Flooding effects on grassland species composition in the Azul creek basin, Argentina

Abstract

From a hydrological point of view, the characteristic of the water behaviour in catchments so depressed as the Azul creek basin (centre of Buenos Aires province, Argentina) is water accumulation above the land surface. Thus, water on the ground does not have a single runoff direction, but moves in a disorderly, indefinite and unpredictable way. Considering that periodic floods are a typical disturbance of the region, the objective of this study is to analyse, under field conditions, the transformative effect of prolonged flooding on floristic composition, taking into account the different vegetation patches and their relative position over the relief, the chemical characteristics and the groundwater fluctuation, and some edaphic properties in each site. Vegetation samplings were performed during three consecutive springs, when the grassland was on different hydrological conditions due to very different rainfall precedent histories. A digital terrain model of the study area was built and a flow accumulation map was created from it. Pits were dug to describe edaphic variables and shallow wells were drilled for monitoring the groundwater characteristics. Flooding, in relation with surface and groundwater dynamics and soil characteristics, is the factor that determines and promotes the differentiation among sites that are relatively close, contiguous and even topographically in almost identical positions. So, some patches of vegetation get their differentiation through the limiting conditions of their soils, while others receive greater influence from the hydrodynamics to which they are subject. Thus, in this study it becomes evident how certain stands are floristically homogenised or differentiated over time according to their flooding conditions and, hence, according to the area from which they receive surface and groundwater flow. Also, results corroborate the way the water regime determines the structure and heterogeneity of plant communities in such environments.