High Central Andes Research Articles

Sustainability of Animal Stocks in Traditional Pastoral Systems of the High-Altitude Andean Altiplano

The main traditional livelihood in the high Central Andes Plateau, locally known as the Puna, is pastoralism. This system is based on llamas (Lama glama) and sheep (Ovis aries), which graze on natural vegetation (shrub steppe, grasslands, and peatlands). So, the livelihood of Andean communities is strongly influenced by the spatial and temporal dynamics of vegetation. Most of the existing models on herbivory used to estimate sustainable livestock loads require information on the nutritional values and primary production of vegetation, which is lacking for the Argentine Puna. This work aimed to fill the gap by implementing a carrying capacity model using data on annual vegetation growth and herbivore feeding strategies collected in the field in different plant communities. The hypothesis that guided this work was that environmental heterogeneity affects management decisions and should be taken into account to model sustainable animal stocks in Andean pastoralist systems. We estimated carrying capacity based on the MSY logistic model, considering biomass and aboveground primary production of vegetation communities, climatic features, and herbivore selective responses. We compared the estimated carrying capacity with real animal stocks in four production units within the study site in 2013, 2014, and 2017. The results indicate that the carrying capacity presents high spatial and temporal variability. In three of the production units assessed, the livestock—mixed herds of sheep and llamas—were above or near the estimated carrying capacity, even when the stocking rates were reduced during dry periods. The high stocking rates, along with evidence of land degradation, may indicate moderate to severe overgrazing. This work demonstrates the usefulness of estimating the carrying capacity to monitor the pastoral systems at fine space and time scales. These monitoring techniques can facilitate decision making among Puna herders in a biocultural context of knowledge dialog that may foster sustainability in pastoralist livelihoods.

Revision of the Neotropical Neuratelia Rondani (Diptera, Mycetophilidae, Sciophilinae): two new species, a new combination, and a new synonym.

We describe two new Neotropical species of Neuratelia Rondani from the high Central Andes of Colombia, N.altoandinasp. nov. and N.colombianasp. nov. The holotype of Eudicranaelegans Lane actually is a species of Neuratelia and a new combination is proposed. Our examination of the holotype of Neurateliasapaici Lane from southeastern Brazil shows this species to be a synonym of N.elegans (Lane), which is formally proposed here. Neurateliasapaici is redescribed. The position of these three species within the genus is discussed. A key for the Neotropical species of Neuratelia is provided.

An inventory of cryospheric landforms in the arid diagonal of South America (high Central Andes, Atacama region, Chile)

Abstract An integral, glacial and periglacial, inventory of the cryosphere in the Atacama region in Chile was carried out to estimate the amount of frozen water reserves in this arid to hyper-arid region. The inventory was carried out with medium to high spatial resolution satellite images and resulted in a total of 168,908 ha of landforms with significant ice content in one of the most fragile Andean ecosystems: the semi-arid – hyper-arid zones. This study shows an integral mapping of the cryosphere including glaciers and ice-bearing periglacial landforms. The different ice landforms in the Atacama region shows a progressive change in the landforms containing ice responding to the northwards increase of aridity upon entry to the South American hyper-arid axis. The main change observed is that uncovered glaciers are not present north of 26.5°S, being replaced by creeping permafrost landforms. Uncovered glaciers may produce more water annually compared to other ice-bearing landforms, but they only represent 2.7% of the cryospheric area. However, cryoforms containing a mixture of ice, sediment and rock fragments could be more important as water reserves because of their large extensions that cover c. 97% of the total area of ice-bearing landforms mapped. It also shows that the water-supply potential of ice-bearing periglacial landforms could be much higher than that of the glaciers in arid zones. The comparison with the previous inventory of 1987 suggests a reduction of 34% of uncovered glaciers, which is consistent with the aridization process observed in almost all of northern Chile. These findings reinforce the fact that glaciers and other types of permanent ice-bearing landforms need to be inventoried together if the objective is to provide the society with a realistic view of the distribution of the cryospheric hydrological resources in desert regions.

