José A. Boninsegna

Recent Trends in Tree-Ring Records from High Elevation Sites in the Andes of Northern Patagonia

A new set of tree-ring records from the Andes of northern Patagonia, Argentina (41° S) was used to evaluate recent (i.e., last 250 years) regional trends in tree growth at upper treeline. Fifteen tree-ring chronologies from 1200 to 1750 m elevation were developed for Nothofagus pumilio, the dominant subalpine species. Samples were collected along three elevational transects located along the steep west-to-east precipitation gradient from the main Cordillera (mean annual precipitation >4000 mm) to an eastern outlier of the Andes (mean annual precipitation >2000 mm). Ring-width variation in higher elevation tree-ring records from the main Cordillera is mainly related to changes in temperature and precipitation during spring and summer. However, the response to climatic variation is also influenced by local site factors of elevation and exposure. Based on the relationships between Nothofagus growth and climate, we reconstructed changes in snow cover duration in late spring and variations in mean annual temperature since A.D. 1750. Abrupt interannual changes in the mean annual temperature reconstruction are associated with strong to very strong El Niño-Southern Oscillation events. At upper treeline, tree growth since 1977 has been anomalously high. A sharp rise in global average tropospheric temperatures has been recorded since the mid-1970s in response to an enhanced tropical hydrologic cycle due to an increase in temperature of the tropical Pacific. Temperatures in northern Patagonia have been anomalously high throughout the 1980s, which is consistent with positive temperature anomalies in the tropical Pacific and along the western coast of the Americas at c.a. 40° S latitude. Our 250-year temperature reconstruction indicates that although the persistently high temperatures of the 1980s are uncommon during this period, they are not unprecedented. Tropical climatic episodes similar to that observed during the 1980s may have occurred in the recent past under pre-industrial carbon dioxide levels.

Tree‐ring evidence for long‐term precipitation changes in subtropical South America

In recent years there has been a notable increase in the number of tree-ring chronologies for the temperate and cold regions of the Americas. In comparison, few advances have been reported for the American tropics and subtropics, where the absence of seasonality appears to be the main reason for the lack of well-defined growth bands in most species. Distinct, annually formed tree-rings have recently been reported for subtropical montane trees on the eastern slope of the Andes (22–28°S). Six absolute-dated chronologies from Juglans australis and Cedrela lilloi at the upper treeline (between 1700 and 2000 m) in the montane forest of north-western Argentina were selected to explore the potential of these records to infer decade- to century-scale climatic variations in the subtropics. These tree-rings capture a significant percentage of the variances in regional temperature and precipitation records and appear to be suitable to reconstruct decade-long changes in large-scale circulation over the South American subtropics. In particular, tree-growth at xeric sites has been strongly influenced by precipitation changes, which in turn respond to alternating patterns of zonal versus meridional flows over subtropical South America. The upper treeline records indicate that the increase in precipitation during the past three decades, caused by an enhanced transport of humid air masses from the Brazilian–Bolivian lowland tropics to the semiarid subtropics, has been unprecedented for the past 200 years. Although this precipitation increase may reflect natural variability in the subtropics, it is also consistent with 2×CO2 climatic simulations from five general circulation models. There is a general agreement among model results about a noticeable increase in precipitation in north-western Argentina due to an intensification of the water transport across subtropical South America in response to a southward displacement of the continental low and an increasing warming at these latitudes. A larger network of tree-ring chronologies will aid efforts to understand long-term climatic interactions between tropical and subtropical regions in South America, and shed light on the role of natural versus anthropogenic forcings on regional climatic changes.

Interdecadal climatic variations in millennial temperature reconstructions from southern South America

Two millennial reconstructions of mean summer temperature departures from the eastern and western slopes of the southern Andes (1120 and 3620 years, respectively) have been spectrally analyzed to determine the most significant modes of climate variations in southern South America during the last millennia. Blackman-Tukey and Maximum Entropy spectral analyses of these reconstructions identify temperature oscillations with periods of 35.8, 23.9, 9.5, 6.0–6.9, 5.4–5.7, 4.5–4.8, and 3.4–3.7 years. Because of the relatively flexible standardization used to derive the temperature reconstructions, a large part of the variance (related to the oscillations of summer temperature) is concentrated in short-term periods. A new set of eight long tree-ring chronologies from Fitzroya cupressoides (ranging from 1120 to 3620 years in length), has been used to develop a regional record of tree-ring variations during recent millennia in the southern Andes. A more conservative detrending than that used for developing the temperature reconstructions has been employed here to preserve the low frequency components in the tree-ring series. Maximum Entropy spectral analyses for different filter orders indicate prominent peaks at around 250, 77, 50, 33, 21, 17, 15, and 11 years. Some power is also seen at 4–7 years, probably associated with El Nino-Southern Oscillation events. The 11, 21 and 77-year terms may be related to long-term solar variations or long-term circulation of the Southern Oceans. Ocean-atmosphere model predictions suggest the possibility of using the South American tree-ring records to reconstruct past changes of the thermohaline circulation in the Atlantic Ocean.

Biogeographical Consequences of Recent Climate Changes in the Southern Andes of Argentina

Long-term trends of temperature variations across the Southern Andes (37–55°S) have been recently examined using a combination of instrumental and proxy records. Tree-ring based reconstructions indicate that the annual temperatures during the 20th century have been anomalously warm across the Southern Andes in the context of the past four centuries. The mean annual temperatures for northern and southern Patagonia during the interval 1900–1990 are 0.53°C and 0.86°C above the AD 1640–1899 means, respectively. Increased temperatures are seriously impacting the physical and biological systems across the Southern Andes.

