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Effects of grazing intensification and abandonment on the composition, diversity and functioning of Mediterranean grasslands

  • Autores: Carlos Pérez Carmona
  • Directores de la Tesis: Begoña Peco (dir. tes.), Francisco Martín Azcárate (dir. tes.)
  • Lectura: En la Universidad Autónoma de Madrid ( España ) en 2012
  • Idioma: español
  • Tribunal Calificador de la Tesis: Concepción López Alados (presid.), Javier Seoane Pinilla (secret.), Francesco Bello De (voc.), Gerardo Moreno Marcos (voc.), Arantza Aldezabal Roteta (voc.)
  • Materias:
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  • Resumen
    • Because of the link between grazing and Mediterranean ecosystems, the consequences of the process of grazing intensi¿cation and the associated grazing abandonment have been identi¿ed as an issue that deserves further research (Peco et al. 2005,2006; de Bello et al. 2005,2006; Golodets et al. 2009b; Tárrega et al. 2009). The motivation of this thesis is to describe the effects of grazing intensi¿cation/abandonment on the composition, functioning and persistence of these grasslands, speci¿cally focusing on Mediterranean dehesas as one of the most representative examples of these ecosystems.

      Although there are great differences in species composition between grazed and ungrazed areas, the mechanisms determining these differences are still far from being unveiled. A recent study has shown that the different germination responses of increaser and decreaser species under changes in light conditions caused by grazing could be one of the determinants of these compositional differences (Dobarro et al. 2010).

      CHAPTER 2 explores the effects that cattle dung leachates have on the germination of species with different responses to grazing. In order to do that, seeds and seedlings of seven confamilial pairs of species with different responses to grazing were exposed to different concentrations of cattle dung leachates. The initial hypothesis was that increaser species can germinate and/or establish better than decreasers in dung-generated environments.

      CHAPTERS 3, 4 and 5 focus on the in¿uence that grazing intensi¿- cation and abandonment have on the diversity of Mediterranean grasslands. Speci¿cally, CHAPTER 3 deals with the changes in species richness and functional trait diversity produced after grazing abandonment.

      The paper presented in that chapter also analyses the impact of change of use on other vegetation descriptors (¿oristic composition and weighted trait average for individual traits) and the environment (soil fertility, bare soil cover and litter cover), integrating this information to provide an overall assessment of the mechanisms by which land use changes in¿uence the system, and the consequences of these changes for its functions.

      The effects of resource limitation and disturbance on plant ecological strategies and community assembly processes are increasingly well known. However, there is a lack of work examining how temporal variation in resource availability interacts with spatial abiotic stress gradients and disturbance to in¿uence assembly processes in plant communities.

      The study presented in CHAPTER 4 uses functional diversity for key plant ecological strategy traits to explore how temporal and spatial variation in water availability interact with grazing disturbance to in¿uence assembly processes in Mediterranean grassland communities.

      Rainfall is predicted to decline in the Mediterranean Basin, as well as in many of the areas of the world where grazing is signi¿cant (Christensen et al. 2007). Therefore, it is necessary to achieve a deeper understanding of the response of plant communities to the interaction between grazing and climate. In CHAPTER 5, the effects of interannual and topographical variations in water availability on the taxonomic and functional diversity responses to grazing are examined at different spatial scales. A strong emphasis is put on how the relationship between functional and taxonomical diversities is affected by grazing and on whether water availability modulates this effect.

      CHAPTERS 6 and 7 deal with the consequences of grazing intensi¿cation at the landscape scale. CHAPTER 6 presents a methodology to monitor grazing effects on herbaceous vegetation in highly heteroge neous Mediterranean grasslands. The proposed technique analyses the difference in photosynthetically active vegetation cover between spring and summer using spectral mixture analysis of very high spatial resolution images. Boosted Regression Trees are used to disentangle the relative in¿uence that physiographic and grazing management related variables have on this difference. Eventually, an example illustrating the applicability of the methodology in a Mediterranean dehesa is provided.

      CHAPTER 7 investigates the lack of regeneration of the tree layer of dehesas in relation to the abandonment of transhumant practices. In order to cope with the problem of the lack of regeneration of the trees in dehesas, several authors have proposed different management alternatives, such as the temporal or permanent abandonment of grazing and the reduction in the stocking rates. However, and despite temporal cessation of grazing has been recently suggested as an option to reverse the global decline in scattered trees (Dorrough 2005; Fischer et al. 2010), the effect of the recovery of non-continuous grazing practices such as transhumance has not been tested to date. In the presented manuscript, the effects that seasonal grazing associated with transhumant pastoralism have on the regeneration and vegetative condition of holm oaks are analysed by comparing different dehesas with contrasting grazing regimes (transhumant seasonal grazing vs. permanent grazing).

      Finally, in CHAPTER 8, the ¿ndings of the preceding chapters are summarised and some lines for future research are proposed.


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