César Mario Rostagno

Mounds associated with shrubs in aridic soils of northeastern Patagonia: Characteristics and probable genesis

Abstract Studies of mounds associated with shrub clumps in northeastern Patagonia indicate that these forms have an average major diameter of 355 cm and an average height of 41 cm. Mound cover and density are 40% and 450 mounds/ha respectively. Mound and intermound soils present a similar textural composition as well as a similar coarse fragment content (10–20%). Root and organic matter contents are significantly higher whereas calcium carbonate is significantly lower in the mound than in the intermound soils. Mounds may represent a relict land surface level, lowered in the intermound area mainly by wash erosion. Mound development is related to discrete shrub clumps that shelter the underlying soil. Once established, the shrub clump and the underlying mound remain as a stable and self maintained system.

Soil Erosion Thresholds and Alternative States in Northeastern Patagonian Rangelands

Abstract In semiarid rangelands, continuous grazing may decrease vegetation cover, accelerating soil erosion and eventually causing a transition to an alternative, degraded state. State-and-transition models invoke process-based explanations of alternative states, but there are few examples that use empirical data on key factors and processes. We used rainfall simulation to determine 1) the relationships between soil surface characteristics and interrill erosion in 3 spatially related plant communities: stable grass with scattered shrubs (GS), degraded grass with scattered shrubs (DGS), and degraded shrub steppes (DSS), and 2) the site conservation threshold (SCT) of this rangeland. We also analyzed the effect of past erosion on soil and vegetation characteristics. In the GS, sediment production and sediment concentration were significantly lower (p < 0.05) than in the DGS and the DSS. The main soil protection factors in the GS and in the DGS were perennial grass and litter cover, while in the DSS, gravel cover became the main soil protection factor. The SCT, the point at which the rate of soil erosion increases markedly, corresponded to a plant-and-litter cover close to 90% and occurred within the DGS. Although this plant community may reverse back to the conserved GS, long-term accelerated erosion may result in enough soil loss to trigger irreversible changes and prompt the transition to the DSS. The threshold underlying this transition would be reached when the A horizon is severely reduced by erosion. Under these conditions, the soil hydrological properties are affected irreversibly, preventing perennial grass establishment. While the GS represents a resource conserving plant community, desirable for both forage production and soil protection, the DSS represents a dysfunctional state with a minimum forage value. The DGS represents an unstable and transitional community that, without management intervention to halt soil erosion, will likely change into the DSS.

Biological Evaluation of Seaweed Composting

Large quantities of green seaweed, linked probably to eutrophication, are cast ashore every summer on the Puerto Madryn beaches (Patagonia, Argentina, 42°S, 65°W). This algal biomass interferes with recreational uses of the beach, and therefore must be periodically collected and disposed. Part of this algal biomass was composted with the objective to produce an amendment to improve physical and nutritional characteristics of some local soils used in intensive horticulture, and at the same time to find a way to reduce environmental pollution. The compost was then biologically evaluated by determining the growth rate of tomato plants cultivated on various substrata (washed sand, sandy loam soil, and sandy loam soil plus inorganic fertilizers) to which different doses of compost were added. Results showed that in all cases the addition of compost increased water holding capacity and plant growth. The increase of tomato plants (Licopersicum esculentum var. platense) was proportional to the compost doses. Also...

Environmental patterns and plant distribution along aprecipitation gradient in western Patagonia

Abstract Plant species distribution along a gradient of precipitation was analysed inwestern Patagonia. First, the affinity of 53 vegetation releves was tested by principal component analysis of the matrix of n × n binary correlations among species cover values. Further, the relationship between plant cover (Festuca pallescens, shrubs, and Stipa spp.) and the environmental factors (precipitation, soil texture, soil depth, altitude, landscape form, aspect, and disturbance) was analysed by stepwise regression analysis. Five homogeneous vegetation units could be identified at several different regions of the gradient. Soil characteristics associated to precipitation levels (i.e. texture, profile depth), but not precipitation per se, were strongly related to the cover of F. pallescens and shrub species. A model of transitions in the composition of vegetation is introduced which includes soil variables, site conditions and the spatial distribution of grazing herds.

