Héctor del Valle

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.

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.

Polycyclic aromatic hydrocarbons (PAHs) and their polar derivatives (oxygenated PAHs, azaarenes) in soils along a climosequence in Argentina.

We evaluated the effects of soil properties and climate on concentrations of parent and oxygenated polycyclic aromatic compounds (PAHs and OPAHs) and azaarenes (AZAs) in topsoil and subsoil at 20 sites along a 2100-km north (N)-south (S) transect in Argentina. The concentrations of Σ29PAHs, Σ15OPAHs and Σ4AZAs ranged 2.4-38 ng g(-1), 0.05-124 ng g(-1) and not detected to 0.97 ng g(-1), respectively. With decreasing anthropogenic influence from N to S, low molecular weight PAHs increasingly dominated. The octanol-water partitioning coefficients correlated significantly with the subsoil to topsoil concentration ratios of most compounds suggesting leaching as the main transport process. Organic C concentrations correlated significantly with those of many compounds typical for atmosphere-soil partitioning. Lighter OPAHs were mainly detected in the S suggesting biological sources and heavier OPAHs in the N suggesting a closer association with parent-PAHs. Decreasing alkyl-naphthalene/naphthalene and 9,10-anthraquinone (9,10-ANQ)/anthracene ratios from N to S indicated that 9,10-ANQ might have originated from low-temperature combustion.

Functional and structural landscape indicators of intensification, resilience and resistance in agroecosystems in southern Argentina based on remotely sensed data

There is increasing interest in developing criteria to evaluate the environmental implications of intensive agricultural land use. This implies discriminating between nature and man-made effects upon structural and functional attributes of agroecosystems. Adequate indicators of these combined effects should be cost efficient yet compatible with the core of ecological theory on biodiversity, spatial organization and ecosystem stability. We developed resistance-resilience metrics of plant growth to evaluate the intensity of agricultural use in a temperate irrigated basin in southern Argentina. The metrics are based on an analysis of the components of a temporal series of vegetation indices computed at a low resolution from available globally remote sensed reflectance imagery. We related the developed metrics to the properties of the soils and plant canopies observed at field scale and high-resolution imagery of the basin. Soil depth, soil erosion status and land fragmentation account for large fractions of the variance of the distribution of functional groups of the plant canopies and are also correlated with smaller scale attributes of land vegetation cover. Resistance-resilience indicators constitute a cost-efficient and adequate approach to evaluate the degree of intensification of land agricultural use.

Semi-automated mapping of burned areas in semi-arid ecosystems using MODIS time-series imagery

Understanding spatial and temporal patterns of burned areas at regional scales, provides a long-term perspective of fire processes and its effects on ecosystems and vegetation recovery patterns, and it is a key factor to design prevention and post-fire restoration plans and strategies. Standard satellite burned area and active fire products derived from the 500-m MODIS and SPOT are avail able to this end. However, prior research caution on the use of these global-scale products for regional and sub-regional applica tions. Consequently, we propose a novel algorithm for automated identification and mapping of burned areas at regional scale in semi-arid shrublands. The algorithm uses a set of the Normalized Burned Ratio Index products derived from MODIS time series; using a two-phased cycle, it firstly detects potentially burned pixels while keeping a low commission error (false detection of burned areas), and subsequently labels them as seed patches. Region growing image segmentation algorithms are applied to the seed patches in the second-phase, to define the perimeter of fire affected areas while decreasing omission errors (missing real burned areas). Independently-derived Landsat ETM+ burned-area reference data was used for validation purposes. The correlation between the size of burnt areas detected by the global fire products and independently-derived Landsat reference data ranged from R² = 0.01 - 0.28, while our algorithm performed showed a stronger correlation coefficient (R² = 0.96). Our findings confirm prior research calling for caution when using the global fire products locally or regionally.

Spatial distribution of wildfire risk in the Monte biome (Patagonia, Argentina)

Abstract Please click here to download the map associated with this article. In many rangeland ecosystems, the role of wildfires has been the object of detailed studies. However, in Argentina and especially in the Monte biome this knowledge is limited. This region is prone to wildfires during late spring and summer, when high winds and temperatures are coupled with low relative humidity. In addition, this region also presents cases of extreme wildfire behaviour. The aim of this study is to develop a probability-based model for estimating wildfire risk at a regional scale based on the Moderate Resolution Imaging Spectroradiometer (MODIS) Hot Spot data. The map presented here covers an area of approximately 246,000 km at a 1:100,000 scale. It is the result of fitting the 2001–2010 hot spot time series with a logistic regression model to 15 potentially explanatory environmental and anthropogenic variables. At a regional scale, mean net primary production, spring temperature, spring and summer precipitation, elevation, slope, distance to nearest road are the most important factors explaining wildfire risk.

Synergistic use of Landsat and Hyperion imageries for ecological site classification in rangelands

The synergistic use of multispectral and hyperspectral data is evaluated for discrimination and mapping of ecological sites in the Patagonian rangelands. The method proposed encompasses two steps. First, mixture tuned matched filtering and logistic regression analyses are used for Hyperion data processing to obtain ecological site probability images in the area covered by hyperspectral imagery. Second, neural networks are used to model the relationships between the spectral response patterns of Landsat and the probability images from Hyperion, and to extrapolate them to the entire study area.

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.

Teledetección satelital cuantitativa para estimar el área basal del bosque de Nothofagus pumilio (Nothofagaceae): El rol del índice de área foliar como información auxiliar

Los bosques de lenga (Nothofagus pumilio) son el recurso forestal mas importante de la region andino patagonica argentina, sin embargo, para implementar planes de manejo en pos de prevenir o revertir su degradacion es necesario disponer de mayor informacion sobre su estructura. Una alternativa para obtener esa informacion es relacionar datos satelitales con las caracteristicas del bosque a traves de modelos fisicos y estadisticos. Pero, ?cual es el metodo mas eficaz? El indice de area foliar (IAF) se encuentra relacionado con la reflectividad del dosel vegetal a traves del modelo de transferencia radiativa PROSAIL, lo que permite desarrollar tecnicas de teledeteccion satelital para estimar el IAF con bases fisicas, en vez de con modelos estadisticos. De ese modo se puede aprovechar la relacion empirico-biologica existente entre el IAF y la estructura del bosque para estimar el area basal. El objetivo principal fue comparar la exactitud de tres metodos para estimar el area basal de la lenga con datos SPOT-5. Los metodos comparados fueron: estadistico directo (ED), estadistico de dos pasos (E2P) y fisico-estadistico de dos pasos (FE2P). La exactitud fue evaluada con 24 parcelas no involucradas en el ajuste o calibracion de los modelos. No se hallaron diferencias significativas en la exactitud de la alternativa fisico-estadistica en comparacion con las netamente estadisticas. Sin embargo, en base al analisis conjunto de los resultados y lo reportado por otros investigadores, se concluye que la ventaja de utilizar modelos fisicos radica en la mayor robustez de la estimacion y no en la mayor exactitud.