Multiple pathways for tree regeneration in anthropogenic savannas: incorporating biotic and abiotic drivers into management schemes

Abstract

Summary1. Oak savannas are biodiversity-rich landscapes allowing sustainable livestock production throughout the world. The long-term persistence of these ecosystems critically depends on the regeneration of the tree layer. Nevertheless, studies addressing the mechanisms involved for conservation planning are of limited value because they tend to focus on single explanatory factors. 2. We evaluated the combined effect of biotic and abiotic factors on recruitment of holm oak Quercus ilex in the Mediterranean savannas of western Spain. Transition probabilities from flower to seed to the established seedling were estimated in grazed, shrub-encroached and cropped plus fenced habitats in two consecutive years. 3. Trees in cropped habitats produced more female flowers and larger acorn crops in both years. The physiological condition of trees was better in cropped habitats and worst in shrub-encroached plots. Overall, resource-mediated effects overrode the effects of biotic damage on tree fecundity in all habitats. 4. Acorn survival and seedling establishment were higher in cropped and shrub-encroached plots, though in cropped plots saplings are predictably destroyed by subsequent grazing and ⁄ or by mechanical treatment used to restart the cropping cycle. 5. Complete regeneration failure was found in 6 out of 24 possible management scenarios, mostly in the presence of large vertebrate herbivores. However, even low positive cumulative transition probabilities between life stages exceeded a safe threshold for early regeneration. 6. Synthesis and applications. Natural early recruitment of oak savannas can be achieved through various management regimes. These include cereal cropping in fenced plots (provided established saplings were not subsequently destroyed) or shrub encroachment in undergrazed or livestockexcluded plots. Among these, natural recruitment after encroachment is a cost-effective tool as compared to artificial plantation.

Spatiotemporal variation in acorn production and damage in a Spanish holm oak (Quercus ilex) dehesa

“…Concerning acorn losses, it must be noted that studies based on acorns collected on the ground tend to overestimate acorn damage, as sound acorns are selected by post-dispersal predators . In our study, based on acorns collected from the canopy, the mean attack rate by both insects was 9.40%, that is well below figures reported in comparable studies in holm-oak in dehesas: 18% (Pulido and Díaz, 2005), 58% , and 20% in Pulido et al (2010). On the other hand, Brenneria infested 10.29% of acorns, a figure lower than the 16.30% found by Pulido and Díaz (2005) and 25.60% reported by Vázquez et al (1999).…”

Persistent Acacia savannas replace Mediterranean sclerophyllous forests in South America

“…A long history of human disturbances has strongly influenced ecosystem functioning and species composition in the five Mediterranean regions around the world (Aschmann, 1973;Conacher and Sala, 1998), i.e. California (Stylinski and Allen, 1999;Franklin, 2010), the European Mediterranean Basin (Barbero et al, 1990;Acácio et al, 2009Acácio et al, , 2010Pulido et al, 2010), central Chile Holmgren, 2002;Figueroa et al, 2004;Armesto et al, 2010), southwest South Africa (Brown and Van Staden, 1997;Alston and Richardson, 2006), and southwest Australia (Hobbs and Huenneke, 1992;Dixon et al, 1995).…”

Shrub encroachment of Iberian dehesas: implications on total forage productivity

“…Other authors, however, have shown the positive effects of encroachment on the functions of silvopastoral systems, such as natural regeneration of trees (Ramírez and Díaz 2008), diversification of animal diet (Patón 2003), erosion control (Lavado et al 2004) and biodiversity conservation (Díaz et al 2003). As a consequence, temporary abandonment and subsequent shrub encroachment have been proposed as a more costeffective tool than artificial plantation to enhance natural recruitment of trees (Pulido et al 2010). This type of dehesa management involves creating a mosaic-type farm and landscape according to a long-term rotation plan, habitual in the past, where forest and encroached plots are mixed with more intensively grazed plots.…”