Study

Carbon storage and sequestration by urban trees in the USA

Nowak, D. J.; Crane, D. E.

Environmental Pollution, 2002

Peer-reviewedmodelling studymixed

What they did

Trees were measured on field plots in a set of cities, their mass estimated with allometric equations, and carbon storage and annual uptake calculated per unit of tree cover and per tree, with adjustments for the poorer growth conditions of urban trees.

Sample size: field plots across ten United States cities

What they found

Storage and annual sequestration per tree varied by more than an order of magnitude with species, diameter, condition and city. The paper reports averages while being explicit that the average is an accounting device across a highly skewed distribution: a small minority of large trees hold most of the carbon.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Urban forests store and sequester a substantial quantity of carbon, and estimates must account for the size distribution and condition of the trees involved.

How NatureHQ reads it

NatureHQ cites this partly for what it measures and mostly for where the famous numbers come from. Figures of the "one tree absorbs so many kilograms of CO₂ a year" kind descend from work of this type, and they are averages over a distribution so skewed that the average describes almost no individual tree. Reproducing a single number without its species, size, age, site and method is not a simplification of this literature; it is a claim this literature does not make.

Limitations

United States cities; species mixes and growing conditions differ elsewhere.
Mass is estimated from equations built mostly on forest-grown trees, then adjusted for urban conditions.
Reports stocks and rates, not a full life-cycle balance including planting, maintenance and eventual decay.