Postfire vegetation dynamics in three rangelands of northeastern patagonia, Argentina

Abstract In many rangeland ecosystems, the role of fire on vegetation dynamics has been the object of detailed studies. In Argentina, and especially in Patagonia, the knowledge of how fire changes vegetation is scarce. In 3 areas affected by wildfires on different dates (in 1988, 1994, and 1998), we determined the structure of the vegetation (plant cover, density, and biomass) and compared it with that of nearby unburned areas. Based upon these data, we present a qualitative state-and-transition model of this rangeland. For the sites burned in 1988 and 1994, aerial biomass, density, and cover of perennial grasses were significantly greater (P ≤ 0.05) for burned than for unburned areas. For the site burned in 1998, although there were no significant differences in perennial grass biomass and cover, density was significantly greater compared to the unburned area. Total shrub cover was significantly lower (P ≤ 0.05) in burned than in the unburned areas, attaining 49.7%, 15.0%, and 5.5% of that of the unburned areas for the sites burned in 1988, 1994, and 1998, respectively. Similar to cover, density for most shrubby species was significantly greater in the unburned than in the burned areas for the 3 sites. Grazing and fires of different intensities, combined with variable rainfall, makes the prediction of postfire vegetation changes difficult. However, it can be generalized that fire changed the vegetation from shrub-dominated steppes, a persistent state in northeastern Patagonia, into a grass-dominated transient state. This change has persisted for more than 10 years after a fire event. From the perspective of sheep raising, the significant postfire increase in perennial grass biomass represents a substantial improvement in the condition of these rangelands. However, repeated fires would be necessary to control the sprouting shrubs and maintain the grass-dominated state.

Runoff and Erosion in Five Land Units of a Closed Basin of Northeastern Patagonia

Soil loss and runoff were assessed in five land units (LUs) of a closed basin by means of 18 experimental plots within a 42 month period . Temporal distribution of runoff and erosion was unequal . Among 120 rainfall events recorded during the study period , only 15 events caused runoff . Maximum percent of runoff (79 . 9 % ) and maximum specific runoff (19.5 Lm-2) were produced by the two events of highest intensity and were recorded in a LU characterized by a crusted and com pacted sandy loam soil . These two events also accounted for more than 60 % of the total soil loss . Soil loss was negatively correlated to percent of sand in the top soil (r=-0.91 and vegetation cover (r=-0.69) and ranged from 37.0 gm-2 (sand) to 1661.7 gm-2 (clay).

Soil–Geomorphology Relationships and Pedogenic Processes in Península Valdés

The soil landscape of the Peninusla Valdes region is review based in soil genesis analysis and soil–geomorphic relationships. The main soil types of the study area are grouped into two Soil Orders: Aridisols and Entisols. The oldest geomorphic surface corresponds to a relict terrace level from the Rodados Patagonicos lithostratigraphic unit, with Xeric Petrocalcids—Xeric Haplocalcids soil complex. Pleistocene landforms as youngest terrace levels, piedmont pediments (endorheic basins and coastal zone), and paleo-beach ridge units, an intricate soil distribution pattern occurs. This soil distribution is registered by a soil complex constituted by Natrargids, Natrigypsids, Calciargids and Haplocalcids, all of them with xeric regime soil moisture. In playa lakes of the endorheic basins, the soils were classified as Calcic Aquisalids. The Entisols are developed on Holocene geomorphic surfaces; Xeric Torripsamments in stabilized aeolian fields, and Typic Torriorthents in bajadas (coalescing alluvial fans associated to piedmont pediments). In salt marshes, geomorphic elements, anoxia degree, and vegetation communities are associated to soil type; Haplic Sulfaquents, Sodic Endoaquents and Sodic Psammaquents, are related to the low salt marsh, while Typic Fluvaquents and Sodic Hydraquents are developed in the high salt marshes. The main pedogenic processes registered in the Peninsula Valdes soils are clay illuviation, calcification, gypsification, and sulfide production—sulfidization. As the calcretization process progresses, a transformation and neoformation of clay minerals occurs in the following sequence: smectite—palygorskite—sepiolite. The isotopic composition of δ13C y δ18O in pedogenic carbonate could be used as paleoecological and plaeoclimate proxy indicators, respectively.

