Estudio de la comunidad vegetal tras la restauración ecológica de una marisma transformada en el Parque Nacional de Doñana

Laburpena

The need to repair habitats of high ecological value degraded by human activity has led to recent conceptual and practical developments in Ecology that constitute Restoration Ecology, From this perspective, actions are currently being undertaken to improve the ecological functioning of the Doñana National Park, including some that seek to reverse processes which served, several decades ago, to drain and transform large areas of marsh into cropland, when marshes were considered unproductive and unhealthy places. Thus the 'Doñana 2005 Project' of the Ministry of Environment has actions intended to respond to the need for regeneration and ecological restoration of the marshes and wetlands of Doñana. Part of the Action n° 6 comprises the restoration of the Travieso Stream and the floodplains on either side of its riverbed, across the Caracoles Estate, isolated and transformed for dry-land agriculture since the 1970s. This thesis is framed within one of the research projects that are being developed: 'Restoration ecology of the fauna and flora in the Caracoles Estate (Action n°6 from the Doñana 2005 Project, Ministry of Environment). Dynamics of the aquatic and terrestrial vegetation. Colonization processes and factors which regulate them. Seed bank dynamics'. The study covers the period since 2004, when the first phase of the restoration was conducted, until 2010. The second phase is taking place now (2014) after being delayed repeatedly. In the first chapter and as introduction, the historical background that justified the project under the Doñana 2005 Project is detailed. Subsequently, the study site, the restoration works and the transformations performed on the estate for its agricultural conversion are described. In this context, the results of literature research concerning the agricultural activity are explained in detail: species cultivated, rotation periods and time since abandonment. Furthermore, the methodological design that underlies the following chapters is explained. Clear objectives and references are essential to evaluate restoration success and to improve the future management of the restored ecosystems. That is why in Chapter 2 the main environmental factors and their effects on the different plant communities that comprise the reference sites used in this study are characterized. Topics of importance in Restoration Ecology are discussed, including the issue of defining reference conditions in spatially and temporally heterogeneous landscapes, such as the study site. Chapters 3 and 4 address the changes undergone by the plant community through spontaneous succession after land abandonment and restoration actions. In the former, the focus is on the part of Caracoles that was not re-excavated, where changes are related to environmental factors (physicochemical, spatiotemporal and agricultural). Two different approaches are considered: one for the dominant annual communities, using a grid of permanent sampling points, and another for the native shrub populations scattered throughout the study site, with the use of 30 x 30 m permanent quadrats. In both cases the dynamics through time are studied. Similarly, annual communities are addressed in Chapter 4, but focusing on the re-excavated channel of the Travieso. Furthermore, here the perennial community (helophytes) that is colonizing the channel was monitored using vertical aerial photographs. This chapter deals especially with the effects of restoration on the hydrological recovery, as well as its consequences for seed dispersal processes and the relevance of stochastic factors in the re-colonization. Given the widespread impoverishment of the native seed bank in abandoned agricultural land, in the last chapter seed banks are assessed in order to determine their conservation status and role as a possible limiting factor for restoration. To do this, seed bank richness, density and species composition were analyzed at various depths and under different moisture regimes. At the end of the thesis, a general discussion and the general conclusions extracted from all the chapters are indicated.