Estructura y estado de conservación de los bosques de Prosopis flexuosa D.C. (Fabaceae, subfamilia: Mimosoideae) en el noreste de Mendoza (Argentina)

La recomendacion de normas de manejo que regulen el uso de los recursos forestales debe estar basada en el conocimiento de la estructura y dinamica de los mismos. El objetivo de este trabajo fue conocer la estructura poblacional de las principales unidades boscosas del bosque de Prosopis flexuosa del noreste de Mendoza, sus condiciones sanitarias y su potencial productivo. Se muestrearon un total de 1.471 algarrobos en las cuatro unidades boscosas mas representativas. La densidad total de algarrobos fue la siguiente: bosque semicerrado de P. flexuosa con Atriplex lampa y Lycium tenuispinosum en valles intermedanos (Bosque 1): 181 arboles ha-1, bosque abierto de P. flexuosa con Trichomaria usillo y Suaeda divaricata en ondulaciones (Bosque 2): 155 arboles ha-1, bosque abierto de P. flexuosa con T. usillo (Bosque 3): 233 arboles ha-1 y bosque abierto de P. flexuosa con A. lampa en ondulaciones suaves (Bosque 4): 215 arboles ha-1. El analisis de componentes principales de la estructura diametrica agrupo los sitios relevados en las distintas unidades boscosas segun la proporcion de arboles de diametro basal mayor a 25 cm. Los sitios del Bosque 1 (mayor proporcion de arboles grandes), se separaron de los sitios con mayor proporcion arboles pequenos (bosques 2 y 4). Debido al habito de crecimiento de los algarrobos, la cantidad de productos maderables de estos bosques es baja. Ademas, P. flexuosa presenta en el area un porcentaje alto de individuos con mas de dos fustes, la forma en muchos casos es decumbente y la altura de los fustes es menor a un metro. Por lo tanto, el potencial forestal del bosque es bajo y el posible aprovechamiento deberia realizarse a escala local, considerando la inclusion de otras actividades complementarias en zonas establecidas para tal fin

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.

Age effects on the climatic signal in Araucaria araucana from xeric sites in Patagonia, Argentina

Background: At different cambial ages, trees experience changes in their structure and interactions with environmental conditions. Reciprocal mechanisms between tree age and physical resources, photosynthetic rates, and xylem production may influence hydraulic resistance and plant water stress. However, it is yet uncertain how these mechanisms are associated with changes in growth sensitivity to biophysical drivers, especially climate. Aim: To establish age-associated climate – growth relationships in growth rings of Araucaria araucana trees from the temperate xeric zones of northern Patagonia, Argentina. Methods: We analysed the growth in 211 A. araucana trees from four sampling sites, in three age classes: young (≤120 years), mature (121–275 years), and old (≥276 years). We explored the correlations between the signal strength of tree growth and climate, based on comparisons between each age-class chronology and monthly mean surface air temperature, total precipitation, and the Southern Annular Mode (SAM) index. Results: The young trees showed higher correlations when their growth was compared with precipitation, air temperature, and the SAM index during austral spring and summer months of the same year. In contrast, growth in mature and old trees showed higher correlations with summer temperatures of the previous growing season. Conclusions: The sensitivity of the radial-growth response of A. araucana to climate varies with age and is strongest in the rings of young trees.

Volcanic Signals in Temperature Reconstructions Based on Tree-Ring Records for North and South America

Publisher Summary High-resolution, well-dated proxy records can extend the climatic information available for analysis for centuries or even millennia. Several reconstructed temperature series covering the past several centuries have been produced for both North, and South America. The injection of dust, sulfur dioxide, and other emissions by explosive volcanic eruptions can affect the temperature at regional, and on occasion global scales. In this chapter, reconstructed temperature series for both hemispheres in a search for signals of volcanic eruptions are compared. The results show a rather weak, but discernible pattern of temperature anomalies, probably linked to volcanic activity in the time, and spatial domains in the Northern Hemisphere. In the Southern Hemisphere, however, no observable pattern can be related to the set of explosions. Only very few eruptions, particularly those produced by southern volcanoes, seem to have some effect there. The relationships between the sulfate volcanic loads, and the reconstructed temperature series are also discussed in the chapter.

Growth rates in diameter, basal area, and height of Pilgerodendron uviferum; relationship between growth index and germination

SummaryIncrement borer samples taken at breast height in Pilgerodendron uviferum (Cupressaceae) trees growing in two Chilean forest stands near latitude 42° south were used to study growth rates in diameter, basal area, and height. Radial average growth is 0.51 mm/year in Santa Luciă 0.44 mm/year in the Piuchué stand; the correlation between diameter and age is 0.79 in Santa Lucía and 0.64 in Piuchué. Similar results were obtained in the basal area/age relationship. Both groups of trees have a low rate of growth compared to rates obtained in other studies. Maximum values for radial growth (culmination age) are reached at 110 years of age in Santa Lucia and 50 years in Piuchue. Growth in height, averaging 5.8 cm/year, appears to be similar in all trees analyzed; occasional differences can be attributed to understory position of some trees. Comparison of radial growth and germination of Pilgerodendron plants shows a synchronous relationship, suggesting an influence of climate on the pattern of forest regeneration.