In 2018, the Intergovernmental Panel on Climate Change highlighted the importance of limiting global temperature rise to 1.5°C above pre-industrial levels to avoid some of the most severe impacts of climate change (IPCC, 2018). As well as a rapid and substantial reduction in carbon emissions associated with global activities, we can look to urban forests to help us limit rising levels of carbon dioxide in the atmosphere. Trees provide a range of intrinsic benefits to people, often referred to as ecosystem services (Millenium Ecosystem Assessment, 2005). Sequestering and storing carbon is just one of the ecosystem services trees provide. There are many other environmental benefits, such as the removal of air pollution and benefits for both human physical and mental health (Nowak and Dwyer, 2007).
The Committee on Climate Change advise in their net-zero report (CCC, 2019) that the UK needs to plant a minimum of 30,000 hectares of woodland annually to achieve net-zero greenhouse gas emissions by the year 2050. Leeds City Council (LCC) have committed to plant 5.8 million trees over the next 25 years as part of the city’s contribution to the UK net-zero targets (LCC, 2020). To ensure that the intended emissions reductions are achieved, in addition to planting more trees, the current level of carbon sequestration from the city’s existing urban forest needs to be retained.
The LCC Natural Resources and Waste Local Plan (LCC, 2013) set out strategies for the management of local resources and waste within the city. The plan called for increased protection of trees affected by development activity, and set a requirement for three trees to be planted for every tree felled (LCC, 2013). This replacement policy does not account for the differences between trees, such as size, species and condition, and lacks the flexibility to address variations in carbon sequestration potential.
UBoC have been working to develop a method for estimating the number of trees that are required to replace a tree of a given condition, species and size, in order to achieve parity in carbon sequestration. This specific focus on carbon sequestration formed the brief for this research, however climate change mitigation is just one of the many environmental benefits provided by the urban forest (Nowak and Dwyer, 2007).
In line with the net-zero commitments of the city, UBoC attempt to inform an updated tree replacement policy for LCC which ensures zero net loss of carbon sequestration in the city attributed to the required felling of trees in planned development works. Our analysis utilises the best available local estimates for rates of carbon sequestration of urban trees outside of woodlands, within the Leeds area, and uses this data to calculate tree replacement rates based on tree condition, size, and species in a new Leeds4Trees method, to provide a scientific underpinning for a revised replacement rate policy.