Chaco Plains Research Articles

Effects of habitat perturbation on lizard assemblages in the center-west of the Arid Chaco region, Argentina.

Several human disturbances contribute to the decrease of vertebrate species' richness and abundance, altering the processes of an ecosystem. We evaluate richness, diversity and relative abundance of species for lizard assemblages at sites with different degrees of perturbation in the center-west of the Arid Chaco region in Argentina. Between 2015 and 2018, six lizard assemblages were sampled monthly -using pitfall traps- in three areas of the Chaco, with a perturbed and an unperturbed (control) replica at each of the areas: (1) Chaco Mountain plain, (2) Chaco Mountain slope, and (3) Chaco Plains, and habitat characteristics of each study site were recorded. We captured 1446 lizards, belonging to 12 species. The perturbed area at the Chaco Mountain plain showed the greatest richness, diversity and abundance of species. In the perturbed Chaco Plains, species abundance decreased by about 50% with respect to the control site. Liolaemus chacoensis was the dominant species at all sites. Some species could be negatively affected by a total loss of arboreal strata, tree trunks and fallen leaves. Structural parameters of lizard assemblages were related to the habitat characteristics; therefore, these results provide information for the conservation and management of lands and lizard assemblages in the Arid Chaco.

Hydrological and productive impacts of recent land‐use and land‐cover changes in the semiarid Chaco: Understanding novel water excess in water scarce farmlands

AbstractOver the last decades, the rapid replacement of native forests by crops and pastures in the Argentinean semiarid Chaco plains has triggered unprecedented groundwater level raises resulting from deep drainage increases, leading to the first massive waterlogging event on records (~25,000 Ha flooded in 2015 near Bandera, one of the most cultivated clusters of the Chaco). In this paper, we link this episode to the ongoing deforestation and cropping scheme shifts through the combined analysis of remote sensing data, agricultural surveys, local farmer information and hydrologic modelling. From 2000 to 2015, the agricultural area of Bandera increased from 21% to 50%, mostly at the expense of dry forests. In this period, agriculture migrated from more intensive (i.e., double‐cropping) to more water‐conservative (i.e., late‐summer single crops) schemes as a general strategy to reduce drought risks. These changes reduced regional evapotranspiration and increased the intensity of deep drainage in wet years. Contrasting cropping schemes displayed significant evapotranspiration differences, but all of them experienced substantial drainage losses (~100–200 mm) during the wettest year (2014/2015), suggesting that cropping adjustments have a limited capacity to halt the generation of water excesses. Nearly 50% of the cropped area in Bandera could not be sown or harvested following the groundwater recharge event of 2014/2015. In the ongoing context of shallow and rising water tables, the introduction of novel cropping schemes that include deep‐rooted perennials, to promote transpirative groundwater discharge, seems crucial to avoid the recurrence of water excesses and their associated dryland salinity risk in the region.

Hydrology-driven seasonal changes in the phytoplankton of a subtropical river (Riacho Formosa, Argentina).

The Riacho Formosa is one of many autochtonous watercourses running along the subtropical region of the Wet Chaco Plains and draining into Paraguay River. Their typical hydrological cycle is characterized by a late winter low phase and a high phase throughout the warm season. As part of a baseline characterization, the composition and structure of the phytoplankton were studied in relation to river depth, water temperature, pH, conductivity and transparency through 4 seasonal samplings between June 2015 and March 2016. A rich phytoplankton community (338 taxa) was revealed in this study. A few species of Cryptophyceae and Euglenophyceae dominated the community, especially during low waters. A cluster analysis showed that community compositions were more dissimilar over time than along the watercourse, and were spatially more homogeneous during high waters. A canonical correspondence analysis showed that environmental features significantly explained 42.6% of the total variance of species data (p= 0.004). We conclude that phytoplankton responds to hydrological changes through a high species turnover, with dominance peaks of euryhaline, shadow adapted and organic matter exploiting taxa during low waters.