Structure and State of Stress of the Chilean Subduction Zone from Terrestrial and Satellite-Derived Gravity and Gravity Gradient Data

It is well known that the quality of gravity modelling of the Earth’s lithosphere is heavily dependent on the limited number of available terrestrial gravity data. More recently, however, interest has grown within the geoscientific community to utilise the homogeneously measured satellite gravity and gravity gradient data for lithospheric scale modelling. Here, we present an interdisciplinary approach to determine the state of stress and rate of deformation in the Central Andean subduction system. We employed gravity data from terrestrial, satellite-based and combined sources using multiple methods to constrain stress, strain and gravitational potential energy (GPE). Well-constrained 3D density models, which were partly optimised using the combined regional gravity model IMOSAGA01C (Hosse et al. in Surv Geophys, 2014, this issue), were used as bases for the computation of stress anomalies on the top of the subducting oceanic Nazca plate and GPE relative to the base of the lithosphere. The geometries and physical parameters of the 3D density models were used for the computation of stresses and uplift rates in the dynamic modelling. The stress distributions, as derived from the static and dynamic modelling, reveal distinct positive anomalies of up to 80 MPa along the coastal Jurassic batholith belt. The anomalies correlate well with major seismicity in the shallow parts of the subduction system. Moreover, the pattern of stress distributions in the Andean convergent zone varies both along the north–south and west–east directions, suggesting that the continental fore-arc is highly segmented. Estimates of GPE show that the high Central Andes might be in a state of horizontal deviatoric tension. Models of gravity gradients from the Gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite mission were used to compute Bouguer-like gradient anomalies at 8 km above sea level. The analysis suggests that data from GOCE add significant value to the interpretation of lithospheric structures, given that the appropriate topographic correction is applied.

A Modeling Study of Stratospheric Waves over the Southern Andes and Drake Passage

Abstract Large-amplitude stratospheric gravity waves over the southern Andes and Drake Passage, as observed by the Atmospheric Infrared Sounder (AIRS) on 8–9 August 2010, are modeled and studied using a deep (0–70 km) version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The simulated tropospheric waves are generated by flow over the high central Andes ridge and the Patagonian peaks in the southern Andes. Some waves emanating from Patagonia propagate southeastward across Drake Passage into the stratosphere over a horizontal distance of more than 1000 km. The wave momentum flux is characterized by a tropospheric maximum over Patagonia that splits into two comparable maxima in the stratosphere: one located directly over the terrain and the other tilting southward with altitude. Using spatial ray-tracing techniques and flow conditions derived from the numerical simulation, the authors find that waves that originate from the high ridge in the Central Andes are absorbed by a critical level in the lower stratosphere. The three-dimensional waves originating from Patagonia could be separated into three families—namely, a northeast-propagating family, which is absorbed by a critical level between 15 and 20 km; a localized family, which breaks down in the stratosphere and lower mesosphere directly above Patagonia; and a southeast-propagating family, which forms the observed linear stratospheric wave patterns oriented across Drake Passage. The southward group propagation, assisted by lateral wave refraction due to persistent meridional shear of the zonal winds, leads to stratospheric wave breaking and drag near 60°S, well south of the parent orography.

Plant Communities and Phytogeographical Units from NW San Juan Province (High Central Andes of Argentina)

Abstract MARTINEZ CARRETERO, E., A. DALMASSO, J. MARQUEZ & M. MARTINELLI (2010). Plant communities and phytogeographical units from NW San Juan Province (High Central Andes of Argentina). Candollea 65: 69–93. In English, English and French abstracts. Vegetation of the High Central Andes of Argentina was studied in NW San Juan Province (MAB Reserve of San Guillermo) from a phytosociological point of view. Four new associations are proposed: two shrubland associations, Artemisietum paramilloensis and Lycio-Ephedretum rupestri, and two grassland associations, Jaravetum chrysophyllae and Jaravetum absconditae. The latitudinal and altitudinal boundaries of the Monte, Puna and High Andean phytogeographic provinces were established for the study area. This area constitutes the floristic and biogeographic transition between northern (Bolivia) and southern (Mendoza, Argentina) Puna elements, belonging to the Calamagrostietea vicunarum class, and to the Stipo-Lycietea class, respectively.

