Species Adapted for Street-Tree Environments in Iowa

Methods

On 1978, The Department of Forestry at Iowa State University in cooperation with the Forestry Section, Iowa Conservation Commission, initiated research on street tree populations in smaller Iowa communities. Forty towns with populations between 500-10,000 (1970 census) were selected at random from the almost 1000 towns in Iowa. For each of the 40 towns, a complete street tree (trees along streets on public property or in publicly owned parking) inventory was done to determine needs and to serve as a basis for management of the trees within each individual community.

Individual tree information included specific location in the community, species, size class of each tree (diameter classes: 0-3, 3-6, 6-12, 12-20, over 20 inches), and condition or health of each tree. There were three condition classes: good, fair, and poor. Good trees were those requiring no care or maintenance or only minimal maintenance such as minor pruning. Any problems associated with trees classified in good condition were not a threat to longevity or usefulness of the trees. Fair trees were those in need of corrective treatments if they are to function for more than 10 additional years. Poor trees were those that probably will not survive longer than 2 years without corrective action.

Results

From the more than 39,000 trees classified in this study, Table 1 lists the percentage of each species classified in good condition for those species observed at least 100 times. Species that had about 90% or more of the individuals classified as good probably are acceptable species for street-tree types of environments. Species with much less than 90% of the individuals in good classification may be less well adapted for street-tree types of environments.

As trees age, their overall condition tends to deteriorate, and all the species observed showed a general decrease in the percentage ranked good as diameter increased. Some species, however showed a much larger decrease than others (Table 2). The species listed in Table 2 may not be good choices as large specimen or landscape trees under the stress conditions found in street-tree environments.

The climate of Iowa has a strong northwest to southeast gradient, being more warm and moist in the southeast. Also, there are major differences in the soils of the northwestern and the southeastern halves of the state. Therefore, a comparison was made to see if the condition of each tree species in the northwestern half of the state was different from that in the southeastern half. For most species there were no major differences, but sugar maple (Acer saccharum), silver maple (Acer saccharinum), and Norway maple (Acer platanoides) are in a substantially better condition in the northwestern half of the state than the southeastern half (Table 3).

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Discussion

The number of acceptable street trees species for any community is small because they must fit situations with many limiting factors. According to Cott (1980), they should: 1) permit free movement of pedestrians and vehicles, 2) not interfere with overhead or underground utility installations, 3) not interfere with sight distance for safe vehicle traffic, 4) provide desired shade and appearance, and 5) be hardy and require minimum maintenance. Because of limited planting space, large trees with high branching habit are generally preferred. Small trees and shrubs are acceptable only on areas that can accommodate the spread of the trees without interfering with both pedestrian and vehicular traffic. In addition, species known to be susceptible to serious insects or diseases should not be used as street trees because of increased maintenance costs.

In 1977, the Iowa Community Tree Program Extension Committee (including representatives from the Departments of Entomology, Forestry, Horticulture, Landscape Architecture, and Plant Pathology at Iowa State University) recommended, based on the recommendations of Cott (1980) and their knowledge of the species that grow in the state, the following species for street tree use: sugar maple, black maple (Acer nigrum), red maple (Acer rubrum), hackberry (Celtis oc- cidentalis), green ash (Fraxinus pennsylvanica), white ash (Fraxinus americana), ginkgo (Ginkgo biioba), sycamore (Platanus occidentalis), white oak (Quercus alba), red oak (Quercus rubra, scarlet oak (Quercus coccinea), bur oak (Quercus macrocarpa), English Oak ((Quercus robur), American linden (Tilia americana), and little leaf linden (Tilia cordata).

All but four of the species recommended by the Extension Committee appear in Table 1 . Less than 100 ginkgo, white oak, scarlet oak, and English oak were examined,so they are not listed, but for all four species, more than 90% of the trees were classified as good. Of the recommended street trees that appear in Table 1, all have more than 88% of the trees classifed as good except for black maple (82%) and white ash (78%). Therefore, the species recommended for street trees in Iowa should continue to be used, though black maple and white ash should be used cautiously.

There are 42 species listed in Table 1. Most of them seem to do well growing in the street-tree environment. Although many of them do not have the characteristics desirable for street trees, they should do well in street-tree-like environments.

As a rule, midwestern communities need more diversity and a better selection of adaptable species. Sugar maple, silver maple, Norway maple, and green ash made up 52% of the street trees that were examined. Only two of the species, sugar maple and green ash, are from the list of recommended street trees, and both are in the middle in terms of percentage of trees classified as good. Urban forest managers in Iowa, as other midwestern states, should strive for the use of well adapted species and increased diversity in street tree populations.