Historical changes in biomass carbon stocks in the Mediterranean (Spain, 1860–2010)

Land-use change was the main source of anthropogenic CO₂ emissions until the mid-twentieth century, especially due to deforestation processes. In recent decades, however, CO₂ sequestration is being induced in some countries where forest area is experiencing net increases. Despite the key role of these processes, we hardly dispose of any empirical evidence of historical changes in biomass stocks, especially in the long-term (over 50 years) and in cultivated areas. In this study, we quantify the evolution of the surface area, carbon stocks (C) and C density of living biomass in Spain (50 provinces) between 1860 and 2010. According to our results, the C stock fell from 340.3 Tg C to 254.2 Tg C between 1860 and 1950, to then reach 844.0 Tg C in 2010. Although the stock began to increase much later than in other European countries, annual growth rates were much more significant. A decomposition analysis allowed us to observe that the increase in stock was mainly due to the change in C density (61.2% of the effect), surface area (35.3%) and, to a lesser extent, to the effect of location in more productive areas (5.7%). Woody crops – which were historically managed as agroforestry systems when combined with other crops – stored 15.8% of total stocks during the period studied. They play a particularly important role in areas with a Mediterranean climate because in these provinces, crops such as olive groves, vineyards or oranges have proliferated. The reasons for C stock increases are: the substitution of firewood with fossil fuels; agricultural intensification; and the outsourcing of land use to other countries through agricultural imports.