Shrub encroachment of Iberian dehesas : implications on total forage productivity

Abstract

The Iberian dehesa, a grazed Mediterranean open woodland, is an outstanding example of a silvopastoral system. Shrub encroachment has been proposed as an effective method to facilitate natural tree regeneration and ensure the sustainability of silvopastoral systems. At the same time, shrubs can affect both pasture understory and tree overstory production. The effects of varying intensities of shrub encroachment on the productivity of four forage components of this system—pasture, acorns, shrub browse and tree browse—were tested and the overall metabolisable energy (ME) under different scenarios of vegetation structure was calculated. Two common shrub species with contrasting plant architectures and ecological functions were studied as models: Cistus ladanifer and Retama sphaerocarpa. Cistus negatively affected acorn, tree browse and pasture production, whereas Retama positively affected pasture and browse production and was neutral for acorn production. The highest ME values were found when tree cover was ca. 70 % and C. ladanifer cover was low or R. sphaerocarpa cover was medium. The authors recommend conserving and promoting patched three-layered plots in Iberian dehesas and similar extensive silvopastoral systems where certain shrub species could simultaneously encourage the sustainability of these systems and reinforce (with R. sphaerocarpa) or maintain (with C. ladanifer) their productivity.