Urban Forestry Management Series: Implementing a Species Diversity Program
by Mark Duntemann
Natural Path Urban Forestry Consultants
July, 2004
Introduction
Species Diversity is a common goal in many community forestry programs. But what does diversity really mean? How do communities invoke a program that enhances diversity? This paper strives to answer these questions by outlining seven steps to achieving a species diversity program that is tailored to each community's unique situation. These steps focus on assessing the current population and program; establishing target numbers for each species; determining annual thresholds for each species being planted; and aggressively rotate species in and out of the annual planting list.
Problem Statement
Over the past twelve years, Natural Path Urban Forestry Consultants, (NPUFC) has provided services inventorying street and park trees and developing management strategies for communities. This has provided an in-depth exposure to a broad range of programs and data from across the country. A number of issues and patterns in historic and current community planting programs have arisen. Upon reviewing the data and policies of these programs, four clear patterns of species distribution are evident. They are:
- A relatively large variety of species can be found in a community. Even for a small community it is possible to identify over eighty species and cultivars.
- Typically, only a handful of species dominate the population. It is not uncommon for a suburb of Chicago to have 25% of their street tree population comprised of one species. In some Montana communities, 70% of the street tree population is comprised of only one species.
- Dominant species are often a result of distant past planting programs, and these tend to be considered high maintenance species by the community.
- Many communities currently focus on providing short lists of acceptable species for planting in streetscapes. In many cases, these very brief lists have not changed in over a decade, and they perpetuate near monoculture populations.
The negative aspects of the patterns identified above are significant. Managers invariably deal with a large number of trees planted decades ago that would now be considered inappropriate because of their high maintenance costs. A large portion of current municipal forestry budgets is devoted to maintaining these trees. Current planting programs that offer a very narrow range for species selection have a tendency to promulgate near monoculture plantings and, over time, actually reduce the number of species in the population.
By analyzing historical data, it is evident that the potential planting palate of a community can be very large. There are many urban-appropriate species that are typically underutilized.
A number of references on urban forestry emphasize the importance of species diversity. However, it is unclear in the literature on how a community actually enacts a diversity program. A strong emphasis is placed on understanding the species composition of a community and at a higher order mapping the trees to understand species distribution. Thresholds are even discussed. A baseline of information is the foundation from which a diversity program is established, but simply summarizing existing data is of little use to the proactive community if basic guidelines of evaluation cannot be implemented to, first, determine diversity; and second, if desired, modify the planting program to enact diversity.
Planning for Species Diversity
Enacting a species diversity program implies three actions: Minimize overused species; increase the planting of underused species are increased in emphasis; and introduce new species into the landscape. To achieve each of these elements, eight clear tasks have been developed by NPFC. The first two tasks focus on evaluating the current and potential tree resource. The remaining six tasks outline a systematic process for establishing thresholds for each species and circulating species in and out of the annual planting list.
Task 1 – Compile and assess data on the current tree
population and program.
An inventory
is the easiest tool for collecting and summarizing this data.ing this
assessment. Necessary information
includes:
- Species count and percent representation in the population.
- Number of vacant planting sites in the community.
- Number of trees planted each year.
- Number of trees removed each year.
- Contents of the annual planting list.
The Village of Flossmoor, IL will be used as an example throughout this discussion. The Village is located in the south suburbs of Chicago. It has 7,644 street trees which are represented by 105 species and cultivars. In addition to the trees, the Village has identified 1,213 available planting sites. The Village's program plants 100 trees a year and removes about 70 trees a year.
For this discussion, fifteen species from the population will be analyzed--the ten most abundant and five underused species. All cultivars have been combined within a species. Table 1 displays the total count and percent of population represented by the fifteen species selected.
| Table 1 - Village of Flossmoor, IL – Species Composition | ||
|---|---|---|
| Species | Count | Percent of Total |
| Maple, Silver | 1,676 | 21.9 |
| Ash, Green | 1,164 | 15.2 |
| Honeylocust | 870 | 11.4 |
| Maple, Norway | 525 | 6.9 |
| Oak, Red | 370 | 4.8 |
| Maple, Red | 368 | 4.8 |
| Linden, American | 252 | 3.3 |
| Ash, White | 238 | 3.1 |
| Linden, Littleleaf | 213 | 2.8 |
| Maple, Sugar | 206 | 2.7 |
| Oak, Bur | 75 | 1.0 |
| Hornbeam, American | 16 | 0.2 |
| Kentucky Coffeetree | 16 | 0.2 |
| Pagodatree, Japanese | 3 | 0.0 |
| Redbud, Eastern | 2 | 0.0 |
| TOTAL | 5,994 | 78.00 |
Table 2 displays the list of trees that are currently available for residents and developers to select from for street tree plantings.
| Table 2 - Village of Flossmoor, IL – Planting List |
|---|
| Species |
| Maple, Norway |
| Ash, Green |
| Crabapple |
| Serviceberry |
| Linden, Littleleaf |
| Oak, Red |
Task 2 – Develop a complete palate of all potential species to
plant.
Create a list of all trees that can potentially be planted
in the area. The list should
include everything from small to large; and abundant to obscure. This list can be derived from a number
of sources:
- Current tree population: Identify all species that currently appear to be doing well.
