Holm oak meadows or oak savannahs are semi-natural savanna‑like systems that result from the thinning of the Mediterranean forest, in which an herbaceous layer and an arboreal canopy (mostly Quercus species) coexist. They represent one of the largest agroforestry systems through Mediterranean climatic regions (also called dehesas in Spain and montados in Portugal), and are ecosystems of high cultural and economic value. However, traditional uses, including production of forage, acorns, timber and cork, which have shaped oak savannahs into a matrix of trees and open grassland, are declining towards intensive farming and grazing, and there is a worrying lack of tree regeneration.
Understanding how the tree – open grassland structure drives ecosystem functioning is essential for oak savannahs preservation and management. Yet, there are no studies integrating the canopy influence on greenhouse gas (GHG) exchange, including CO2, CH4 and N2O, mediated by the herbaceous layer structure and composition. In addition, little is known about the effect of different tree species (Quercus ilex, Quercus suber and Pinus pinea) on GHG exchange.
Two field campaigns were performed (spring and autumn) to record GHG fluxes by discrete chamber based measurements, and to sample aboveground biomass, which was afterwards separated into plant functional types (grasses, forbs and legumes). GHG fluxes well modeled as function of the season, tree canopy and the herbaceous layer structure and composition by a diversity-interaction model approach.
Results/Conclusions
Our study provides insight into oak savannahs functioning, showing that the tree – open grassland structure modified the herbaceous layer structure, composition and diversity, both in terms of plant functional types (PFT) and species. Moreover, the tree – open grassland structure especially drove CO2 and N2O fluxes; emissions under the canopy of P. pinea being higher than under Quercus species. Legumes were key stone drivers of CO2 and N2O exchange, enhancing CO2 uptake and N2O emissions.
Interestingly, the integrated analysis of the canopy and the herbaceous layer improved the understanding of mechanisms affecting GHG exchange, and provided some keys to determine the optimum oak savannah structure to improve ecosystem services, and guarantee the preservation of these systems.