CONICET | Buscador de Institutos y Recursos Humanos

Alluvial fans are depositional landforms that occur where confined stream channels emerge from mountain catchments into zones of reduced stream power. The abrupt reduction of stream power results in the deposition especially of the coarse fraction of the sediment load (Bull 1977; Harvey 2011). The study sites are located within the arid region of Monte desert phytogeographic province, which extends throughout western Argentina (Morello 1958, Cabrera 1994). More specifically, between the parallels 31° 40' and 31° 30' S and the meridians 68° 50' and 68° 41' W. The Central district is an area characterized by a great diversity of landforms created by fluvial, alluvial, and neotectonic processes to which the vegetation patterns are related. The present work has the following objectives: 1: determine whether geomorphological and superficial characteristics of the landforms influence the vegetation patterns, and 2: if the vegetation properties are linked to the soil texture across tree differents alluvial fans in an arid landscape.Google Earth software was used for the identification and delimitation of the different study sites. In addition, a digital elevation model (DEM) was obtained as raster data whit a spatial resolution of 5 m from the MDE-Ar program of the Instituto Geográfico Nacional of Argentina. Subsequently, this DEM was pre-processed using the SAGA GIS v.2.1.2 software using the method proposed by Planchon and Darboux (2002). Then, the digital analysis of the terrain was carried out to obtain the following terrain parameters and morphometric indices: topographic wetness index (TWI), topographic roughness index (TRI), slope, hillslope aspect (HA), wind effect (WE), and vertical dissection index (DI). The proportion of rock fragments and fine sediment coverage comprising the surface cover of alluvial fans was assessed using the Point Quadrat modified method. Observations and descriptions of the soil profiles were restricted to the uppermost 50 cm following the method used by Schoeneberger et al. (2012). For the textural analysis, the samples obtained in the field were air-dried and subsequently weighed. They were then sieved using mechanical separation with a 200-mesh sieve. With the material smaller than 2 mm (40 g), the Bouyoucos assay (1962) was carried out under the determinations of textural fractions for soils of semi-arid regions provided by Barbeito and Bono (2006). The relationship between geomorphometric, soil, and vegetation variables was evaluated through multivariate statistical analysis.The main results show the presence of 45 plant species distributed in 19 families, where shrubs are dominant. Larrea cuneifolia is the dominant species in both bajadas and is distributed throughout the entire extension of these units, regardless of the topographic position in which it is found. In the bajada of Cerro Zonda in particular, a clear decrease in the vegetation coverage, total richness, and diversity is observed. This may be due to the high erosion rates directly correlated with topographic parameters that vary according to tectonic activity. The Río Ancho alluvial fan is dominated by fluvial processes with intensity and erosive capacity directly related to the size of the basin, as a consequence, the dominant species is Larrea divaricata. The high proportion of rock fragment coverage and the low availability of fine sediments exert a negative control on the development of vegetation. In addition, the low coverage of fine sediments does not help water retention, decreasing richness through a possible decrease in the germination of the seed bank. At a subsurface level, the sandy textures found in the first centimeters of the soil do not facilitate the establishment and development of species as do those clayey-loamy soils that benefit diversity. The sandy textures of the soils of the alluvial fans do not retain water but help its rapid infiltration instead. However, soils with a content of the clay fraction increase the retention and accumulation of water, thus favoring vegetation coverage and diversity (Fig 1).At a regional scale, the vegetation community of the studied alluvial fans presents the shrub stratum as dominant over herbs, cacti, and trees. Moreover, the variations in the coverage, richness, and diversity patterns show differences associated with variables such as vertical dissection, slope, terrain roughness, and topographic wetness. This is due to the control these variables exert on the frequency and intensity of hydro-geomorphological processes such as erosion and accumulation of sediments

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