Interacting effects of soil degradation and precipitation on plant productivity in NE Patagonia, Argentina

ABSTRACT Our objective was to examine the effects of inter-annual variation of precipitation on productivity of two dominant species (Chuquiraga avellanedae, an evergreen shrub, and Nassella tenuis, a perennial grass) in two communities of contrasting soil degradation: a herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Data were collected during two consecutive years with different annual precipitation. Aboveground productivity was determined nondestructively using a double sampling approach. The number of inflorescences per plant was recorded too. Perennial grass productivity was lower in SS than in HSS in both years, while shrub productivity was lower in SS only during the year of below average precipitation. With rising precipitation the perennial grass increased the number of inflorescences while the evergreen shrub augmented vegetative biomass. In summary, the effects of precipitation on plant productivity are community dependent; abiotic factors, such as superficial and sub-superficial soil characteristics, and biotic factors, such as leaf area index (LAI) or tussock sizes, may interact to influence the responses of species to precipitation. Our results suggest that if precipitation increased, this would favor the dominance of shrubs over grasses.

Late Cenozoic Landforms and Landscape Evolution of Península Valdés

The present landscape of the Peninsula Valdes is the result of a complex interrelation between climatic (aeolian deposition, windblown processes, glacial and interglacial cycles, pluvial and fluvial processes), tectonic, and eustatic controls that had work in the Andean foreland during the late Cenozoic. Based on a geomorphological approach, which includes new descriptions, interpretations, and hierarchically classification of the main landforms of this region, together with previous geomorphological surveys, the Peninsula Valdes area was grouped in three major geomorphologic systems: Uplands and Plains, Great Endorheic Basins, and Coastal Zone. Based on the interrelationship among these three geomorphological systems the landscape evolution of the late Cenozoic of Peninsula Valdes could be summarized in five main stages: (1) development of fluvial and alluvial systems during the Pliocene early Pleistocene; (2) closed basin formation associated to tectonic processes during the early middle Plesitocene; (3) first marine transgressions during the late Pleistocene; (4) flooding of the gulfs and construction of the peninsula in the late Plesitocene–Holocene; (5) final flooding in the region during the middle Holocene.

Soil Degradation in Peninsula Valdes: Causes, Factors, Processes, and Assessment Methods

In semiarid rangelands where the anthropogenic impact is currently increasing, as occurs in the rangelands of the Peninsula Valdes, the detrimental impacts of soil degradation on land resources became really dramatic. This chapter presents a review on the current knowledge of soil degradation in the Peninsula Valdes rangelands. Section 1 introduces the chapter, Sect. 2 focuses on soil degradation main processes, factors and causes, and Sect. 3 presents a review of soil degradation assessment methods and several soil degradation studies carried out since 1990 in the Peninsula Valdes region. Water and wind erosion are the degradation processes that are most strongly evidenced. Major causes of soil degradation are attributed to a combination of climatic and anthropic factors, with overgrazing being perceived to be a major factor. Four key causes associated with overgrazing in the Peninsula Valdes region rangelands are described: (1) Poor range management with respect to flock distribution and overstocking, (2) Limited access to information, (3) Top-down and largely ineffective government policy, and (4) Overdependence on grazing systems for sustained livelihoods. Assessment methods for assessing soil degradation include: expert judgment, remote sensing, productivity changes, field monitoring, pilot studies at farm level based on field criteria and expert opinion, and modeling.