Abstract
Iberian holm oak meadows are savannah-like ecosystems that result from traditional silvo-pastoral practices. However, such traditional uses are declining, driving changes in the typical tree—open grassland structure of these systems. Yet, there are no studies integrating the whole ecosystem—including the arboreal and the herbaceous layer—as drivers of greenhouse gas (GHG: CO2, CH4 and N2O) dynamics. Here, we aimed at integrating the influence of tree canopies and interactions among plant functional types (PFT: grasses, forbs, and legumes) of the herbaceous layer as GHG exchange drivers. For that purpose, we performed chamber-based GHG surveys in plots dominated by representative canopy types of Iberian holm oak meadows, including Quercus species and Pinus pinea stands, the last a common tree plantation replacing traditional stands, and unraveled GHG drivers through a diversity-interaction model approach. Our results show the tree–open grassland structure, especially drove CO2 and N2O fluxes, with higher emissions under the canopy than in the open grassland. Emissions under P. pinea canopies are higher than those under Quercus species. In addition, the inclusion of diversity and compositional terms of the herbaceous layer improve the explained variability, with legumes enhancing CO2 uptake and N2O emissions. Changes in the tree cover and tree species composition, in combination with changes in the structure and composition of the herbaceous layer, will imply deep changes in the GHG exchange of Iberian holm oak meadows. These results may provide some guidelines to perform better management strategies of this vast but vulnerable ecosystem.
Highlights
Holm oak meadows, called dehesas in Spain and montados in Portugal, are seminatural savannah-like agroecosystems that result from the thinning of the Mediterranean forest, in which an herbaceous and an arboreal layer coexist
Reduction that was mostly due to a change in the plant functional types (PFT) composition, with the biomass of forbs and legumes decreasing under the canopy compared to the open grassland (Figure S3 and Table S2)
The litter fraction was markedly higher under the canopy than in the open grassland, this difference almost disappeared in autumn (Figure S2 and Table S2)
Summary
Called dehesas in Spain and montados in Portugal, are seminatural savannah-like agroecosystems that result from the thinning of the Mediterranean forest, in which an herbaceous and an arboreal (mostly Quercus species) layer coexist. In particular, have traditionally provided a wide variety of goods and services, including pasture for livestock, acorns, timber, and cork Such traditional uses that have shaped holm oak meadows into a matrix of trees and open grassland are changing, with the consequent implications that this may have on ecosystem functioning. Extensive grazing is declining towards intensive farming; plantations of fast-growing trees, mostly Eucalyptus and Pinus species; shrub encroachment due to land abandonment; and there is a worrying lack of tree regeneration [7,8]
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