Juan Carlos Aravena

Snowpack Variations in the Central Andes of Argentina and Chile, 1951–2005: Large-Scale Atmospheric Influences and Implications for Water Resources in the Region

Abstract The snowpack in the central Andes (30°–37°S) is the primary source for streamflow in central Chile and central-western Argentina, but few published studies are available on snowpack variability in the region. This paper develops the first regional snowpack series (1951–2005) from Chilean and Argentinean snow course records. This series shows a strong regional signal, marked interannual variability, and a positive, though nonsignificant, linear trend. Correlations with local precipitation and temperature records reveal a marked association with conditions in central Chile. High snow accumulation is generally concurrent with El Nino events in the tropical Pacific, but only 5 of the 10 driest years coincided with La Nina events. Evaluation of 500-hPa geopotential height anomaly maps during extreme snow years highlights the crucial significance of tropospheric conditions in the subtropical and southeast Pacific in modulating snowfall. Correlations with gridded SST and SLP data and multiple regression...

Ancient Austrocedrus Tree-Ring Chronologies Used to Reconstruct Central Chile Precipitation Variability from A.D. 1200 to 2000

An expanded network of moisture-sensitive tree-ring chronologies has been developed for central Chile from long-lived cypress trees in the Andean Cordillera. A regional ring width chronology of cypress sites has been used to develop well-calibrated and verified estimates of June–December precipitation totals for central Chile extending from A.D. 1200 to 2000. These reconstructions are confirmed in part by historical references to drought in the seventeenth and eighteenth centuries and by nineteenth-century observations on the position of the Rio Cipreses glacier. Analyses of the return intervals between droughts in the instrumental and reconstructed precipitation series indicate that the probability of drought has increased dramatically during the late nineteenth and twentieth centuries, consistent with selected long instrumental precipitation records and with the general recession of glaciers in the Andean Cordillera. This increased drought risk has occurred along with the growing demand on surface water resources and may heighten socioeconomic sensitivity to climate variability in central Chile.

Fire and the dynamics of Fitzroya cupressoides (alerce) forests of Chile's Cordillera Pelada.

AbstractWidespread mortality of Fitzroya cupressoides (alerce) is found throughout the Coastal Range of south-central Chile. The main explanations for tree mortality have been fire and climate chan...

Fire History in Central Chile: Tree-Ring Evidence and Modern Records

Wildfires in Chile are believed to have originated primarily from fires set by humans to clear natural vegetation to permit agriculture (also see Montenegro et al., Chapter 14, this volume). Besides the intentionally set fires, rapid population growth during the last several decades of the twentieth century has further contributed to an exponential increase in fires set accidentally by motor vehicles along roadsides and by careless campers, especially in central Chile where the human population is concentrated (CONAF 2000). Of course, some fires are ignited naturally by lightning and volcanic eruptions, but the frequency of such ignitions is relatively low in comparison with human-set fires. It is likely that such a low-frequency fire regime may have prevailed before the arrival of indigenous populations to this region, as suggested by sedimentary records of charcoal and fossil pollen covering the past 40,000 years (Heusser 1994). Effects of fires on the sclerophyllous vegetation of central Chile have been thoroughly studied from a physiological and ecological point of view (Montenegro et al., Chapter 14, this volume). However, long-term records of fire history, derived either from sedimentary charcoal or from tree rings are extremely scarce for Chile, in general, including central Chile which is the focus of this chapter (Heusser 1994). Tree rings permit the dating of past fires to an annual resolution and thus can provide useful information on the frequency and recurrence intervals of past fires. Additional information on fire extent, intensity, and direction of spread may be derived from data on spatial patterns of fire-scarred trees. In central Chile (32° to 38°S) several woody species produce annual tree

Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across the Western Americas

Common patterns of climatic variability across the Western Americas are modulated by tropical and extra-tropical oscillatory modes operating at different temporal scales. Interannual climatic variations in the tropics and subtropics of the Western Americas are largely regulated by El Nino-Southern Oscillation (ENSO), whereas decadal-scale variations are induced by long-term Pacific modes of climate variability such as the Pacific Decadal Oscillation (PDO). At higher latitudes, climate variations are dominated by oscillations in the Annular Modes (the Arctic and Antarctic Oscillations) which show both interannual and longer-scale temporal oscillations. Here we use a recently-developed network of tree-ring chronologies to document past climatic variations along the length of the Western Cordilleras. The local and regional characterization of the relationships between climate and tree-growth provide the basis to compare climatic variations in temperature- and precipitation-sensitive records in the Western Americas over the past 3–4 centuries. Upper-elevation records from tree-ring sites in the Gulf of Alaska and Patagonia reveal the occurrence of concurrent decade-scale oscillations in temperature during the last 400 years modulated by PDO. The most recent fluctuation from the cold- to the warm-phase of the PDO in the mid 1970s induced marked changes in tree growth in most extratropical temperature-sensitive chronologies in the Western Cordilleras of both Hemispheres. Common patterns of interannual variations in tree-ring chronologies from the relatively-dry subtropics in western North and South America are largely modulated by ENSO. We used an independent reconstruction of Nino-3 sea surface temperature (SST) to document relationships to tree growth in the southwestern US, the Bolivian Altiplano and Central Chile and also to show strong correlations between these regions. These results further document the strong influence of SSTs in the tropical Pacific as a common forcing of precipitation variations in the subtropical Western America during the past 3–4 centuries. Common patterns of interdecadal or longer-scale variability in tree-ring chronologies from the subarctic and subantarctic regions also suggest common forcings for the annular modes of high-latitude climate variability. A clear separation of the relative influence of tropical versus high-latitude modes of variability is currently difficult to establish: discriminating between tropical and extra-tropical influences on tree growth still remains elusive, particularly in subtropical and temperate regions along our transect. We still need independent reconstructions of tropical and polar modes of climate variability to gain insight into past forcing interactions and the combined effect on climates of the Western Americas. Finally, we also include a series of brief examples (as ‘boxes’) illustrating some of the major regional developments in dendrochronology over this global transect in the last 10 years.

Fire Regimes and Forest Dynamics in the Lake Region of South-Central Chile

Fire is one of the major disturbances shaping the vegetation and landscape patterns in the Lake Region of south-central Chile (39°30¢–43°30¢S). Most of these fires occurred after the European settlement in the area, which started ca. 1750, but it was not until the 1850s that extensive settlement took place which led to massive burning and clearing of forests for agriculture and pasture land (Elizalde 1970; Wilhelm 1968). Recent research from tree rings in the Cordillera Pelada, (ca. 40°S) has documented fires in the last 600 years, that may be attributed to both lightning and the native human population (Lara et al. 1999a). Research from pollen records and Quaternary stratigraphy indicates the extensive occurrence of fire in southern South America, since about 13,000BP (Heusser 1994). A long history of fire occurrence has also been found in central Chile (see Aravena et al., Chapter 12, this volume) and in Patagonia, Argentina (Veblen et al., Chapter 9, Kitzberger and Veblen, Chapter 10, Huber and Markgraf, Chapter 13, this volume). Forest dynamics of various vegetation types in the region and their relation to different kinds of disturbances—especially volcanism, landslides, logging, and fire—have been described by several studies (Veblen and Ashton 1978, 1982; Veblen et al. 1981; Veblen 1983, 1985; Veblen et al. 1996). Nevertheless, the detailed study of fire regimes, and their relation to forest dynamics is only incipient in the Chilean Lake Region (Lara et al. 1999a). In contrast, the ecological role of fire has received substantial research attention in the forests of northern Patagonia, Argentina (Veblen et al. 1995, Veblen et al., Chapter 9, this volume).

