Native Trees and Provenance Lead to Increased Longevityby Doug Chapman
From City Trees, The Journal of The Society of Municipal Arborists
Now is the time to consider use of regional native trees. Arboretums have recently spearheaded plant conservation and use of native trees. We have known for many years that native plants are better adapted to local conditions, yet we continue trying to use the same several hundred tree cultivars throughout all of North America.
I recently visited Salt Lake City noting the home and street landscapes were comprised of many trees we grow in the Midwest. I could say the same about many dissimilar parts of the country.
This observation was not true as I drove through Provo Canyon on the way to Park City, Utah. The native plants in the landscape were unique to the area.
How similar are continental environmental conditions? Should we consider growing a Sugar Maple (Acer saccharum) native to Vermont in Denver, CO just because both places have mountains?
We can reduce maintenance while planting the right tree for any area of the country, i.e. Blue Spruce (Picea pungens) growing in well drained soils at 6,000 - 10,000 feet elevation or White Spruce (P. glauca) in cold climates from Northern Michigan to Northern New England. In Michigan, Blue Spruce starts declining at 20 years and frequently contacts Cytospora Canker. This is not so in the Western Rockies.
The University of Minnesota introduction program is focused and based upon good science. One can cite Harold Pellet's plant introduction program, which is focused on selecting, developing, and introducing trees for cold continental climates. Many northern natives are cold temperature hardy, i.e. White Pine (Pinus strobus), Paper Birch (Betula papyrifera), and White Spruce, but are not warm temperature tolerant. It has been proven that native White Birch will decline if the mean July temperature is above 70° F. Further, it should be noted that entomologist Neilson studying white bark birches reported native Betula papyrifera showed less susceptibility to Bronze Birch Borer than some Asian species. Others reported native birch from the southern range of B. papyrifera were more tolerant of higher temperature than plants from a more northern provenance.
Provenance, defined as the local origin of trees within their native range, will exhibit local adaptation. Many of our favorite tree species have a wide North/South native range, but provenances from Northern latitudes can't survive high temperatures in the South and conversely southern provenance trees within the same species are not cold temperature hardy. It has been shown that plant selections in northern latitudes of the species native range are more photo period responsive and cold temperature hardy.
The selection and introduction of regional native trees could lead to a boon for the nursery industry. No longer would the same cultivars be offered for use across the entire North American landscape. Regional seedlings and/or cultivars could lead to increased longevity of trees in our urban landscapes. Further the rootstock and scion would be of the same provenance avoiding the incongenality with Red Maple cultivars. Lastly, regional native trees based upon provenance would mean that every part of the country could again be unique. Think of that innovative McDonald's Restaurant in Freeport, Maine that matched its architecture with that of the region. One knew you were vacationing in Maine! The aesthetic qualities of regional native trees may display some unique characteristics, but the genetics would definitely be locally adapted with greater diversity.
Today we may use forty tree species comprised of some five hundred cultivars or clones. Thus of the millions of trees planted annually, it is possible 85% are cultivars. This means that the composition of the many thousand individual trees being planted annually in urban landscapes across the country could be simply 500 clones. Picture crabapple cultivars when many were first introduced, i.e. Malus 'Radiant', M. 'Dorothea', and M. 'Snowdrift'. They exhibited resistance to fire blight and apple scab, but after a number of years and many being planted, these cultivars no longer posses resistance to these diseases and we had to develop new cultivars. Professor Lester Nichols from Penn State spent a professional career testing and reporting on disease resistant cultivars of crabapple, a situation that was in a continuously dynamic state requiring a new report every few years. This fact alone favors seedling selection to increase diversity. Yet, we should be able to produce seedlings as well as some superior regional cultivars.
No one technique is perfect but an emphasis on multiple solutions should lead to a healthier lower maintenance landscape that is based upon good science.
Douglas J. Chapman is a horticulturist and Director of Dow Gardens
in Midland, Michigan.
Society of Municipal Arborists
Trees are the City