The forests and agricultural lansdscapes of the Basque Autonomous Community have undergone significant transformations since the Middle Ages, starting with forest clearing and terracing. The traditional agro-silvo-pastoral systems distributed around the family farmsteads, the baserria, were implemented at least from the Late Middle Ages (Narbarte-Hernandez et al., 2024). Communal lands that were used for pastures or forests were very often transformed in the Modern period with the introduction of American crops (mainly maize) and agricultural practices like liming (Narbarte-Hernandez et al., 2024). At the same time, forest plantations of native species (oaks, beech, chestnuts) were very frequent from the XVII century due to the high demand of firewood and charcoal by the forge and naval industries, as well as for domestic use. In the late XIX century the plantations of Monterrey pine (Pinus radiata D. Don) start their implementation in the Basque country, with their later expansion during the second half of the XX century. 

Twenty years ago, Merino et al. (2004) informed of the risk of long-term accelerated erosion, carbon and nutrient loss caused by intensive forestry management on acidic soils of the Basque Country. Intensive practices for site preparation (removal of organic residues, ploughing, and scalping) can deplete the soil of base cations, reduce cation exchange capacity, loss of organic matter, subsoil compaction, reduction of microbial biomass and activity (Merino et al., 2004). In the mid- to long-term the loss of soil fertility can reduce tree growth and nutrient status, jeopardizing the economic and environmental sustainability of the forest plantations. Intensive forestry practices which include clearcutting and mechanical site preparation in steep slopes, on soils that are intrinsically acidic and nutrient poor, make them very vulnerable to several risks soil degradation: acidification, loss of organic carbon, loss of structure and compaction, depletion of nutrients, and erosion. 

The objectives of this working package were:

You can check the results of this assessment with an interactive web app!

As an example of the soil properties that were explored in this working package, the figure above shows the relationship between effective cation exchange capacity (CECef) and soil pH at different depth intervals and by vegetation type. "Native" refers to semi-natural native forests (oaks, beech, etc.) that are used as reference while "plantation" indicate commercial forest plantations. You can see that the CECef is lower at plantations compared to native forests for pH between 6-8, whereas at pH lower than 6 the CECef is low irrespective of the vegetation. CECef is a proxy of the capacity of the soil to hold cations (positive charge) in the soil exchange solution. Soils with higher CEC can retain more nutrients. The CECef is controlled by soil clay content and mineralogy (negative charge), content or organic matter (mainly with negative charge), and soil pH.

Forest plantations decrease the content of organic matter (and hence reduce CECef), acidify the soil (the pH is lower), and hence we move towards the left in the graph, lower CECef. This means that the condition of the plantations' soils for holding and exchanging cations (calcium 2+, magnesium 2+, potassium+) is lower compared to semi-natural forests