Equilibrium line altitudes along the Andes during the Last millennium: Paleoclimatic implications

Deciphering the climate changes that influenced the glacial fluctuations of the last millennium requires documenting the spatial and temporal patterns of these glacial events. Here, we estimate the change in equilibrium line altitudes (ELAs) between the most prominent glacial advance of the last millennium and the present for four alpine glaciers located in different climatic regimes along the Andes. For each glacier, we reconstruct scenarios of climatic conditions (temperature and precipitation anomalies) that accommodate the observed ELA changes. We focus on the following glaciers: an alpine glacier in the Cordillera Vilcanota (13°S), Tapado glacier (30°S), Cipreses glacier (34°S), and Tranquilo glacier (47°S). Our results show that the range of possible temperature and precipitation anomalies that accommodate the observed ELA changes overlap significantly at three of the four sites (i.e. Vilcanota, Cipreses, and Tranquilo). Only Tapado glacier exhibits a set of climate anomalies that differs from the other three sites. Assuming no change in precipitation, the estimated ELA changes require a cooling of at least 0.7°C in the Cordillera Vilcanota, 1.0°C at Tapado glacier, 0.6°C at Cipreses glacier, and 0.7°C at Tranquilo glacier. Conversely, assuming no change in temperature, the estimated ELA changes are explained by increases in precipitation exceeding 0.52 m yr−1 (64% of the annual precipitation) in the Cordillera Vilcanota, 0.31 m yr−1 (89%) at Tapado glacier, 0.22 m yr−1 (27%) at Cipreses glacier, and 0.3 m yr−1 (27%) at Tranquilo glacier. By mapping the ELA changes and modeling the potential climate forcing across diverse climate settings, we aim to contribute toward documenting the spatial variability of climate conditions during the last millennium, a key step to decipher the mechanisms underlying the glacial fluctuation that occurred during this period.

Patterns of ecosystem development in glacial foreland chronosequences: a comparative analysis of Chile and New Zealand

ABSTRACT After catastrophic disturbances, depleted substrates are readily colonised by organisms that capture nitrogen from the atmosphere and extract phosphorus from minerals. Our main objective was to compare the pattern of ecosystem development following deglaciation in Chile and New Zealand. Results show a similar pattern of C and N accumulation and decline in soil chronosequences, similar decline in biological nitrogen fixation (BNF) and similar δ15N-enriched signal at later stages, providing evidence for the existence of progressive, maximal and retrogressive phases of ecosystem development. However, contrasting patterns between Chilean and New Zealand sites are evident during the progressive phase, when higher C/N, C/P and N/P ratios are found in soils and leaves in Chile than in New Zealand, suggesting a higher nutrient limitation and nutrient use efficiency in the former. Highest rates of BNF were found at the early stages of both the Chilean and New Zealand chronosequences. Contrasting patterns across regions were the lack of a decline in soil total P, and the depleted values in soils of 15N during the progressive phase in the Chilean chronosequences, but enriched values, suggesting an open nitrogen cycle, during retrogression in both the Chilean and the New Zealand chronosequences. Overall, these results provide evidence for the existence of retrogression with ecosystem development in the sub-Antarctic region of the world, even when comparing contrasting biomes, climatic regions and geological substrates.

Limitations and Relevance of Biological Nitrogen Fixation during Postglacial Succession in Cordillera Darwin, Tierra del Fuego, Chile

ABSTRACT We tested the main hypothesis that nutrient accumulation during late stages of postglacial succession would decrease nutrient limitation of diazotrophic activity. We tested this hypothesis by adding carbon (C), phosphorus (P), and molybdenum (Mo) independently or in combination, and nitrogen (N) only to symbiotic, epiphylls on bryophytes, and free-living diazotrophs in three stages of glacier foreland succession in Cordillera Darwin (55°S), southern South America. Experiments were run in spring 2013 and 2014 and in autumn 2015. Diazotrophic activity (DA) was assessed by the acetylene reduction assay. Results showed no effect of C, P, or Mo added either singly or in combination in the spring incubations. During autumn, DA was enhanced by adding a mix of C, P, and Mo to the symbiotic N2-fixing Gunnera magellanica from young successional sites, while in the late successional sites, adding C and Mo alone to the diverse bryophyte carpet on the forest floor enhanced DA. Nitrogen added as ammonium sulfate had a strong negative effect on N2 fixation by free-living diazotrophs in the spring and autumn samples from the late successional site, in the bryophyte carpet from the early successional site (autumn), and in Pseudocyphellaria freycinetii of the midsuccessional site (spring). As in other high-latitude biomes, symbiotic and epiphyllous associations and free-living diazotrophs play a crucial role in the incorporation of new N to postglacial subantarctic forest ecosystems, especially in recently exposed substrates that are strongly limited by nutrient availability in soils. The increasing rates of glacier melting in southern South America is exposing new substrates to microbial colonization, including diazotrophic bacteria. In this environment, largely free of reactive N from atmospheric sources, new ecosystems are rapidly developing on deglaciated surfaces, provided that key elements such as Mo and P and C are present in the substrates.

