The Hardest-Working Employees in the City

ACity accountants have learned to talk about trees in an unfamiliar way: as infrastructure. New York's parks department maintains a public database in which every one of the city's hundreds of thousands of street trees has an identity number and an annual dollar value, calculated from the stormwater it intercepts, the energy its shade saves, and the air pollutants its leaves remove. The totals across the fleet run into tens of millions of dollars a year — and the accounting, foresters argue, still flatters the city rather than the tree, since the hardest benefits to price are the largest ones.

BThe measurable services are impressive enough. A mature canopy intercepts rain, slowing the rush of water into drains and cutting the flash floods that paved cities suffer; a single large tree can hold back thousands of litres in a storm. Shade lowers surface temperatures beneath it by ten degrees or more, and neighbourhoods with generous canopy run measurably cooler than bare ones during heatwaves — a difference that, as mortality statistics show, is counted in lives rather than comfort. Leaves filter fine dust; roots stabilise soil; and along busy roads, well-placed greenery even dampens noise. None of this requires a tree to do anything but stand there.

CThe subtler evidence concerns people. Hospital patients whose windows faced trees, in a study now classic in its field, recovered faster and requested less pain relief than otherwise identical patients facing a brick wall. House prices rise on greener streets; shoppers linger longer; drivers, curiously, slow down. Some of the most striking findings are epidemiological: research following the loss of tens of millions of ash trees to an invasive beetle in the American Midwest found that counties which lost their trees subsequently recorded elevated deaths from heart and respiratory disease — as if the removal of trees had been, in effect, subtracted medicine.

DYet a street is close to the worst place a tree can live. Beneath the pavement, roots that want metres of open soil get a coffin-sized pit of compacted rubble; above it, branches meet delivery trucks, power lines and pruning crews. Road salt poisons the soil each winter; summer radiates heat from asphalt on both sides. The result is written in the statistics: urban street trees live a fraction of their natural lifespans, and many die within the first decade — which is why foresters repeat that the point is not planting but keeping. A sapling photographed at a ribbon-cutting ceremony delivers almost nothing; the services accrue over decades, in exactly the years when maintenance budgets have moved on.

EThe consequence, in city after city, is that canopy maps and income maps look uncomfortably alike: mature trees line the avenues of wealthy districts, while the neighbourhoods that suffer most from heat have the least shade. Campaigns to correct this — from million-tree drives to rules requiring shade over playgrounds — increasingly treat canopy as a matter of equity rather than decoration. The shift in language may be the most important development of all. A decoration is the first thing cut from a budget. Infrastructure that saves lives, hospital bills and flood repairs is rather harder to cancel — and that, say the accountants counting leaves, is precisely the point.