In the arboricultural industry there is a great deal of talk about the necessity of planting a diversity of species of trees in the urban forest. The theory, if you will, is to avoid catastrophic loss in case of disease or insect pressures that target a single species. Common examples cited are a number of historic and devastating events that wiped out, literally, millions of trees.
Dutch Elm Disease, Ophiostoma ulmi and O. novo-ulmi – Primarily targets native elm trees which were a preferred species in urban settings as a majestic street and landscape tree. This introduced pathogen was first reported in 1928 and by 1989 75% of the estimated 77 million elm trees in North America were gone.
This devastating disease has slowly but surely made its way across the nation. Yet, here and there, you will still see survivors such as this large specimen in downtown Hamilton, Montana. There are still several in the area and are always a treat to see.
Trees killed by this disease are still capable of re-sprouting and sending up shoots, as seen in the photo below. This tree was listed as “dead” on an inventory taken in 1992; however, when we re-inventoried the property in 2008 regrowth from the stump had put on significant growth. “Never say never.”
American Chestnut blight – First reported in New York in 1904, within 50 years this introduced pathogen, Cryphonectria parasitica, had decimated American Chestnut trees in the eastern North American forests, a number estimated to be some 4 billion trees. This species was the dominant tree in many of the ecosystems in the Appalachians comprising approximately 25% of the forest.
Emerald Ash Borer, Agrilus planipennis – First reported in 2002, this invasive insect is thought to have been in this country since the late 1980s. As the name implies, it targets ash trees both native and non-native. In its native environment it is not considered a serious pest as there are natural controls; however, outside of its native range those controls are not present. It has killed tens of millions of ash trees to date and is a serious threat to the 8.7 billion ash trees in North America. Ash trees comprise large sections of natural forests but also have been prominent urban forest and landscape selections.
These disasters, affecting both natural forests and planted areas, resulted in various formulas being suggested for diversifying city tree inventories in an effort to reduce the potential for catastrophic loss when, not if, events such as these happen again. These include the “10%-20%-30% rule” (no more than 10% of any species, 20% of any genus or 30% of any family) generally attributed to the late Dr. Frank Santamour. However, Dr. Santamour stated in a paper he gave that he didn’t know who first came up with this proposal, but whatever.
More recently another approach has been suggested by Dr. John Ball out of South Dakota State University: “The 5% Solution”. His proposal suggests genera with limited numbers of species and limited global distribution have less pest potential, and, therefore, should be less apt to be susceptible to this kind of devastating attack. Hence, the suggestion to limit genus and species to 5% of the tree population.
As with any solutions to a given problem, there are pros and cons. The 10-20-30 rule can still end up with too many of one species in any of those groups and the 5% solution may be too restrictive in areas with limited species selection options due to climatic constraints.
Factors determining selection are often what is currently popular as well as what is readily available at the local nursery. Nurseries stock what is going to sell and people tend to buy what they have seen and like. Consumer demand drives stocking decisions by nurseries, which in turn, limits diverse options. It is a classic “Catch-22”. I have even read suggestions by professionals telling people to drive around their neighborhood and see what their neighbors have planted with the presumption that if it worked for them, it will work for you. Not the way to encourage diversity.
In order to branch out and contribute to diversification, you need to do some homework. Consider not only what is suitable in your hardiness zone but also with your water capabilities, site aspect, soil conditions and level of care you are willing to expend on its maintenance. Consider also what function the tree is to provide: shade, aesthetic appeal, flowering, and/or fruiting. You can then make a list of potential candidates. At this point, you may want to contact a local nursery or professional arborist and get their feedback. Don’t be shy about getting a second opinion; everyone has bias.
If your nursery of choice doesn’t carry the species you are looking for, ask if they can order one. If they can, how far in advance do you need to place the order. Do you have the right of refusal if it is damaged or the wrong tree, assuming that was not your mistake.
Remembering all plants in a developed area contribute to the urban forest, the individual homeowner can do a lot to help diversification. And by selecting a non-target tree for primary pests, you are hedging your bets on longer term survival. We need more trees to be planted but they also have to survive. It does the environment no good if trees cannot survive for longer than 20 years.