David Nowak

The problem Nowak solved is one of accounting. Everyone accepted that urban trees are beneficial, but since the benefit appeared in no budget line, the canopy competed at a disadvantage against any construction project in municipal spending. Nowak developed the models that turn crown, species, diameter and location into tonnes of pollutant removed, kilowatts saved on heating and cooling, cubic metres of avoided runoff and carbon stored.

The result is i-Tree, the free toolset the US Forest Service released in 2006, now used by thousands of cities worldwide to inventory their trees and calculate their returns. It is probably the instrument that has done most to let a city hall defend a pruning budget or a planting scheme with a figure on the table.

Nowak is a senior scientist at the US Forest Service's Northern Research Station in Syracuse, and has spent his career measuring the structure and function of the urban forest. His studies on the removal of particulates and ozone by urban vegetation are among the most cited in the field.

His work also includes the caveats, and they are worth reading: benefits depend on species, crown size and long-term survival, so planting a lot and maintaining little produces pretty numbers and no results. One large old tree does the work of dozens of young ones, which is why Nowak insists the priority is conserving mature trees rather than announcing mass plantings.