Pine afforestation of treeless Mediterranean heathlands reduces productivity of neighbouring cork oak woodlands

Afforestation programs as a strategy to address the climate crisis are on the rise worldwide. Although concerns exist about the effects of afforesting treeless habitats on their biodiversity and ecosystem services, potential impacts on areas beyond afforestation boundaries have been largely overlooked. Cork oak (Quercus suber) woodlands in southern Spain are regarded as a successful example of sustainability. However, the afforestation of their neighbouring treeless habitats may compromise their productivity through indirect effects that are not fully understood. Using a multi-method approach, we studied the effects of the afforestation of Mediterranean heathland (herriza) areas with pine tree species on the productivity of neighbouring cork oak woodlands downslope over a 37-year period. The differences in Normalized Difference Vegetation Index (NDVI) and Basal Area Increment (BAI) between cork oak woodlands below open herriza and below afforested herriza became apparent approximately ten years after pine afforestation. Specifically, NDVI was significantly reduced in cork oak stands below afforested herriza areas for the remaining years, while a substantial decrease in BAI of cork oak trees was also associated with afforested herriza areas upslope. Moreover, the NDVI and BAI trends of cork oak stands below afforested herriza reached the minimum levels of the time series following an extreme drought event. Our results reveal that the ecological impacts of afforestation of naturally treeless habitats can expand beyond the plantation area. We propose that the restoration of treeless habitats, such as the herriza, in areas where pine plantations are no longer productive, may enhance their ecological services, particularly in the context of climate change. Therefore, afforestation programs worldwide should carefully consider the trade-offs between carbon mitigation and other ecosystem services at the landscape scale.