What proportion of the costs of urban trees can be justified by the carbon sequestration and air-quality benefits they provide?

Abstract

The i-Tree Eco programme estimates carbon sequestration and air pollution removal by urban forests. Previously we applied i-Tree Eco to the Torbay area of England; in this paper we use these results to assess the extent to which these benefits justify the costs of urban trees. We combined our benefits estimates with cost data to produce a single year “snapshot” comparison of costs and benefits for the urban forest. We then extended the discussion by modelling the lifetime benefits of four species of tree in the Torbay context. Two were assumed to be street trees: Lime (Tilia cordata) and cherry (Prunus avium). Two were assumed to be park trees: Maritime pine (Pinus nigra var maritima) and English oak (Quercus robur). We used this to produce an illustrative, partial, cost: benefit ratio for these trees. The central estimate of the “snapshot” comparison significantly exceeded costs and this remained the case when we sensitivity tested by halving the air-quality benefit to account for an uncertainty in the input data. The central cost-benefit ratios for the individual trees were cherry 1:0.01, lime 1:0.07, pine 1:0.12, and oak 1:0.21. We tested these results for sensitivity to the discount rate; reanalysing with a discount rate of 2.1% as opposed to the standard 3.5%. This produced the following results: Cherry 1:0.01, lime 1:0.76, pine 1:0.97, oak 1:4.96. These results suggest that, in common with other projects, cost-benefit analysis of investment in trees is very sensitive to the discount rate used, but that whilst carbon sequestration and air pollution removal may not justify tree planting by themselves, they do add to the justification. The significance of these benefits vary by tree, location and assumptions used, but larger and longer-lived trees appear to offer the greatest carbon sequestration and air-quality benefit, with the native oak performing particularly strongly.