Multiple pathways for tree regeneration in anthropogenic savannas: incorporating biotic and abiotic drivers into management schemes.
Abstract
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. 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. 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. 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. 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. 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 livestock-excluded plots. Among these, natural recruitment after encroachment is a cost-effective tool as compared to artificial plantation.