Control of forearc density structure on megathrust shear strength along the Chilean subduction zone

Unveiling how the interaction of friction ( μ), pore pressure ( p) and normal stress ( σ n) controls the shear strength distribution along megathrust faults at convergent plate boundaries is crucial to our understanding of processes leading to the occurrence of devastating earthquake and the construction of mountains. An existing density model of the Chilean margin (18°–45°S) allows the computation of the so-called Vertical Stress Anomaly (VSA) that accounts for the component of normal stress due to the weight of the overlying crustal column. A spatial correlation between VSA, surface geology and several earthquake parameters suggests a significant control exerted by the geologically-dominated density structure of the crust on megathrust shear strength. The very seismogenic forearc in front of the high Central Andes (18°–34°S) has positive VSA values (up to 60 MPa) and a mafic-dominated crust, which suggest high values of shear strength at the megathrust that are partially controlled by the weight of a dense forearc crustal column, in addition to a likely dry subduction channel characterized by high- μ and low- p. In contrast, the Southern Andean forearc (34°–45°S) shows neutral-to-negative VSA (down to −30 MPa) correlated with a felsic-dominated crust, which added to a presumably wet subduction channel (low- μ and high-p) configures a rather weak megathrust below a light forearc. Earthquakes here are concentrated around the Arauco peninsula, a first-order morpho-structural anomaly characterized by comparatively higher values of VSA than the rest of this segment, which is the nest of the giant (Mw 9.5) Valdivia 1960 earthquake and the southern rupture termination of the recent Mw 8.8 Maule earthquake. The ultimately giant size of the 1960 earthquake resulted from rupture propagation along a 1000 km long region characterized by negative VSA values. The correlation between VSA and the along-strike segmentation of the Andean orogen indicates that the geologically-inherited density structure of the forearc also rules long-term mechanical coupling between converging plates and prescribes the possible structure of the orogen. Because Central and Southern Andes represent two extremes in terms of orogenic structure and seismogenic behavior, results presented here have significant implications for the general comprehension of processes occurring at short and long time scale at convergent margins.

Phylogeny and evolution of Perezia (Asteraceae: Mutisieae: Nassauviinae)

Abstract A molecular phylogenetic analysis of most of the species of Perezia reveals that, as traditionally defined, the genus is not monophyletic with two species more closely related to Nassauvia than to Perezia. In addition, our results show that Burkartia (Perezia) lanigera is related to Acourtia and is the only member of that clade in South America. The remaining species are monophyletic and show a pattern of an early split between a western temperate and an eastern subtropical clade of species. Within the western clade, the phylogeny indicates a pattern of diversification that proceeded from southern, comparatively low‐elevation habitats to southern high‐elevation habitats, and ultimately into more northern high‐elevation habitats. The most derived clades are found in the high central Andes, where significant radiation has occurred.

What drives orogeny in the Andes?

Abstract The Andes, the world's second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates' interface. The model with a thick (40–45 km at 30 Ma) South American crust and relatively high friction coefficient (0.05) at the Nazca–South American interface generates >300 km of tectonic shortening during 30–35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner (<40 km) continental crust and lower friction coefficient (<0.015) results in <40 km of South American plate shortening, replicating the situation in the southern Andes. Our modeling also demonstrates the important role of the processes leading to mechanical weakening of the overriding plate during tectonic shortening, such as lithospheric delamination, triggered by the gabbro-eclogite transformation in the thickened continental lower crust, and mechanical failure of the sediment cover at the shield margin.

What drives orogeny in the Andes?

The Andes, the world’s second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates’ interface. The model with a thick (40‐45 km at 30 Ma) South American crust and relatively high friction coefficient (0.05) at the Nazca‐South American interface generates ! 300 km of tectonic shortening during 30‐35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner ( 40 km) continental crust and lower friction coefficient ( 0.015) results in 40 km of South American plate shortening, replicating the situation in the southern Andes. Our modeling also demonstrates the important role of the processes leading to mechanical weakening of the overriding plate during tectonic shortening, such as lithospheric delamination, triggered by the gabbro-eclogite transformation in the thickened continental lower crust, and mechanical failure of the sediment cover at the shield margin.

