Abstract. Forest expansion can make an important contribution to the 2015 Paris Agreement, through offsetting Greenhouse gas (GHG) emissions. EU, UK and Scottish forest policy encourages substantial forest expansion. Unfortunately, policy is still inadequately informed by high resolution data, and often assumes a fairly homogenous landscape, uniformly suitable soil types and idealised ‘average’ tree timber yields, while carbon emissions caused by soil disturbance during planting, and changes in climate are rarely adequately considered. Also, the proportional contribution of afforestation targets to national mitigation needs is often overlooked which could lead to over-reliance on tree planting. We address these shortcomings through an integrated modelling approach which estimates net carbon gain for eleven tree species accounting for the interactions between climate, soil and planting practices. We present detailed spatial results for a case study area (Scotland), showing where forest expansion would be likely to result in overall carbon gains, accounting for the differentiated spatial variability of timber yield classes for each one of the species considered including present and future climate. The results showed that upland ecosystems, whose soils are rich in carbon, were vulnerable to net carbon loss, particularly with intensive ground preparation and planting practices. While the prevalence of mineral soils in the lowlands makes them a safer option for planting in theory, these are also areas which might conflict with agricultural activities. Our findings strongly support the notion that both “the right tree in the right place” and “no trees in the wrong place” are important messages for practitioners. In terms of the total UK and Scottish carbon footprints, the magnitude of the offset obtained in 30 years if afforestation goals were fully reached would likely be around 1% of the UK total business as usual GHG footprint and around 10% of the Scottish footprint. Our results can help to improve the targeting of incentives and investments in forest and woodland expansion, but also reinforce the need to pursue emissions reductions in a variety of ways throughout all sectors.

Keywords: Net zero; Soil organic carbon; Ground preparation methods; Carbon footprint; Mitigation; Climate change