Facilitation of holm oak recruitment through two contrasted shrubs species in Mediterranean grazed woodlands

Abstract

Question Regeneration failure and gradual tree dieback are major threats for the persistence of savanna‐like grazed oak woodlands. Current research has argued that the scarcity of ‘safe sites’, in particular shrubs, is the main cause of the lack of effective tree recruitment. But can different shrub species be considered as safe sites generally? Do two distinct shrub species, with contrasted life strategies, affect several life stages of tree regeneration in similar ways or do they specifically influence the recruitment process? Location Holm oak woodlands of SW Iberian Peninsula (40°02′ N, 06°06′ W). Methods We surveyed densities of recently emerged and surviving seedlings as well as small and large saplings over two consecutive years in 40 sites that were independently managed, comparing plots encroached by either Cistus ladanifer (a shallow‐rooted shrub, forming dense populations, with reported allelopathic compounds) or Retama sphaerocarpa (a N2‐fixing, deep‐rooted shrub that forms scattered populations) vs their respective control plots (without shrubs). To assess the effect of mature trees and both shrub species on the performance and survival of recently emerged oak seedlings, we established an acorn sowing experiment in the same surveyed microhabitats (open spaces, shrub, tree and tree–shrub). Results The survey showed that both shrubs species had a positive effect at early recruitment stages. At later life stages, this effect weakened under Cistus whereas it strengthened under Retama. The acorn sowing experiment showed that both shrub species buffered abiotic conditions and enhanced seedling functioning similarly, but Retama enhanced seedling survival to a higher extent than Cistus. Conclusions The two shrub species impose a specific template that is able to affect the long‐term dynamics of Mediterranean oak woodlands. Cistus shrubs are effective in protecting seedlings physically against herbivores and facilitate early survival, but may compete with older stages of oak regeneration. In contrast, Retama shrubs exert stronger biological facilitation and guarantee long‐term persistence of surviving seedlings. We argue that improved understanding of the effectiveness of different nurse plants and their contrasting factors is of major interest for the conservation and restoration of degraded oak woodlands.

Exploring the causes of high biodiversity of Iberian dehesas: the importance of wood pastures and marginal habitats

“…Earthworm diversity did not differ among GHC at the plot scale, but were more abundant in open pastures and near water bodies than in woody habitats, a fact probably explained by the lower moisture content usually found underneath trees and shrubs in Iberian dehesas (Cubera and Moreno 2007a, b;Rolo et al 2013). By contrast, at the landscape scale, earthworm diversity was higher in woody habitats and agricultural crops, reflecting the higher spatial heterogeneity of earthworm community in woody habitats compared to open pastures and wet areas, as reported for cold-humid mixed deciduous forest in Canada (Whalen 2004) and for sub-humid tropical gallery forests in Colombia (Jiménez et al 2011).…”

Facilitating the afforestation of Mediterranean polluted soils by nurse shrubs

“…As a secondary objective, we aimed to test whether the intensity of the facilitation is higher when a pioneer shrub species (Retama sphaerocarpa), rather than a late-successional shrub (Phillyrea angustifolia), is used as a nurse in these harsh environments. Understanding the effectiveness of different nurse plants constitutes an issue of major interest for the conservation and restoration of degraded ecosystems (G omezAparicio, 2009;Rolo et al, 2013).…”

Addressing post-transplant summer water stress in Pinus pinea and Quercus ilex seedlings

“…In addition, the beneficial effect of the below-crown environment is part of the regeneration ecology of stone pine (Manso et al 2012, Gordo et al 2012. Likewise, shrubs, particularly rockrose and broom, play an important role in facilitating holm oak seedling survival, not only by protecting the seedlings from herbivores but also as buffers to stressful factors (Rolo et al 2013), thereby improving the abiotic conditions. However, this may be insufficient to prevent desiccation in the summer (Badano et al 2009).…”