Carbon stock and partitioning in coexisting forests with different management history in Monfragüe National Park (western Spain)

Forests store carbon in wood, forest floor, and soil. Carbon stock and distribution between these compartments differ across forests. The Monfragüe National Park (West Spain) is home to a variety of forests resulting from contrasting land use histories. These include holm and cork oak (Quercus ilex and Q. suber) dehesas (open forests with low-intensity agro-silvo-pastoral use), and pine (Pinus pinaster) and eucalypt (Eucalyptus camaldulensis) plantations, which pursued wood production. These uses ceased decades ago. Part of the former eucalypt plantations was removed and planted with native oaks, but the restoration failed leading to a Cistus ladanifer shrubland. In these five vegetation types, we assessed carbon stock and partitioning among above- (tree and shrub canopy, and forest floor) and belowground compartments (roots and soil); we also assessed tree health, and potential for tree regeneration as indicators of carbon stock persistence. We measured tree dimensions, shrub cover and height, and collected forest floor and soil samples for organic carbon analyses. Allometric equations were used to estimate tree and shrub carbon stocks. Plantations were expected to show a less diverse undercanopy but higher carbon stocks than dehesas, whereas dehesas were expected to allocate a greater proportion of carbon belowground, and to exhibit better tree health and regeneration. Total carbon was the highest in the pine and eucalypt plantations, followed by oak dehesas and minimum in the C. ladanifer shrubland. The shrub layer contributed more to the total carbon stock in the C. ladanifer shrubland and in the eucalypt plantation (13-14%), but ≤ 2% in the pine plantation and in the dehesas. The belowground carbon fraction was greatest in the oak dehesas (c.a. 60%), followed by the pine (55%) and eucalypt (46%) plantations. C. ladanifer shrubland showed an unexpectedly high proportion of carbon in the soil, probably due to legacy effects of the former plantation. Tree health was poorest in eucalypts, followed by cork oaks, while pines and holm oaks showed a better condition. Tree regeneration was poor in all forest types, likely due to canopy competition (plantations) and herbivory (dehesas). Although pine and eucalypt plantations store more total carbon, oak dehesas offer more stable storage due to higher belowground carbon. However, regeneration failure threatens their long-term persistence. Our results show that past land use largely determines forest capacity to store and retain carbon over the long term. Enhancing forest resilience should be a central goal of management aimed at strengthening carbon sink services.