The ecohydrological imprint of deforestation in the semiarid Chaco: insights from the last forest remnants of a highly cultivated landscape

AbstractThe semiarid Chaco plains present one of the highest rates of forest clearing and agricultural expansion of the world. In other semiarid plains, such massive vegetation replacements initiated a groundwater recharge and salt mobilization process that, after decades, raised regional water tables and salts to the surface, degrading agricultural and natural ecosystems. Indirect evidence suggests that this process (known as dryland salinity) began in the Chaco plains. Multiple approaches (deep soil profiles, geoelectric surveys and monitoring of groundwater salinity, level and isotopic composition) were combined to assess the dryland salinity status in one of the oldest and most active agricultural hotspots of the region, where isolated forest remnants occupy an extremely flat cultivated matrix. Full vadose moisture and chloride profiles from paired agriculture‐forest stands (17 profiles, six sites) revealed the following: a generalized onset of deep drainage with cultivation (32 to >87 mm year−1), full leaching of native chloride pools (13.7 ± 2.5 kg m−2) down to the water table after >40 years following clearing and differential groundwater table rises (0.7 to 2 m shallower water tables under agriculture than under neighbouring forests). Continuous level monitoring showed abrupt water table rises under annual crops (up to 2.6 m in 15 days) not seen under forests or pastures. Varying deep drainage rates and groundwater isotopic composition under agricultural plots suggest that these pulses are strongly modulated by crop choices and sequences. In contrast to other dryland salinity‐affected areas of the world, forest remnants in the study area (10–20% of the area) are not only surviving the observed hydrological shifts but also sustaining active salty groundwater transpirative discharge, as evidenced by continuous water table records. The overall impact of these forest remnants on lowering neighbouring water tables would be limited by the low hydraulic conductivity of the sediments. As highly cultivated areas of the Chaco evolve to new hydrological conditions of shallower saline water tables, innovative crop rotations that minimize recharge, enhance transpirative discharge and tolerate salinity will be needed. Copyright © 2016 John Wiley & Sons, Ltd.

Fish fauna from Río Pilcomayo National Park and Ramsar Site and its surroundings, Formosa, Argentina

The first list of fish species from the Río Pilcomayo National Park and Ramsar Site (RPNP), located in the Wet Chaco plains of northern Argentina is presented here. Samplings were carried out bimonthly from December 2006 to January 2008 in sites representative of the different landscapes, in both dry and wet seasons. A total of 77 species were collected, belonging to 23 families and eight orders and covering 54% of the province ichthyofauna. New records to the Province of Formosa are Brachyhypopomus gauderio Giora & Malabarba, 2009; Cyanocharax sp. and Hypophthalmus edentatus Spix & Agassiz, 1829.

Balancing agricultural and hydrologic risk in farming systems of the Chaco plains

Like in other semiarid areas of the world, farming systems in semiarid Chaco tend to use water-conservative crop systems to minimize production risks associated to water stress. While this strategy aims to stabilize crop yields and farmers income, the underutilization of water resources in wet years may result in heavy deep drainage water losses which could potentially lead to the development of dryland salinity. Conversely, more intensive crop systems that consume water exhaustively present lower drainage rates but are more prone to crop failure. We employed a monthly soil water balance approach to analyze the productive and ecohydrologic effects of five different farming systems across the region (winter, spring, summer, late-summer and a winter–summer double crop system) and to assess the possibility of minimizing emerging trade-offs between them through flexible water-informed cropping sequences. Our results indicate that water stress diminishes as crop systems are delayed towards the rainy season (winter > spring > summer > late-summer), but the productively safer late-summer strategy is the one with highest drainage rates. In most of the region, the relatively high production risk and insignificant drainage probability generally determine the convenience of conservative late-summer systems. However, in areas (or years) with higher amount and/or seasonality of rainfall, more intensive double-crop systems are necessary to minimize the likely high drainage fluxes. As rainfall is highly variable from one year to the other, the knowledge of soil water content at the onset of the season is useful to predict part of the available water offer and to asses expected production and ecohydrologic risks. In the most drainage-prone areas the implementation of flexible sequences that alternate conservative and intensive crop systems depending on soil water status, significantly reduced mean annual drainage with an acceptable increase in mean water stress index.

Onset of deep drainage and salt mobilization following forest clearing and cultivation in the Chaco plains (Argentina)

moisture content and 94% lower chloride stocks compared to dry forest (0.05 6 0.04 kg/m 2 versus 0.77 6 0.4 kg/m 2 ). Estimates of groundwater recharge based on the displacement of chloride peaks suggested values of 27–87 mm yr � 1 for agricultural soybean stands, and 10.4 mm yr � 1 for pastures. While hydrological shifts in the regional groundwater system are poorly monitored and understood, our findings show that it is potentially sensitive to land use changes and to salinization processes.

