Tree scattering and long-term persistence of dehesas: patterns and processes

Abstract

Díaz, M. 2014. Tree scattering and long-term persistence of dehesas: patterns and processes. Ecosistemas 23(2): 5-12. Doi.: 10.7818/ECOS.2014.23-2.02

High biodiversity in Spanish dehesas is due to intimate mixtures of habitat types and habitat elements at several spatial and temporal scales. Dehesa management in Spain maintains a mosaic of vegetation that includes scattered trees on shrubby and/or grasslands matrices. Intimate mixtures due to tree scattering are key to maintain both agricultural productivity and high local diversity, but compromise tree regeneration due to conflicting effects of tree scattering on several stages of the regeneration cycle. I review how tree scattering influences both local diversity and the outcomes of regeneration stages, from flower production to early establishment, aiming at understanding how and why dehesa management creates a productive and species-rich system that is however not sustainable in the long term. Preliminary results form large-scale landscape experiments and monitoring projects indicate that tree scattering tends to favour transitions in the earlier stages of the regeneration cycle (pollination, abortion, and pre- and postdispersal seed predation), apparently because of reduced plant-plant competition and decreased detection by insect, and even rodent and ungulate, seed predators. However, tree isolation collapses seed dispersal to safe sites, apparently due to negative effects of predation risk on both the quantitative and the qualitative components of seed dispersal effectiveness. Ensuring oak recruitment in dehesas should be based on the landscape-scale management of foraging costs for dispersers rather than of tree-level management of foraging benefits. Detailed knowledge needed to implement this general recommendation at local scales is being increasingly available in Spain.