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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 forest 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) when it was first found in Michigan in 2003 and since 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 as south as Arkansas, where it was found in 2013 and 2014, respectively.

PTStreediversity

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 the 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 palette of species or genera developed in the hopes to avoid
future catastrophic tree loss.
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
bmps 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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