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

Woodlands sequester carbon dioxide from the atmosphere, which could help mitigate climate change. As part of an effort 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 also have the potential to affect air quality, in part due to the emission of biogenic volatile organic compounds (BVOCs) which are precursors to major atmospheric pollutants, ozone (O₃) and particulate matter (PM). This study presents for the first-time estimates of BVOC emissions that are consistent with net-zero aligned afforestation in the UK. The BVOC emission scenarios consider suitability of tree species for the UK coupled with regionally appropriate emissions potentials. 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 the variation in BVOC emissions potentials between and within plant functional types (PFTs) on estimates of BVOC emissions from UK land cover, to understand the scale of change associated with afforestation to 19% woodland cover by the year 2050.  

Our estimate of current annual UK BVOC emissions is 40 kt yr^-1 of isoprene and 46 kt yr^-1 of total monoterpenes. Broadleaf afforestation results in a change in UK isoprene emission of between -4% and +131%, and a change in total monoterpene emission of between +6% and +52%. Needleleaf afforestation leads to a change in UK isoprene emission of between -3% and +20%, and a change in total monoterpene emission of between +66% and +95%.   

Our study highlights the potential for net-zero aligned afforestation to have substantial impacts on UK BVOC emissions, and therefore air quality, but also demonstrates routes to minimising these impacts through consideration of the emissions potentials of tree species planted. We show that incorporating regionally appropriate emissions factors, information about present day abundance of tree species, and the likely role of different species in the UK’s future forests, can substantially alter estimates of emissions. This study highlights an important interaction between the land and the atmosphere, for climate change mitigation options, specifically afforestation, to hold the potential to impact air quality.