Assessing street tree diversity in four Ohio communities using the weighted Simpson index

Diversity is a key factor which should be considered during planning street replacements and involves assessment of existing street population for various factors especially resistance of trees to pests. Street tree replacement histories in the United States reveal extensive replacement of American elms killed by Dutch elm disease (DED) with maples, or ashes. In turn, these trees are now under attack by Asian longhorn beetle (ALB) and emerald ash borer (EAB). Two commonly used evaluation measure of diversity namely the 10:20:30 rule and the Simpson index takes into account only the number and evenness of the taxa in a street tree population. However these indexes fail to consider the differential functional values of trees which are critical in building a sustainable urban forest while attempting to increase diversity. The proposed weighted Simpson index which weights the absolute abundance of tree taxa based on some functional variable could be used as a guide to identify areas of focus for a community. A case study involving four communities in Ohio namely Toledo, Westerville, Dublin and Yellow Springs was conducted to examine the usefulness of the weighted Simpson Index. Three functional variables including environmental benefits, pest vulnerability and taxon adaptability at three different taxonomic levels, family, genus and species, were used to arrive at the weighted Simpson indexes for the four communities. Results revealed the usefulness of weighted Simpson Index in exposing areas of concern for each community. Generally all the four communities were lacking larger statured trees with greater environmental benefits.

► Diversity in street trees is a key to maintaining sustainable urban forest.
► Diversity measures fail to consider crucial functional variables used in planning.
► Weighted Simpson index was computed using 3 functional variables for 4 communities.
► Simpson index weighted by function value assists in identifying areas of focus.
► Generally the communities should consider trees with higher environmental benefits.