Collapse of the Pilcomayo River

The Pilcomayo River flows south-eastwards from the Bolivian Andes across the Chaco Plains, setting the border between Argentina and Paraguay. It flows down along 1000km, in principle, to finally join the Paraguay River. It spills over the plains during the rainy season from January to March. The sediment load of the Pilcomayo is one of the largest in the world: 140milliontons per year, which is mostly wash load from the upland Andes. The mean concentration of suspended sediment is 15g/l. The maximum recorded concentration is as high as 60g/l. The river has built a large fan covering a surface of 210,000km2, with many abandoned channels. Today, it is a river prone to avulsion, raising border disputes between the two lowland countries, Argentina and Paraguay. Moreover, the very special feature of Pilcomayo River is that it does not actually flow into the Paraguay River. Very far upstream of the mouth in the Paraguay the channel blocks itself with sediment and wood debris forcing water and sediment to spread across the plains. Moreover, the point of blockage has moved hundreds of kilometers upstream throughout the 20th century. Many environmental issues arise because of this collapse (channel discontinuity), not the least of them is the migration of fish. The future of the river concerns Bolivia and the two lowland countries.

Stable isotope composition of middle Miocene carbonates of the Frontal Cordillera and Sierras Pampeanas: Did the Paranaense seaway flood western and central Argentina?

The geographic extent, interconnectedness and chronology of the Miocene “Paranaense” epeiric seaway have been the subject of considerable debate. Understanding the timing and location of this marine incursion is significant for documenting sea level changes, paleoclimatic changes, and surface uplift or subsidence. Stable isotope analyses of carbonate strata in the flat-slab segment of the Sierras Pampeanas and high Andean Cordillera, previously purported to be of marine origin, provide evidence that the Paraná seaway did not inundate this portion of west and central Argentina. No unambiguously marine facies or isotopic signatures were recognized for five representative stratigraphic units: the Saguión Formation ESE of Salinas Grande in Córdoba Province; Anta Formation in the Quebrada de la Yesera and Lerma Valley of Salta Province; Del Buey and Del Abra Formations within the Famatina Ranges, La Rioja Province; and Chinches Formation in the Manantiales foreland basin in the Frontal Cordillera, San Juan Province. Paleontologic and lithologic features in support of a marine origin are reconsidered herein. Instead, a lacustrine origin is inferred for these formations, their contemporaneity perhaps related to concurrent global climate conditions and broad tectonic setting. Until more substantive evidence of marine deposition is found, we reject correlation of these units with the Paraná seaway, preserved in the Chaco-Pampean Plain subsurface, and discourage mapping of the seaway as extending into the central Sierras Pampeanas or Andean foreland. Our findings suggest the Miocene Andean foreland was elevated above sea level although it remained as a largely flat-lying area west of the Pampas and Chaco plains, which were inundated by the Paraná seaway.

Moho topography in the central Andes and its geodynamic implications

P-to-S converted waves at the continental Moho together with waves multiply reflected between the Earth’s surface and the Moho have been used to estimate the Moho depth and average crustal Vp/Vs variations in the central Andes. Our analysis confirms and significantly complements the Moho depth estimates previously obtained from wide-angle seismic studies and receiver functions. The resulting crustal thickness varies from about 35 km in the forearc region to more than 70 km beneath the plateau and thins (30 km) further to the east in the Chaco plains. Beneath the Andean plateau, the Moho is deeper in the north (Altiplano) and shallower in the south (Puna), where the plateau attains its maximum elevation. A non-linear relation exists between crustal thickness and elevation (and Bouguer gravity), suggesting that the crust shallower than 50–55 km is predominately felsic in contrast to a predominately mafic crust below. Such a relation also implies a 100 km thick thermal lithosphere beneath the Altiplano and with a lithospheric thinning of a few tens of kilometers beneath the Puna. Absence of expected increase in lithospheric thickness in regions of almost doubled crust strongly suggests partial removal of the mantle lithosphere beneath the entire plateau. In the Subandean ranges at 19–20°S, the relation between altitude and crustal thickness indicates a thick lithosphere (up to 130–150 km) and lithospheric flexure. Beneath a relative topographic low at the Salar de Atacama, a thick crust (67 km) suggests that the lithosphere in this region is abnormally cold and dynamically subsided, possibly due to coupling with the subducting plate. This may be related to the strongest (Ms=8.0) known intra-slab earthquake in the central Andes that happened very close to this region in 1950. The average crustal Vp/Vs ratio is about 1.77 for the Altiplano–Puna and it reaches the highest values (1.80–1.85) beneath the volcanic arc, indicating high ambient crustal temperatures and wide-spread intra-crustal melting.

