Change In Street-Tree Composition Of Two Urbana, Illinois Neighborhoods After Fifty Years: 1932-1982

Results and Discussion

Between 1932 and 1982 American elm and short-lived trees such as eastern cottonwood and box elder virtually disappeared from the parkways (Tables 1 and 2). The disappearance of American elm is attributable to Dutch elm disease, which eliminated elms in Urbana during the 1950’s and 1960’s. Tables 1 and 2 reveal that the total number of trees decreased by 41% between 1932 and 1982, while the total basal area decreased by only 12% and the average tree’s stem basal area increased by 76%. There were fewer but larger trees in these two Urbana neighborhoods fifty years after the original measurements. The dominance of elm trees was changed after fifty years to a more uniform distribution and a greater variety of tree species.

Silver maples, sugar maples, sycamores and hackberries survived the 50-year period in greatest numbers (Table 3). The rapid growth rate of silver maple relative to other survivors is evident in the greater increase in basal area of this species over fifty years (Table 3). The total proportion of silver maples that survived was greaterthan that for sugar maple (Tables 1, 2, and 3). Sugar maples are long-lived and sturdy, and they are regarded as desirable parkway trees, while silver maples are highly susceptible to wind damage and considered undesirable.

In 1932, more than half of the total basal area for all street trees in the two neighborhoods was attributed to elm and cottonwood, with elms accounting for most of the basal area. After 50 years the neighborhoods had become less dominated by a single street-tree species. Tree species with the greatest total stem basal areas in 1932 were, in decreasing order, American and red elms, eastern cottonwood, silver maple, and sugar maple. By 1982 silver maple followed in order by sugar maple, sycamore, hackberry, and Siberian elm, which is resistant to Dutch elm disease, dominated the parkways within the boundaries of the two neighborhoods. Siberian elm and silver maple are both susceptible to wind damage, making them undesirable street-tree selections for Urbana. New plantings of both species are banned from city property by current ordinance.

The tree species found on Urbana parkways in 1932 were rapidly-growing, easily propagated species, many of which were locally available as forest wildings or volunteers on disturbed land. The trend, as illustrated in tables and appendices, is toward a greater diversity of trees, with a larger number of native and exotic species selections, and more cultivated varieties selected for special attributes. This emerging trend seems to be, in part, the result of city ordinances establishing street-tree planting and maintenance policies and an Arbor Division established in 1974 that is empowered to implement street-tree plans and regulations.

Dramatic changes in street tree composition occurred in these two Urbana neighborhoods over 50 years. Dutch elm disease, construction and utility company activities, different growth rates of trees, different mortality patterns of tree species, and changing selections of tree species for planting probably account for the changes, though historical planting and. mortality data are almost completely lacking. Insofar as these neighborhood parkways are representative of street tree populations in general, parkway “forests” must be regarded as very dynamic ecosystems. Much can be done to produce stability and lower maintenance costs of urban parkway forests by more judicious selection of tree species for planting. This seems to be the trend in Urbana.