Achieving tree canopy cover targets: A case study of Bristol, UK

Abstract

Many cities are increasing urban canopy cover to mitigate the impacts of urbanisation and climate change. Ambitious canopy cover targets are being implemented, with little consideration on how best to achieve them. The City of Bristol has recently published a One City Plan, which includes a target to double tree canopy cover by 2045. This study aims to investigate its feasibility, and explore different planting scenarios to achieve the target and maximise ecosystem service delivery. Using an existing i-Tree Eco assessment of the urban forest, i-Tree Forecast was used to project future urban forest growth under a number of different user-defined planting scenarios. Sixteen scenarios were forecasted over 27 years to test a variety of approaches. Tree stock size and number, timing of planting and annual mortality rates were varied to test the performance of each scenario. Planting 18,000 large tree stock, equivalent to ‘heavy standards’ every year for 27 years provided the most feasible scenario in balancing canopy cover, leaf area index and pollutant removal whilst providing a stable population. This increased to 44,000 trees per year assuming an annual mortality rate of 3 % more typical of urban areas, demonstrating the importance of good stewardship. The size of planted tree and timing of planting had a strong impact on the development of canopy cover. It was concluded that the One City Plan target is feasible if the planting rate is increased to at least 18,000 large tree stock from the current 10,000 trees per year.