The late miocene climatic change—persistence of a climatic signal through the orogenic stratigraphic record in northwestern Argentina

From the analysis of stratigraphic sections located in different areas of northwestern Argentina, an important climatic change was defined. This change, which took place during the late Miocene, is widely represented in the whole region (and in other latitudes). It is best displayed in the Calchaquı́ Valley, across the contact between the Angastaco and Palo Pintado Formations. The Angastaco Formation is composed of sandstones and conglomerates deposited in alluvial, fluvial, and eolian environments, under extremely arid climatic conditions, with practically no fossil content. Conversely, the Palo Pintado Formation was deposited in lacustrine and fluvial settings, under wetter climatic conditions, comparable with the present prevailing conditions in the Andean eastern foothills and in the ‘Chaco Salteño’ (foreland Chaco Plains). Matching this climatic amelioration, the Palo Pintado Formation supports an important fossil content (vertebrates, plants, and palinomorphs). The persistence of this climatic change (although not so clear) in other parts of NW Argentina remarks its importance and shows that the climatic signal in question was strong enough to avoid being masked by the facial variations linked to the important tectonic signature that characterizes the Cenozoic orogenic successions.

The annual cycle of the Rufous‐collared sparrow (Zonotrichia capensis) in three biotopes in north‐western Argentina

This investigation delineates the phenology of the annual cycle (gonadal activity, nesting, moult, behavioural events) in the Rufous‐collared sparrow and attempts to identify the environmental factors involved in the temporal coordination of the annual cycle with environmental seasonality. Populations of sparrows were studied in arid chaco plains, subtropical foothill transition zone, and mountain semi‐desert along a 100‐km east‐west transect at 26.9d̀S in the eastern piedmont of the Andes. This is a region of summer rainfall and winter drought characterized by a temporally unpredictable onset of vernal rains and vegetative rejuvenation.All three populations have a strong gonadal cycle including a period of histological collapse and reorganization. Measureable gonadal growth began in spring concurrently in all three populations when the daily photoperiod surpassed about 12 hours. The rates of gonadal growth in the log‐linear phase were alike in the chaco and mountain populations, but were only 35–45% of the rate in the intervening foothill population. The earliest nestings in all populations were closely correlated with the first growth of vegetative ground cover, but less closely correlated with other environmental variables. The growth of ground cover to an arbitrary index stage differed by several weeks in the three biotopes, and the onset of the nesting season differed accordingly. Rufous‐collared sparrows in this region nest almost exclusively on the ground in dense grass, and are evidently dependent on its concealment. Gonadal regression began at different times in the three populations and was not uniformly correlated with any obvious changes in the condition of the habitat. In all cases, regression began after the summer solstice, and thus coincided with decreasing daylength.Postnuptial moult in adults began almost simultaneously in all three populations regardless of the phase of the nesting cycle. Moult (suspended temporarily in some individuals) thus overlapped the end of the nesting season in the chaco and foothill populations, but not in the mountain population. Postjuvenal moult in immatures also began almost simultaneously in the three populations, about two weeks later than the onset of postnuptial moult.The temporal coordination of the annual cycle of Rufous‐collared sparrows in northwestern Argentina appears to be controlled basically by the annual photocycle, with habitat conditions (probably vegetative growth in spring) acting as essential modifiers in the final adjustment of nesting to local conditions. This system allows the sparrows to exploit suitable nesting conditions as soon as they occur each year in a temporally unpredictable environment in which an exclusively photoperiod control of the gonadal cycle would be deleterious in a large proportion of years.