Revision ofDoniophyton (Compositae, Barnadesioideae)

This revision describes, illustrates and documents morphological variation inDoniophyton (Compositae, Barnadesioideae), restricted to Argentina and Chile. Two species are recognized,D. anomalum andD. weddellii (sp. nova), possessing distinct morphological and chromosomal features, elevational tolerances, and nearly allopatric distributions.Doniophyton weddellii occurs primarily in central to northern Andean Chile and Argentina from 1900–4000 m a. s. l.;D. anomalum is found principally in centralwestern Argentina and south into Patagonia at 0–1800 m a. s. l. Close relationship exists withChuquiraga of subfam.Barnadesioideae. It is hypothesized thatDoniophyton evolved out ofChuquiraga in the high central Andes between Chile and Argentina. It is suggested thatD. weddellii differentiated first, correlating with an aneuploid chromosomal decrease from n = 27 (inChuquiraga) to n = 25. Further evolution and chromosomal decrease to n = 24 resulted inD. anomalum, with accompanying migration into southern Andes and Patagonia. Nomenclatural changes result from examination of protologues and type specimens:Doniophyton anomalum replaces the commonly used nameD. patagonicum, and a new species,D. weddellii, is described for the taxon masquerading under the routinely used superfluous nameD. andicola.

Glaciological Studies in the High Central Andes Using Digital Processing of Satellite Images

Inaccessibility and the high cost of systematic survey have made remote sensing the most appropriate monitoring technique to study the glaciers in the high Central Andes. The sparse information that is available on the glaciers of the Argentine Central Andes is mainly concentrated on the Plomo River headwaters. Through digital processing of satellite images, the positions of glacier fronts in the Plomo catchment were updated for the 1974-1992 period; this completed the mapping of the frontal positions that began in 1909. Digital processing included histogram equalization and normalization, as well as color composites; the images were first geometrically corrected. The results indicate that during recent years most of the glaciers have been advancing, within a long-term trend of generalized retreat.

La Vegetación de los Altos Andes Centrales: Bardas Blancas-Paso Pehuenche (Malargüe, Mendoza, Argentina)

Se estudió la vegetación entre Bardas Blancas y Paso Pehuenche en el Sur Mendocino a fin de obtener información florístico-ecológica de las comunidades vegetales andinas. Se trabajó en un gradiente altitudinal de 1420 a 2800 m s.n.m. y se utilizó el método fitosociológico. Se describen las principales comunidades vegetales que surgen de los análisis de tablas comparativas de relevamientos. Se revela a 19 comunidades vegetales, incluídas las vegas de altura y baja altura, y 24 grupos florísticos caracterizadas por especies diagnósticas, exclusivas, preferenciales y acompañantes. El análisis de dispersión y de conglomerados jerárquicos reafirma las estrechas relaciones de las comunidades vegetales con las temperaturas y altitudes. El análisis corológico identifica a 2 provincias fitogeográficas: Patagonica y Altoandina. Hay una relación florística-ecológica con los ambientes fitogeográficos y las formas de vida. El estudio fitosociológico determina a 3 pisos de vegetación: 1- de los matorrales de Neosparton ephedroides, Larrea nitida, Colliguaja integerrima y Fabiana imbricata, 2- de los matorrales de Adesmia volckmannii, Chuquiraga oppositifolia, Adesmia pinifolia y Adesmia obovata y 3-. de las estepas de Azorella monantha, Nassauvia revoluta, Empetrum rubrum y Ochetophila nana. Los resultados probablemente podrían extrapolarse al N y S del área.

The grain chenopods of the tropical American highlands

A general review, incorporating many new observations, of the American grain chenopods is given. The species areCh. quinoa (Quinoa),Ch. nuttalliae (Huauzontle) andCh. pallidicaule (Canahua); the first two are related tetraploids (2n = 36), and the third is a quite distinct diploid. Huauzontle is Mexican, and the other two are Andean plants. Brief surveys of systematics, biology, cultivation and utilization of the three species are given. Quinoa is the principal native grain of the high central Andes and is still of considerable local importance.