Conduct a basic inventory of public trees, noting typical characteristics of each tree – species, height, diameter, age and condition. Area-based random sampling works fine.
In many cases, volunteers or students can be recruited and trained to conduct the inventory.
Analyze your inventory data and calculate the percentage of all trees deemed to be in “good” or better condition.
Now calculate the percentage of trees of each species deemed to be in “good” or better condition.
Calculate, for each species, the Relative Performance Index. Divide the percentage of trees of each common species in good condition, by the percentage of trees in the entire community deemed in good condition:
percentage of trees in a single species in good condition
percentage of trees of all species in good condition
For example, if 60 percent of white mulberry trees [morus alba] are classified in good condition, and 60 percent of the total population of trees are likewise classified in good condition – the Relative Performance Index value for that species would be 1. Species with RPI’s greater than 1 generally provide more benefits to a community. Those with RPI’s less than 1 may, for reasons [including but not limited to senescence] will likely provide less, and require more costly maintenance.
Click the “Access Resource” link for a more detailed explanation of the Relative Performance Index.
Pacific Southwest Research Station, US Forest ServiceView Resource