How a City Learned to Count Its Trees
[1] In 2015 the city of Halvorsen set out to answer a question that sounds simple: how many trees did it own? The parks department estimated 80,000. The public works department, who maintained the streets, estimated 140,000. Neither number was based on a count. Both were based on multiplying the length of the city's streets by a guess about how many trees stood along a typical block.
[2] The city hired a small team of arborists to conduct a true inventory. Over eighteen months, the team walked every public street and recorded the species, trunk diameter, and condition of each tree, assigning it a numbered aluminum tag. The final count was 96,412. More surprising than the total, however, was what the inventory revealed about the trees themselves: nearly a third belonged to a single species, the Norway maple, and most of those were of roughly the same age.
[3] That uniformity was a legacy of a planting campaign in the 1960s, when Norway maples were cheap, fast-growing, and fashionable. Fifty years later, the trees were reaching the end of their lives at the same time, a disease that targeted the species could, in principle, strip a third of the city's canopy in a decade. The inventory made the risk visible; the estimates never could have.
[4] Armed with the data, the city rewrote its planting rules. No single species may now make up more than ten percent of new plantings on any street, and no genus more than twenty. Foresters have began favoring oaks, lindens, and native hackberries. Residents can look up any street tree on a public map, learn its species, and report problems. The tags, once a curiosity, are now how neighbors talk about particular trees.
[5] The inventory cost the city about $1.2 million. Officials estimate that, in avoided emergency removals and storm damage, it paid for itself within four years. Meanwhile, other cities have adopted similar programs. Halvorsen's arborists now field calls from municipalities across the region asking how to begin. Their advice is always the same: stop estimating and start counting.