Shrubs affect soil nutrients availability with contrasting consequences for pasture understory and tree overstory production and nutrient status in Mediterranean grazed open woodlands

Increases of woody plant cover in former savannas and managed savanna-like systems are currently considered one of the major components of global environmental change. Besides, shrub encroachment is frequently recommended as a practical tool to ensure tree regeneration. However, there is little evidence that this process affects neighboring plants-tree and pasture-production and nutrient status in Mediterranean open woodlands through the modification of soil properties. The effect of two contrasting, in terms relevant to nutrient cycling, shrub species, Cistus ladanifer and Retama sphaerocarpa, were investigated. Cistus, a shallow rooted shrub, reduced top-soil water content and enhanced P and K but depleted N and Mg availability under its canopy. It worsened consistently both tree overstory and pasture understory production and nutrient content. By contrast, Retama, a deep-rooted, N-2-fixing shrub, increased top-soil water content and N and Mg availability, but diminished P availability. In addition, it affected distinctively to pasture (facilitation) and trees (competition) biomass production. Overall, pasture production, irrespective of the habitat, was mainly limited by P, whereas trees growing with Cistus were limited by Ca and N and those trees growing with Retama by P availability. Our results showed that shrubs are capable to affect consistently both pasture and tree production and nutrient status through the modification of soil nutrient availability. However, these effects depended on specific shrub-understory and shrub-overstory combinations, which have implications for managing grazed open woodlands.