The role of a native tussock grass (Paspalum quadrifarium Lam.) in structuring plant communities in the Flooding Pampa grasslands, Argentina
The patches of Paspalum quadrifarium-dominated grasslands found atpresent could be remnants of the vegetation that defined the Pampas landscape inthe past. The mere presence of such physical structures should lead to systemsin which many other species are dependent upon both the autogenic creation ofsurface area for living space and the autogenic and allogenic modulation ofresources controlled by these structures. We carried out amensurative–comparative study in naturally occurring sites dominated byP. quadrifarium that occupy different positions within thelandscape in the Flooding Pampa. We found different species assemblages in thesestructurally homogeneous stands, related to the edaphic and topographicgradients. The comparison with sites from similar positions in the landscapewhere this native tussock-grass was absent showed that the dominance ofP. quadrifarium is a minor determinant of floristicdifferences, which are mainly controlled by abiotic factors. We assigned thesedominated grassland stands to previously delimited vegetation units for theFlooding Pampa grasslands and we compared them with the vegetation unitsassigned, in terms of composite properties of the community that may beindicators of ecosystem processes, such as biological invasions andproductivity. Our results show that this native tussock grass is associated withhigher floristic richness, higher representation of perennials and grasses andlower presence of alien plants. This homogeneous pattern of variation across thevegetation units would suggest that, despite the absence of dramatic changes incommunity composition, the dominance of this tussock grass plays an importantrole in structuring species diversity patterns.