| dc.description.abstracteng | The riparian zones are essential to environmental quality, providing human society with several ecosystems services. Nonetheless, in tropical ecosystems, an intensive overexploitation of resources can hinder their resilience thresholds, compromising key ecological mechanisms that are arguably not yet fully understood. In the Brazilian Cerrado savannah, which is connected to many river basins with abundant groundwater, knowledge gaps on the relationships between hydrological parameters and plant diversity still exist. In intensively cultivated agricultural areas with remaining riparian vegetation (RV), the ways in which vegetation distribution and plant diversity are related to groundwater dynamics (seasonal groundwater level change) as well as the nutrient content of different water fluxes and how RV maintain water quality are topics that remain unclear. Previous studies have been conducted on the diversity and distribution of Cerrado gallery forest plants, but specific research related to vegetation and groundwater nutrients and dynamics is scarce. To bridge this gap, this thesis, under the umbrella of the CarBioCial project (Gerold, 2017), investigated the ecohydrological relationships between vegetation, groundwater dynamics and nutrients in a riparian zone of the Cerrado in the Campo Verde municipality. Groundwater depth, as well as variations in nutrient content were measured over a two-year period (2014-2016), comprised of both wet and dry seasons, together with a phytosociological and floristic survey. Relationships between the measured variables of interest were then statistically evaluated by clustering and fitting environmental variables with a detrended correspondence analysis.
Diversity, floristic and structural patterns of riparian zones in the Cerrado at four different transects were analysed first (Chapter 4.1 and 5.1). Three different types of vegetation inside the riparian areas were compared, namely the campo de murundus, cerrado sensu strictu, and gallery forest, giving a total of 38 plots of 20 x 30 meters. The data analysis was based on α-diversity indexes and structural patterns obtained from phytosociological measures (abundance, frequency, and dominance). In the campo de murundus, a total of 706 individuals were registered, distributed among 32 families, 46 genera and 64 species. At the Cerrado sensu strictu, 1937 individuals were sampled, distributed among 35 families, 62 genera and 99 species. In the gallery forest, a total of 370 individuals were sampled and distributed among 34 families, 62 genera and 82 species. The α-diversity values in campo de murundus and in the cerrado sensu strictu were within the expected range, despite that the patches that were surrounded by cropland and/or pasture. In gallery forests, the diversity of plants suffers from human activities such as livestock grazing and selective logging. Our results show that riparian zones do not maintain their plant diversity in Cerrado regions, due to such human activities. Within our study area, the riparian zone was colonized by plant species usually found in Cerrado, Atlantic rainforest, Amazon rainforest, and Caatinga ecosystems. This plant-based colonization developed a heterogenic environment adapted to the conditions of the landscape. The waterlogged conditions within the campo de murundus vegetation type, located between earth mounds, formed a habitat for plant species that can survive in saturated and flooded conditions. The earth mounds (islands) themselves provide a drier environment for the typical Cerrado species to flourish.
Secondly, we assessed whether or not the water quality is maintained in the riparian zone (Chapter 4.2 and Chapter 5.2). We compared water nutrients from base flow and stormflow levels of the river discharge, overflow and groundwater from the inner (gallery forest and campo de murundus) and surrounding sites (cropland area = PLU) of the riparian zone. The water quality was analysed separately according to hydrological pathways and grouped by events and compared pair-wise using the non-parametric Mann-Whitney test. We found that the riparian zone contributes to the maintenance of water quality, as the concentration of several nutrients (e.g., DIC, SO42-, S, K, P, Mg) was reduced along the hydrological pathways from cropland to the river, passing through the RZ. The vegetation in the riparian zone of Transect 1 was formed by several pioneer plants that regenerated degraded areas in the Cerrado region.
Thirdly, we analysed possible relationships between plant diversity, groundwater table depth and change, and nutrient concentrations (Chapter 4.3 and 5.3). For this, 18 wells were placed in 18 plots within Transects 1 and 2, which consisted of vegetation from the gallery forest and campo de murundus vegetation zones. The main findings highlighted relationships between the groundwater table depth, TIC and the campo de murundus vegetation zone, as well as copper and zinc with the gallery forest vegetation zone. The results for the gallery forest indicated no groundwater level variables, which was correlated with this vegetation type. This underlines a strong difference between the two vegetation types. Groundwater variation is shown to be a dominant factor in the campo de murundus zone and its plant diversity. In addition, the presence of one plant species in particular, Tachigali vulgaris, indicates a specific hydrological regime, i.e. that the groundwater level reaches the soil surface layer, maintaining waterlogged conditions for several months. In addition, the Miconia albicans species was strongly correlated with total inorganic carbon. Also, Tibouchina stenocarpa, Maprounea guianensis, Xylopia aromatica, and Vismia guianensis were strongly correlated with campo de murundus. Two other plant species (Cordia bicolor and Sacoglottis guianensis) were correlated with the gallery forest vegetation type. We concluded that studies which can establish functional approaches between these variables are necessary to better understand the extent to which the groundwater can shape the plant diversity in the riparian zones of the Cerrado. | de |