Demonstrating diversity

The summer of 2012 marked the 10-year anniversary of the discovery of emerald ash borer (EAB) in southeast Michigan. The ash borer is a phloem-feeding beetle from Asia that girdles and kills host ash (Fraxinus sp.) trees. Since its introduction near Detroit, EAB has continued to spread rapidly across the Upper Midwest and eastern United States, often with the unknowing aid of campers and others moving firewood. As of Aug. 1, EAB has been confirmed in 16 U.S. states from Minnesota to Connecticut and two Canadian provinces. Although researchers have made progress on a variety of fronts in the battle against the beetle, the future of most native Fraxinus species in North America remains in doubt.

Green and white ash were popular choices in landscapes and as street trees because of their good growth rates, fall color and adaptability to a wide range of site conditions. The rapid demise of ash trees in the wake of EAB has been truly remarkable and has resulted in a significant loss of urban tree canopy in many communities. A recent study projected that EAB will result in the loss of 17 million ash trees in urban and community forests by 2019. As with Dutch elm disease before it, the EAB saga demonstrates the vulnerability of native trees to exotic pests for which they have not evolved resistance. As the likelihood of additional exotic pest introduction increases with global trade, the need to diversify our landscapes and reduce the risk of catastrophic tree loss is greater than ever.

Ash alternatives
To evaluate ash alternatives and promote increased species diversity in areas affected by EAB, we established an arboretum at MSU Tollgate Education Center in Novi, Mich., near the epicenter of the original EAB outbreak. In the spring of 2003, a small army of volunteers descended upon a hillside at the Tollgate Center armed with shovels, rakes and water hoses. The volunteers planted nearly 200 1½-inch bare-root shade trees representing 37 different ash alternative selections (five trees per selection). The trees were donated by J. Frank Schmidt and Sons Nursery and Carlton Plants. Additional support was provided by the Michigan Nursery and Landscape Association and MSU Project GREEEN. After planting, the trees were mulched with 3 inches of wood chips and irrigated during the first summer to aid in establishment. Initial survival of the planting was excellent although deer have taken their toll despite the addition of plastic guards to the tree trunks. Listed below are some of the selections that have proven to be solid performers in the ash alternative arboretum during the past decade.

State street maple provides a winning combination of growth rate and dark green foliage.

State Street maple Acer miyabei ‘Morton’
Acer miyabei is native to Japan and this selection was made by the Morton arboretum. In the Tollgate planting, trees have a good growth rate (25’ after 10 years) with a dense crown and dark green foliage. We have seen little evidence of pest problems. USDA Hardiness Zone 4.

Hardy rubber tree Eucommia ulmoides
Hardy rubber tree is native to China, where it is relatively rare in the wild. It is hardy to Zone 5 and considered to be relatively pest free. At Tollgate, trees have shown good form and characteristic glossy green leaves. Growth rate is intermediate (20 feet) among trees in the arboretum.

American Sentry linden Tilia americana ‘McKSentry’
Redmond linden Tilia americana euchlora ‘Redmond’
Greenspire linden Tilia cordata ‘Greenspire’
We included three linden selections at Tollgate and all three have been outstanding. Average heights after 10 years were 20 feet for Redmond linden, 22 feet for Greenspire and 23 feet for American Sentry. All three have great pyramidal form and excellent, dark-green leaf color. American Sentry hardy to Zone 3; Redmond to Zone 3; and Greenspire to Zone 4.

American Hophornbeam Ostrya virginiana
Hophornbeam is among the slower-growing trees in the planting (14 feet after 10 years) but it has been a steady performer with good leaf color and form. The hop-like flowers add to its visual appeal. Hardy to Zone 4.

Greenspire linden

Northern pin oak Quercus ellipsodalis
Northern pin oak is another tree in the “slow-but-steady” category. (11 feet after 10 years). The alkaline soils at Tollgate (pH = 7.5) turned out to be a perfect site to demonstrate the greater pH tolerance of Quercus ellipsodalis compared to the standard pin oak (Q. palustris). Northern pin oak maintained dark green leaves while pin oaks on the site became extremely chlorotic. Hardy to Zone 5.

Shingle oak Quercus imbricaria
Shingle oak is primarily a southern species and Michigan is on the northern fringe of its native range. Nevertheless, shingle oak was an intermediate grower in the arboretum, averaging 19 feet at 10 years. The trees had dark green glossy leaves. Hardy to Zone 5.

Sawtooth oak Quercus accutissima
Like shingle oak, sawtooth oak doesn’t have deep lobes on its leaves like most oaks. Sawtooth oaks grew slowly (15 feet at 10 years) in the arboretum, but their sharply serrated leaves still provide plenty of interest. Hardy to Zone 5.

Accolade elm Ulmus japonica × wilsoniana ‘Morton’
One of the ironies of the EAB story is that we are now recommending elms to replace ashes, many of which were planted to replace elms lost to Dutch elm disease. Accolade elm is part of new group of hybrid elms that have been selected for tolerance to Dutch elm disease. Accolade elms were among the fastest growing trees at Tollgate, averaging 27 feet at age 10. Hardy to Zone 4.

Triumph elm Ulmus ‘Morton Glossy’
As with Accolade elm, Triumph elm is hybrid elm selected at the Morton arboretum for Dutch elm disease tolerance. Triumph is also a fast grower (27 feet at age 10). In fact, Triumph elms grew so fast in the arboretum that they literally blew the plastic dear guards off their trunks. Hardy to Zone 5.

Yellow buckeye Aesculus flava
Although we don’t usually say good things about buckeyes at Michigan State, yellow buckeye has proven to be a good performer in the Tollgate planting. Growth rate has been intermediate (19 feet at ten years) and the trees have maintained good form and remained pest-free. Hardy to Zone 4.
 

Top Left: That’s an oak?  Shingle oak demonstrates not all oak leaves are lobed. Top Right: Northern pine oak (R) was able to maintain deep green leaf color, while pin oak (L) became extremely chlorotic. Bottom left: Accolade elm was one of the fastest growing trees in the demonstration planting. At age 10, the trees are developing attractive vase-shaped crowns. Bottom Right: Hophornbeam fruit provides added visual interest to this small to medium sized tree.

Bert Cregg is associate professor, Plant & Soil Sciences, Michigan State University, cregg@msu.edu; Robert Schutzki is associate professor, Plant & Soil Sciences, Michigan State University, schutzki@msu.edu; and Roy Prentice is farm manager at Tollgate Education Center, prentic1@msu.edu.