Forest expansion in abandoned agricultural lands has limited effect to offset carbon emissions from Central-North Spain

We assessed the process of carbon (C) accumulation as a consequence of forest expansion in abandoned agricultural lands over the period 1977–2017 in a vast (9.4 million ha) area of Mediterranean continental environment in Central-North Spain. We achieved this objective, through obtaining AGC and BGC estimations based on direct field measurements taken in 30 plots (25 m × 25 m), and extrapolating to the landscape using Synthetic Aperture Radar (SAR) satellite data from 2018. Using aerial photographs and forest maps, we found that 145,193 ha of agricultural land in 1957 (1.54% of the study regions’ total area) has since then regenerated naturally to forests and woodlands. Although mean AGC and BGC densities were modest (i.e. 18.04 and 6.78 Mg C ha−1), they reached relatively large maximum values (i.e. 60 and 21 Mg C ha−1). The BGC stock was also very large, representing 37.3% of the total C stock (10 Tg) accumulated. However, we detected a mean annual C sink of 0.25 Tg C·year−1 which barely offset 1.22% of the total regional CO2 emissions. Our findings point to a smaller sequestration potential under Mediterranean continental than under temperate-cold conditions. Nonetheless, the area affected by this process could be larger than detected and many of the recovering lands might have not still reached their C uptake peak. If such lands are to be used to store C, we strongly advocate for the application of active forest management measures to increase their CO2 sequestration potential.