Variabilidad de la precipitación en la ciudad de Punta Arenas, Chile, desde principios del siglo XX / Rainfall variability at Punta Arenas City, Chile, since beginning of 20th century

Registros de precipitacion instrumental extensos y continuos son escasos en America del Sur, y aun mas en altas latitudes. Estos registros son utiles para contrastar reconstrucciones climaticas del pasado y calibrar modelos climaticos regionales. Pocas ciudades de Chile (e.g. Santiago, Concepcion, Valdivia, Punta Arenas), poseen observaciones meteorologicas extensas desde mediados del siglo XIX o principios del siglo XX. El analisis de estos registros es tambien fundamental para evaluar los cambios recientes en el clima respecto de periodos previos, informacion especialmente util para delinear politicas de adaptacion. Este trabajo analizo la pluviometria de Punta Arenas y tuvo como objetivos: i) evaluar los cambios mensuales, estacionales y anuales de la precipitacion en los periodos 1900-2014 y 1990-2014, ii) evaluar la recurrencia de eventos de sequias, y iii) analizar la relacion entre la precipitacion y el Modo Anular del Sur (SAM). Los resultados evidenciaron una fuerte variabilidad decadal a multi-decadal en la precipitacion anual y en cada estacion del ano. Reducciones significativas se observaron en la precipitacion anual durante el periodo 1900- 2014, siendo mas acentuadas entre 1990- 2014. En este ultimo periodo, la precipitacion de primavera y verano presento una reduccion significativa, sugiriendo una extension de las condiciones estivales. En invierno, un significativo incremento fue observado desde el ano 1990. La recurrencia de eventos extremos de sequias severas tambien se incremento posterior al ano 1990. Relaciones negativas y significativas entre la actividad SAM y la precipitacion anual se observaron en marzo y en el intervalo septiembre-diciembre. La reduccion en la precipitacion de la ciudad de Punta Arenas estaria estrechamente relacionada con la actividad SAM, el cual modularia la variabilidad de las precipitaciones a escalas temporales interanuales e interdecadales. Abstract Extended and continuous instrumental records of precipitation are scarce in South America, even more at high latitudes. These records are useful to validate past climatic reconstructions and calibrate regional climatic models. Few Chilean cities (e.g. Santiago, Concepcion, and Valdivia) present long meteorological observations, since mid-19th century and/or the beginning of 20th century. The analysis of these records is also key to assessing recent changes In the climate with respect to previous periods. This Information is especially useful for delineating adaptation policies. This paper studied the Punta Arenas rainfall record seeking for the following aims: i) to evaluate monthly, seasonal, and annual variations of rainfall records over the 1900- 2014 and 1990-2014 interval, ii) to evaluate the recurrence of drought extreme events in annual precipitation, and iii) to analyze the relationship between precipitation and the Southern Annular Mode (SAM). The findings evidenced strong decadal to multi-decadal variability in the annual and seasonal precipitation. A significant decrease of the annual precipitation was observed across the 1900-2014 period, becoming more severe in the 1990-2014 period. In this last period, spring and summer precipitation shows also a significant negative trend, suggesting an extension of summer conditions. In winter season, a significant increase was observed since 1990. Extreme drought events have increased since the 1990 year as well. Significant and negative relationships were observed between SAM activity and annual precipitation in March, and September to December months. Decreasing rainfall pattern in Punta Arenas is closely associated with the SAM variability at inter-annual to inter-decadal time scales.