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The variability of water year precipitation and selected blue oak tree-ring chronologies in California are both dominated by heavy precipitation delivered during just a few days each year. These heavy precipitation events can spell the difference between surplus or deficit water supply and elevated flood risk. Some blue oak chronologies are highly correlated with water year precipitation (r = 0.84) but are equally well correlated (r = 0.82) with heavy precipitation totals ≥25.4 mm (1 in., ≈95th percentile of daily totals, 1949–2004). The blue oak correlation with nonheavy daily totals is much weaker (r = 0.55). Consequently, some blue oak chronologies represent selective proxies for the temporal and spatial variability of heavy precipitation totals and are used to reconstruct the amount and number of days with heavy precipitation in northern California from 1582 to 2021. Instrumental and reconstructed heavy precipitation totals are strongly correlated with gridded atmospheric river–related precipitation over the western United States, especially in central California. Spectral analysis indicates that instrumental heavy precipitation totals may be dominated by high-frequency variability and the nonheavy totals by low-frequency variance. The reconstruction of heavy precipitation is coherent with instrumental heavy totals across the frequency domain and include concentrations of variance at ENSO and biennial frequencies. Return period analyses calculated using instrumental heavy precipitation totals are representative of the return periods in the blue oak reconstruction despite the large differences in series length. Decadal surges in the amount, frequency, and interannual volatility of heavy precipitation totals are reconstructed, likely reflecting episodes of elevated atmospheric river activity in the past. Significance Statement Tree-ring chronologies of blue oak are highly correlated with precipitation delivered to northern California during just the heaviest days of precipitation each year. The reconstruction of heavy precipitation indicates decadal episodes with a high frequency of extreme precipitation. These episodes of frequent heavy precipitation likely arose because of elevated atmospheric river activity and are relevant to the analysis of water supply and flood hazard in California.

Antecedentes: Los modelos para determinar la idoneidad de hábitat de especies arbóreas es actualmente una herramienta geoespacial relevante en la toma de decisiones. Hipótesis: La idoneidad de hábitat actual y futura permite identificar zonas potenciales para conservación del ahuehuete en México. Especie de estudio: Taxodium mucronatum Ten. Sitio de estudio: México. Métodos: Se emplearon registros de ahuehuete y variables climáticas, edafológicas y topográficas, las cuales se procesaron a través del algoritmo MaxEnt; se estimaron superficies actuales y futuras para los escenarios 2050, 2070 con dos criterios de distribución, idoneidad de hábitat y ecosistema actual o ripario y se identificaron áreas potenciales para conservación. Resultados: Se estimó una superficie de 11,217,706.2 y 48,958.5 ha como áreas de idoneidad de hábitat, bajo los dos criterios, respectivamente. La variable más importante en el modelo fue la temperatura promedio del trimestre más frío. El modelo predice una reducción en superficie potencial de 2,989,580.9 ha y de 7,158,9829.96 ha para 2050 y 2070, respectivamente. La reducción estimada en la superficie actual es de 12,415.3 ha y 32,037.1 ha, para 2050 y 2070, respectivamente. Se identificaron diversas zonas para conservación en los estados de Puebla, Guanajuato, Estado de México y Chiapas. Conclusiones: En el presente estudio, se generó una propuesta para selección de sitios y desarrollo de actividades de conservación y restauración del ahuehuete en México, mediante el algoritmo MaxEnt.

Season-to-season persistence of soil moisture drought varies across North America. Such interseasonal autocorrelation can have modest skill in forecasting future conditions several months in advance. Because robust instrumental observations of precipitation span less than 100 years, the temporal stability of the relationship between seasonal moisture anomalies is uncertain. The North American Seasonal Precipitation Atlas (NASPA) is a gridded network of separately reconstructed cool-season (December–April) and warm-season (May–July) precipitation series and offers new insights on the intra-annual changes in drought for up to 2000 years. Here, the NASPA precipitation reconstructions are rescaled to represent the long-term soil moisture balance during the cool season and 3-month-long atmospheric moisture during the warm season. These rescaled seasonal reconstructions are then used to quantify the frequency, magnitude, and spatial extent of cool-season drought that was relieved or reversed during the following summer months. The adjusted seasonal reconstructions reproduce the general patterns of large-scale drought amelioration and termination in the instrumental record during the twentieth century and are used to estimate relief and reversals for the most skillfully reconstructed past 500 years. Subcontinental-to-continental-scale reversals of cool-season drought in the following warm season have been rare, but the reconstructions display periods prior to the instrumental data of increased reversal probabilities for the mid-Atlantic region and the U.S. Southwest. Drought relief at the continental scale may arise in part from macroscale ocean–atmosphere processes, whereas the smaller-scale regional reversals may reflect land surface feedbacks and stochastic variability.

