared up. According to the authors, both GRIN and The Flora of China have regrouped at least four species of Asian elms, considering them to be one species, Ulmus davidiana, or the David elm. Included in this new grouping are what were once known as the chalkbark elm, the David elm, the Japanese elm, and the Wilson elm. Emerald Sunshine, a cultivar, was derived from the chalk bark elm and seems to be discussed under the name U. propinqua JFA-Bieberich. This tree, included in the Tree for Seattle list, is said to grow to a height of 35 feet with a spread of 25 feet and is pest resistant.
To confuse matters, other sources list Ulmus propinqua/Ulmus davidiana var. japonica as the Japanese elm. Another source states Japanese elms include 6 genera and 35 species. According to them, Ulmus davidiana var japonica is the most resistant to Dutch elm disease. Dirr and Warren* state the oldest Japanese elm (Ulmus japonica, now considered U. davidiana var japonica) in the US was planted in 1890 on the campus of UMass Amherst. They also mention this cultivar is known as Discovery “in the trade,” and both Discovery and Emerald Sunshine are listed under the cultivars under the David Elm. I’m not sure if this says we have three of the same tree species in our arboretum or if all three are so closely related they might as well be the same? Most sources also mention Discovery is highly resistant to Dutch Elm Disease.
What does seem to be true is that many cultivars of Asian elms have some defense against the pests that devastated the American elm. Some of the other traits of these trees, such as smaller size or a more upright trunk with less branching, may make them better choices for yards and roadways.
One type of elm which is usually not recommended is the Siberian Elm (Ulmus pumila.) You can read more about that species on the USDA plant page where it is stated this elm is invasive in New Mexico. It also mentions it is a brittle tree subject to breakage. For a period of time some friends had the National Champion Siberian Elm in their front yard, but a storm about ten years ago broke enough branches it was demoted. With a more recent storm, its size has probably been further diminished. It is possible**, though, that this same tree was still the #2 Siberian elm in the state of Colorado in 2018. The current champion is in the state of Utah.
The Discovery Elm is in the group of trees West of Sheldon Drive and North of Mulberry while one of the two David Elms on the tour is near the entry to the swimming pool. The Chalkbark elm is near the reservable shelter across from the lake and is one of the smaller deciduous trees in that area. The Siberian Elm is not part of the tree tour in the park.
*Diff, Michael A. and Warren, Keith S., The Tree Book: Superior Selections for Landscapes, Streetscapes, and Gardens. Timber Press, 2019
**The champion trees are listed only by city so it is difficult to know for certain if this is the same tree.