Street Tree Diversity in Chicago:

1
Street Tree Diversity in Chicago Chicagos
Response to the Asian Longhorned Beetle
Infestation Kevin B. Dick Northeastern Illinois
University
Introduction
The Present Study
Results (cont.)
Recent invasive species infestations in urban
forests have led to an increased awareness of the
need to improve the diversity of street trees.
One guideline for desired frequencies or
biodiversity in city trees, the 10-20-30
formula1, provides a benchmark for street tree
frequency. Of related interest in Chicago is the
Asian Longhorned Beetle (ALB) Project, which has
given researchers the opportunity to increase the
frequency of uncommon species in one area in
particular, the Ravenswood ALB Quarantine Area
(quarantine area). By comparing the diversity
of the quarantine area to that of the surrounding
area, the Citys response to the ALB infestation
can be both quantified and assessed. In addition,
the Citys policies before the ALB infestation
can be compared to those after, to assess whether
increased diversity has become integrated into
policy. This leads to the question of whether
Chicago meets its own standard of diversity, or
has done so in a scenario such as the ALB
replanting.
ALB Study Area
Figure 3 Highest Frequency Trees that exceed the
10-20-30 formula.
Prior Research

• Dutch Elm Disease decimated 77 million trees
  before 1970 (McCombs, 2001), and recent
  infestations of Asian Longhorned Beetle and
  Emerald Ash Borer have raised alarms in urban
  forests around the country .
• Diversity has been championed as an invasive
  species prevention mechanism for several years
  (Santamour, 1990 Galvin, 1999 Bassuk, 1990
  Dwyer et al, 2003).
• Diversity can be assessed by inventory of an
  urban forest population and measuring against the
  10-20-30 formula or another benchmark of
  diversity.
• By promoting diversity, damage of future invasive
  species infestations can be minimized, as
  monoculture of trees eases the spread of
  invasives such as the ALB.
• Recognizing this, the City of Chicago planted for
  diversity when replanting trees devastated by the
  beetle.

Matched Sample Area
Figure 2. Study Area and Matched Sample. Two
smaller pictures show a part of the study area
the larger picture shows the matched sample. Note
the lack of canopy in the study area.

• The City of Chicago established a quarantine area
  (Figure 1a) in which over 1,700 infested trees
  were removed between 1998 and 2003. A select list
  of non-ALB host trees were replanted, mostly in
  the Study Area (Figure 1b).
• Trees were surveyed in the ALB Study Area
  (n3,555), an area that was most devastated by
  the beetle and therefore had replacement trees
  planted from the approved list. A matched sample
  area was surveyed for comparison (n3,423). The
  sampling method was consistent with Jaenson and
  Bassuk (1992).
• The City of Chicago Bureau of Forestry (Streets
  Sanitation) answered a questionnaire related to
  its current diversity action plan and tree
  planting practices. Results of 10 year
  inventories performed by the City were compared
  to the results from the sampling.

Results

• As shown in Figures 4, 5, and 6, Maples4,
  especially Silver, were the most devastated by
  the beetle. These were replanted with Honey
  Locust, American Linden, Swamp White Oak,
  Catalpa, Black Locust, Dawn Redwood, English
  Oak, and Shingle Oak among others.
• Replanted trees were a mix of trees common to
  Chicagos urban forest (Honey Locust, Amer.
  Linden, Catalpa) and some new arrivals (to an
  extent Black Locust, Dawn Redwood, and various
  Oaks).
• 66 of Chicagos street trees are represented by
  just 4 species
• 73 are from just three genera/families
• Only 6 families are represented in the top ten
  most frequently planted trees
• Chicago outsources the planting of the
  approximately 4-5,000 new plantings per year. An
  approved tree list is given to developers as part
  of the landscaping ordinance. Chicago publishes
  guidelines to promote diversity, and currently
  has a goal of no more than 15 of its inventory
  from one species. Norway Maple, Silver Maple, and
  Honey Locust account for 21, 16, and 15
  respectively as of 2003 (See Figure 7).
• Green and White Ash are no longer planted due to
  the Emerald Ash Borer.

Figure 7 Results of Chicagos 2003 Inventory
(City of Chicago). Compare to the 10-20-30
formula.
Discussion

• Chicago is beginning to plant for diversity,
  though time will tell if this makes a significant
  dent in the current uniform plantings.
• The ALB study area still has taxa that exceed the
  10-20-30 formula despite the replanting.
• Younger trees, however, are more diverse
• New species planted in the ALB Quarantine Areas,
  such as Swamp White Oak, are being used elseware
  in the City.
• Chicago currently does not meet its own diversity
  requirement or the more stringent benchmark of
  the 10-20-30 formula. Targeted reductions in
  certain species can change this over time

Dawn Redwood
Swamp White Oak
Figure 1a. The ALB Ravenswood Quarantine Area
Showing Tree Removal2 Figure 1b Species for
Replanting of Street Trees in Chicago3
References Bassuk, N.L. (1990). Street Tree
Diversity Making Better Choices for the Urban
Landscape. METRIA 7 Proceedings. 71-78. Dwyer,
J. F., D. J. Nowak. and M. H. Noble. (2003).
Sustaining Urban Forests. J. Arboric.
29(1)49-55. Galvin, Michael F. (1999). A
Methodology for Assessing and Managing
Biodiversity in Street Tree Populations A Case
Study J. Arboric. 25 (3) 124-128. Jaenson, R.,
N. Bassuk, S. Schwager, and D. Headley. (1992). A
statistical method for the accurate and rapid
sampling of urban street tree populations. J.
Arboric. 18(4)171183. McCombs, Phil, July 30,
(2001). Making a Stand. The Washington Post C10
Retrieved April 12th, 2005 from
http//www.washingtonpost.com/ac2/wp-dyn?pagename
articlenodecontentIdA3982-2001Jul29 Santamour
, Frank S., Jr. (1990). Trees for Urban Planting
Diversity, Uniformity, and Common Sense. Proc.
7th Conf. Metropolitan Tree Improvement Alliance
(METRIA) 7 57-65 Beetle Photos
http//www.aces.uiuc.edu/longhorned_beetle/ Tree
Photos by Author (October, 2005)
4For an ALB host list, see http//www.na.fs.fed.us
/fhp/alb/general/hostlist.shtm
1 The 10-20-30 formula states that no more than
10 of trees should be representatives of one
species, 20 of one genus, and 30 of one
family. 2http//www.na.fs.fed.us/fhp/alb/index.sht
m 3Reproduced from http//www.na.fs.fed.us/fhp/alb
/general/replanting.shtm
Correspondence can be addressed to Kevin B.
Dick Department of Geography Environmental
Studies Northeastern Illinois University k-dick_at_ne
iu.edu www.neiu.edu/deptges