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Diversity Recommendations for the Urban Forest

By Micah Pace, ISA Certified Arborist, Professional Urban Forester

For decades, researchers, educators, urban foresters, arborists, and tree managers have agreed on the importance of species diversity within the urban forest.

The catalyst for promoting this accepted management philosophy for our urban forests is primarily the result of historical and devastating population losses attributed to an exotic pest/disease. Such was the case of the loss of one of the greatest forests types in North America, the American chestnut (Castanea dentata). According to the American Chestnut Foundation, during the first half of the 20th century, approximately 4 billion trees or nearly ¼ of the population of the eastern hardwood forests of the U.S. were killed by the deadly fungal disease later called chestnut blight. Similarly, since 1930 when Dutch Elm Disease (DED) was first discovered in Ohio, DED spread up and down the U.S. East Coast and then west across the continent, reaching the West Coast in 1973 and ultimately killing some 40 million elm trees. Currently tree professionals have been struggling with the advancement of the Emerald Ash Borer (EAB) since 2003 where it was first found in Michigan and has killed
tens of millions of ash trees alone. EAB has killed tens of millions more trees from the east coast to as far west as Colorado and south to Arkansas, where it was found in 2013 and 2014, respectively.

While the loss of the American chestnut was not an urban forestry loss, per se, the loss of tens of millions of elms and ash trees along our city and suburban streets was a result of over planting of a single genus and in some instances even single species.

So far, the best management practice (recommendation) of using a more diverse palate of species or genera developed in the hopes to avoid future catastrophic losses of trees.

However, while the concept of increasing genus/species diversity is widely accepted, there have been various recommendations of how to implement such diversity. A 2013 ISA conference proceeding paper by Dr. Wade summarized recommendations for urban tree species diversity.

In general, there is a range of percentages from more conservative to less conservative diversity recommendations. The most common bmp typically used is from Santamour’s 1990 work, which states that an urban forest population should consist of no more than 10% and 20% of the same species or genera, respectively. For example, a population of 10,000 trees should have no more than 1000 shumard oaks (less in my opinion) or no more than 2,000 oaks of any species. However,
a more recent perspective being promoted by researchers claims this approach to be outdated and suggests that a new and more conservative strategy which focuses on non-species-specific threats such as EAB, verticillium wilt, and gypsy moth should be used. Dr. Ball, of South Dakota State University, discussed this management approach with attendees at the 2014 Texas Tree Conference. Dr. Ball (and others) now recommend that communities should limit a single genus to only 10
percent of full stocking (rather than as a percentage of the total tree population). This approach, while perhaps more comprehensive, would also
require a full inventory to calculate, including plantable space, something most communities here in Texas do not currently have. Here is Dr. Ball’s paper from 2007.

Finally, a 2013 paper from McPherson and Kotow that “grades” California’s municipal urban forests and discusses this important topic. Of the 29 California municipal forest inventories (836,943 trees) evaluated or “graded”, reducing pest threats was the top priority recommendation in 18 inventories. The study emphasizes the importance of managing for multi-host pests and planting of vacant spots with species not vulnerable to the most abundant and severe pests within the given community/region.

Ultimately, all the bmp’s presented here are helpful ways to improve the long term health and success of our urban forests, but the choice of what and how many to plant should be decided at the local level with regional concerns in mind.
Every community has limitations but education and knowledge should not be one of them. There is a wealth of information that we as tree professionals have to offer. Make sure to speak with your clients and community leaders about proper planning and management and how species selection in terms of present
and future risk to pest and disease is an important part. I think each community should assess their susceptibility to some of these more problematic pest/diseases and decide how aggressive their urban forestry diversity strategy needs be.


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