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Mean daily to monthly precipitation averages peak in late July over eastern Colorado and some of the most damaging Front Range flash floods have occurred because of extreme 1-day rainfall events during this period. Tree-ring chronologies of adjusted latewood width in ponderosa pine from eastern Colorado are highly correlated with the highest 1-day rainfall totals occurring during this 2-week precipitation maximum in late July. A regional average of four adjusted latewood chronologies from eastern Colorado was used to reconstruct the single wettest day observed during the last two weeks of July. The regional chronology was calibrated with the CPC 0.25° × 0.25° Daily U.S. Unified Gauge-Based Analysis of Precipitation dataset and explains 65% of the variance in the highest 1-day late July precipitation totals in the instrumental data from 1948 to 1997. The reconstruction and instrumental data extend fully from 1779 to 2019 and indicate that the frequency of 1-day rainfall extremes in late July has increased since the late eighteenth century. The largest instrumental and reconstructed 1-day precipitation extremes are most commonly associated with the intrusion of a major frontal system into a deep layer of atmospheric moisture across eastern Colorado. These general synoptic conditions have been previously linked to extreme localized rainfall totals and widespread thunderstorm activity over Colorado during the summer season. Chronologies of adjusted latewood width in semiarid eastern Colorado constitute a proxy of weather time-scale rainfall events useful for investigations of long-term variability and for framing natural and potential anthropogenic forcing of precipitation extremes during this 2-week precipitation maximum in a long historical perspective.

Instrumental records indicate a century-long trend towards drying over western North America and wetting over eastern North America. A continuation of these trends into the future would have significant hydroclimatic and socioeconomic consequences in both the semi-arid Southwest and humid East. Using tree-ring reconstructions and hydrologic simulations of summer soil moisture, we evaluate and contextualize the modern summer aridity gradient within its natural range of variability established over the past 600 years and evaluate the effects of observed and anthropogenic precipitation, temperature, and humidity trends. The 2001–2020 positive (wet east-dry west) aridity gradient was larger than any 20 year period since 1400 CE, preceded by the most negative (wet west-dry east) aridity gradient during 1976–1995, leading to a strong multi-decade reversal in aridity gradient anomalies that was rivaled only by a similar event in the late-16th century. The 2001–2020 aridity gradient was dominated by long-term summer precipitation increases in the Midwest and Northeast, with smaller contributions from more warming in the West than the East and spring precipitation decreases in the Southwest. Multi-model mean climate simulations from Coupled Model Intercomparison Project 6 experiments suggest anthropogenic climate trends should not have strongly affected the aridity gradient thus far. However, there is high uncertainty due to inter-model disagreement on anthropogenic precipitation trends. The recent strengthening of the observed aridity gradient, its increasing dependence on precipitation variability, and disagreement in modeled anthropogenic precipitation trends reveal significant uncertainties in how water resource availability will change across North America in the coming decades.

La cuenca hidrológica Lerma-Chapala-Santiago que drena hacia el Lago de Chapala ha sido intensamente impactada por las actividades humanas, lo cual ha alterado el ciclo hidrológico y afectado la recuperación anual del mismo. Para determinar la fluctuación histórica de sus volúmenes se desarrolló una red de nueve cronologías de poblaciones de Taxodium mucronatum distribuidas en bosques de galería de dicha cuenca. Las series de tiempo se analizaron mediante Componentes Principales y ello derivó en una cronología regional de 547 años (1462-2008). La asociación entre los índices dendrocronológicos y los datos del restablecimiento de los niveles del lago del año previo fue significativo, por lo que se creó un modelo de regresión para su reconstrucción. Los valores reconstruidos mostraron una alta variabilidad interanual y multianual que se asocia con sucesos extremos de El Niño-Oscilación del Sur. No obstante, el cambio de uso del suelo y el aprovechamiento indiscriminado del agua para diversos fines, en las últimas décadas, han enmascarado su efecto. Sequías de alta intensidad se detectaron en los períodos 1508-1560, 1581-1608, 1685-1725, 1770-1840, 1916-1924 y 1988-2000. De igual manera, algunas etapas húmedas notables ocurrieron en 1561-1578, 1610-1616, 1760-1769, 1842-1850, 1863-1893 y 1926-1963. De 1960 a la fecha, la recuperación anual del lago disminuyó a causa de la influencia humana. El entendimiento de las oscilaciones históricas en la profundidad del lago permitirá fundamentar acciones para un mejor uso del agua y la conservación de los ecosistemas rivereños que dependen de los recursos hídricos generados en la cuenca Lerma-Chapala-Santiago para su supervivencia.

