Growing trees in Colorado, especially big shade trees, can be very challenging. Few of the specimens in our community “forests” are native to Colorado, and areas where big trees are abundant are often so different from our conditions that those trees do not adapt well here. Some of the difficulties trees face here are: alkaline soils, nutrient-poor and shallow soils, low humidity, hot and drying winter sun, strong winds, untimely wet snows in spring and fall, rapidly changing temperatures, and low rainfall. Add to these the confining root zones in which many trees are growing in urban environments, and it is easy to understand why our trees are often stressed, subject to borer and fungus problems, broken and short-lived.
So it is not surprising that the few well-known tree varieties that generally survive here comprise the vast majority of trees we see growing. Although there are over 100 species of trees growing in Boulder alone, only about 5-10 species dominate the urban forests. For example, in Denver, approximately 40% of the street trees are silver maples, elms and ashes. This lack of diversity can lead to many problems. We have learned from agribusiness that monocultures will eventually magnetize and support growing populations of pests and diseases specific to those species. If that should happen, an entire population of trees can be killed off, leaving devastated neighborhoods, cities and even countries, as occurred with Dutch Elm Disease, and could happen again with the new ash borer. Therefore we want to have a diverse population of trees, so as not to have all our eggs in one basket, to discourage infestations and to provide a richer and more varied beauty. This is one of the reasons why the Champion Trees Program is so valuable: it demonstrates the far broader range of possible species that can be grown in Colorado.
The Champion Trees Program is conducted by the Colorado Tree Coalition which is itself related to, and partially funded by, the Colorado State Forestry Department. In addition, Forestry Dept. employees serve as leaders and volunteer on its committees, providing valuable expertise and dedication to a good cause. The Colorado Tree Coalition is a non-profit organization whose mission is to be a leader in preserving, renewing and enhancing the community “forests” in Colorado. The Champion Trees Program identifies, records and preserves the largest trees of each species growing in Colorado. The size of each tree is determined by measuring the trunk at 4 ½’ above ground level, the height and the crown or branch spread. Anyone can nominate a potential State Champion through their website.
The Colorado Tree Coalition is also collecting a photo library of these champions, some of which are highlighted each year in their calendar “Notable Trees of Colorado”. This year’s calendar features: American Elm, Ohio Buckeye, Bristlecone Pine, Plains Cottonwood, Japanese Tree Lilac, Douglas Fir, Ponderosa Pine, White Mulberry, Lanceleaf Cottonwood, White Oak, Rocky Mt. Juniper, and Kentucky Coffeetree. By arousing appreciation of these and other varieties, they hope to encourage more diversity in our urban forests.
Does inclusion of a tree species among the champion trees mean it is a safe bet to plant in your yard? Not really. For example, there are big specimens of Pin Oak growing in both Green Mt. Cemetery and Chautauqua Park in Boulder, but elsewhere in Boulder, Pin Oaks fail miserably. Kathleen Alexander, City of Boulder Forestry, suggests that the reason for this difference is that Boulder soils near the foothills and in the Boulder Creek floodplain are much more acidic (as low as 5.5pH). There, many oaks, maples, beeches and sycamore do well, whereas they will not thrive in the more alkaline north and east parts of town where pH can be 7.5-8.5. In Denver too, soils can vary widely as to alkalinity. Soil fertility, soil depth, wind and sun exposure are other influential factors. Phil Hoefer, currently on the Champion Tree Committee and 30 years with Colorado State Forestry, has observed that once communities have established initial forests, it is much easier for new trees to get started in the settings improved by wind protection, shade and more organic matter.
So we cannot judge the worthiness of a particular species without considering the conditions in which it is thriving. In past calendars and literature, the Colorado Tree Coalition has not included information on each tree’s needs of pH, soil type, wind and sun exposure. However they have plans to include this type of information in future publications. This Colorado-specific knowledge is hard to find, and will be very beneficial to those of us trying to establish some of the more unusual species.
A few notable Champion Trees are the 115’ Sycamore on the Naropa University Campus in Boulder, the 100’ Shumard Oak at Denver Botanic Gardens, the 75’ Sugar Maple in Ft. Collins, the 73’ Kentucky Coffeetree in Washington Park in Denver, and the National Champion Plains Cottonwood that is 105’ high with a trunk 11’ in diameter, on Boulder County Open Space.
Amongst the Champions are some wonderful and unusual varieties: Copper Beech, Cucumber Magnolia, Willow Oak, Sweetgum, Lacebark Pine, Tulip Tree, Yellowwood, Umbrella Pine, Amur Cork Tree, Giant Sequoia and Dawn Redwood, to name just a few.
However here is a list of trees that are also unusual, but are better adapted to Colorado, and should be planted more in our area: Burr Oak, Gamble Oak, Swamp White Oak, English Oak, Scotch Pine, Bald Cypress, Ohio Buckeye, Common Horsechestnut, White Mulberry, Kentucky Coffeetree, Ptelea Hop Tree (aka Wafer Ash), Tartarian Maple, Apricot, Russian Hawthorn, Thornless Cockspur Hawthorn, Arizona Cypress, Catalpa, Chanticleer Pear, Hackberry and Golden Rain Tree. These recommendations were made by several local foresters including Michael Swanson, Kathleen Alexander, Keith Wood and Phil Hoefer.
Some of the Champion Trees are located at undisclosed private addresses, but many can be seen in public parks, right-of-ways, cemeteries and Denver Botanic Gardens. For more information, or to order a calendar, go to the Colorado Tree Coalition website at www.coloradotrees.org, or call 303-438-9338.