Future forests: estimating biogenic emissions from net-zero aligned afforestation pathways in the UK

Abstract. Woodlands sequester carbon dioxide from the atmosphere, which could help mitigate climate change. As part of efforts to reach net-zero greenhouse gas emissions by the year 2050, the UK's Climate Change Committee (CCC) recommend increasing woodland cover from a UK average of 13 % to 17 %–19 %. Woodlands can also have benefits for air quality. However, they emit biogenic volatile organic compounds (BVOCs) which are precursors to atmospheric pollutants, such as ozone (O3) and particulate matter (PM), which have the potential to degrade air quality. Here we make an estimate of the potential impact of afforestation in the UK on BVOC emissions, coupling information on tree species' emissions potential, planting suitability and policy-informed land cover change. We quantify the potential emission of BVOCs from five afforestation experiments using the Model of Emissions of Gases and Aerosols from Nature (MEGAN) (v2.1) in the Community Land Model (CLM) (v4.5) for the year 2050. Experiments were designed to explore the impact of variation in BVOC emissions potentials between and within plant functional types (PFTs) on estimates of BVOC emissions from UK land cover, in a future warmer climate under elevated atmospheric CO2 concentrations, to understand the scale of change associated with afforestation to 19 % woodland cover by the year 2050. Our estimate of current annual UK emissions is 39 kt yr−1 for isoprene and 46 kt yr−1 for total monoterpenes. Broadleaf afforestation results in a change to UK isoprene emission of between −3 % and +123 %, and a change to total monoterpene emission of between +5 % and +48 %. Needleleaf afforestation leads to a change in UK isoprene emission of between −3 % and +22 %, and a change to total monoterpene emission of between +60 % and +86 %. Our study highlights the potential for net-zero aligned afforestation, in a likely warmer and drier future UK climate, to have substantial impacts on BVOC emissions. However, the results highlight possible pathways to achieving 19 % woodland cover without requiring large increases in isoprene emissions. The emissions estimates presented here provide the opportunity to quantify future impacts on air pollution associated with changes in biogenic emissions, as well as how these impacts would be affected by concurrent changes in anthropogenic emissions.