- Planting lists from neighboring communities.
- Local arboreta or botanic garden lists.
- Tree, shrub, and landscape books.
- Nursery catalogs that match zone restrictions for your area.
Task 3 – Establish a target for each species on your current
list.
To minimize an over emphasis of any one
species on the list, a cap for each species should be established. This cap should be a percentage of the
total possible number of trees that can be present in the community (existing
number of trees plus the number of vacant planting sites). In Flossmoor, a total potential of 8,857
trees exists.
| Table 3 - Village of Flossmoor, IL – Species Targets | ||||
|---|---|---|---|---|
| Current | Target | |||
| Species | Count | Percent of Total |
Count | Percent of Total |
| Maple, Silver | 1,676 | 18.9 | 709 | 8.0 |
| Ash, Green | 1,164 | 13.1 | 709 | 8.0 |
| Honeylocust | 870 | 9.8 | 709 | 8.0 |
| Maple, Norway | 525 | 5.9 | 531 | 6.0 |
| Oak, Red | 370 | 4.2 | 531 | 6.0 |
| Maple, Red | 368 | 4.2 | 443 | 5.0 |
| Linden, American | 252 | 2.8 | 266 | 3.0 |
| Ash, White | 238 | 2.9 | 355 | 4.0 |
| Linden, Littleleaf | 213 | 2.4 | 266 | 3.0 |
| Maple, Sugar | 206 | 2.3 | 355 | 4.0 |
| Oak, Bur | 75 | 0.8 | 531 | 6.0 |
| Hornbeam, American | 16 | 0.2 | 266 | 3.0 |
| Kentucky Coffeetree | 16 | 0.2 | 266 | 3.0 |
| Pagodatree, Japanese | 3 | 0.0 | 89 | 1.0 |
| Redbud, Eastern | 2 | 0.0 | 89 | 1.0 |
| TOTAL | 5,994 | 67.7 | 6,115 | 69.0 |
There are 75 Bur oak, representing 0.8% of the population. If the community wants to target the increase of Bur Oak to 3.0%, a total of 300 more Bur oak are needed to meet this target. (What about incorporating number of trees removed each year?)
Task 4 – Develop a short list of species to plant each year.
Develop
a short list of anywhere from five to twenty species. This is the list of species that are going to be emphasized
in the current year's planting program. The list should incorporate a variety of species in each of the various
size categories determined in the community. (Number
of species in each size category should be proportional to the number of planting
sites in each size category)
| Table 4 - Planting Site Limitations | |||
|---|---|---|---|
| Greenbelt Width | Overhead Utilities | Total | |
| Yes | No | ||
| 0 to 4' | S | S | |
| 5 to 9' | S | M | |
| >9' | M | L | |
| TOTAL | 8,857 | ||
Task 5 – Create a threshold for each species on your annual
planting list.
To minimize an over emphasis of any one
species on the annual list, a cap for each species should be established. For example, if 100 trees are being
planted this year by the Village, a maximum number of any one species from your
list should be established. As
that threshold is reached, that particular species is taken off of the list for
the remainder of the year.
| Table 5 - Village of Flossmoor, IL – Planting List Annual caps | |
|---|---|
| Species | Cap |
| Maple, Norway | 15 |
| Ash, Green | 10 |
| Crabapple | 25 |
| Serviceberry | 25 |
| Linden, Littleleaf | 10 |
| Oak, Red | 25 |
| TOTAL | 100 |
This does not mean that the first development that gets planted can use up all of the Norway Maple for the year. A careful balance of all of suggested species for the year will guarantee an even and equitable distribution of the 100 trees.
Task 6 – Establish a planting cycle for each species.
Each
species stays on the annual list for a few years. The duration--anywhere from three to five--is a function of
how much emphasis will be placed on that particular species (the current versus the target). For example, Kentucky Coffeetree,
currently represented by 16 trees, has a target of 266, its time on the
planting list will be longer.
Task 7 – Rotate species in and out of annual list.
Once
a species has reached the end of its planting cycle, it is taken off the list
for a few years. Other species are
then placed on the annual list to encourage their emphasis. This down time is temporary and should
last anywhere from three to five years for any one species.
Task 8 – Evaluate the program.
No
program is complete without the ability to gauge whether or not the program is
successful or not. The best way to
assess the success or failure of a species diversity program is to compare the
change over time to the individual species count and compare that count against
the targets you have established for each species. Continue to assess the suitability of species, incorporate
changes due to insect/disease, cultural, or other realizations.
Discussion
This narrative should be viewed more as an exercise in understanding the overall concept we are trying to present rather than getting entrenched in numbers. By enacting some of the tasks outlined above, the long-term results will be that overtime as attrition takes hold and a greater variety of species are planted, the species that are overused will be reduced in the total population, and underused species will have greater representation. While the example presented here is from a community that has a fairly substantial planting budget for its size, the concepts can be applied to communities that annually plant ten trees or 1,000 trees. The only difference is that small communities may only emphasize two to five species each year, while larger communities may have substantial annual lists. In either case the development of thresholds and the rotation of species in and out of the annual list is imperative.
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