The two most severe droughts to impact the Great Plains in the twentieth century, the 1930s Dust Bowl and 1950s Drought, were the result of multiyear moisture deficits during the spring and especially the summer season. Tree-ring reconstructions of the Palmer Drought Severity Index indicate similar droughts in magnitude have occurred in previous centuries, but these reconstructions do not capture the potential distinct seasonal drought characteristics like those of the 1930s and 1950s. Separate tree-ring reconstructions of the spring and summer Z-index based on earlywood, latewood, and adjusted latewood width chronologies have been developed for two regions in the northern and southern Great Plains of the US. The reconstructions extend from 1651 to 1990 and 1698–1990, respectively, with instrumental data added from 1991 to 2017. The four reconstructions explain from 39 to 56% of the variance during the 1945–1990 calibration interval and are significantly correlated with independent moisture balance observations during the 1900–1944 validation period. The reconstructions reproduce similar seasonal sea-surface temperature and 500 mb geopotential height spatial correlation patterns detected with the instrumental data. The 1930s is estimated to have been the most extreme decadal summer drought to impact the two regions concurrently in the last few centuries. On average, spring moisture deficits were more severe during the multidecadal droughts of the mid- to late-nineteenth century, but the timing of drought onset and termination differed between the study regions. In the recent two decades the spring moisture balances for the two study regions have largely been opposite, and this has been one of the most extreme periods of anti-phasing in the last few centuries. Seasonal moisture reversals are not randomly distributed in time based on the reconstructed estimates and are related to sea-surface temperature anomalies in the tropical Pacific and to mid-tropospheric circulation changes over the North Pacific–North American sector during May and June.

Land surface feedbacks impart a significant degree of persistence between cool and warm season moisture availability in the central United States. However, the degree of correlation between these two variables is subject to major changes that appear to occur on decadal to multidecadal time scales, even in the relatively short 115-yr instrumental record. Tree-ring reconstructions have extended the limited observational record of long-term soil moisture levels, but such reconstructions do not resolve the seasonal differences in moisture conditions. We present two separate 331-yr-long seasonal moisture reconstructions for the central United States, based on sensitive subannual and annual tree-ring chronologies that have strong and separate seasonal moisture signals: an estimate of the long-term May soil moisture balance and a second estimate of the short-term June–August atmospheric moisture balance. The predictors used in each seasonal reconstruction are not significantly correlated with the alternate season target. Both reconstructions capture over 70% of the interannual variance in the instrumental data for the calibration period and also share significant decadal and multidecadal variability with the instrumental record in both the calibration and validation periods. The instrumental and reconstructed moisture levels are both positively correlated between spring and summer strongly enough to have potential value in seasonal prediction. However, the relationship between spring and summer moisture exhibits major decadal changes in strength and even sign that appear to be related to large-scale ocean–atmosphere dynamics associated with the Atlantic multidecadal oscillation.

The Sao Francisco River basin is one of the most drought-prone regions of Brazil. Seasonally dry tropical forests (SDTF) are widely distributed in the basin and we developed a short chronology of Cedrela fissilis annual ring widths from SDTF fragments based on 89 cores from 44 trees dating from 1961 to 2015. The average correlation among all radii (RBAR) is 0.52. The tree-ring chronology is correlated with wet season precipitation totals, must strongly and consistently near the beginning of the wet season. The spatial pattern of correlation covers most of the southern portion of the Brazilian Drought Polygon and the sub-basins of the two largest tributaries of the Sao Francisco River, in some areas exceeding r = 0.60. The chronology is also correlated with total annual discharge of the Rio Sao Francisco River measured at Barra (r = 0.489; 1961–2015), which is very promising in a country that generates two thirds of its electricity from hydroelectric power plants, particularly if this short chronology can be extended with trees exceeding 150-years old known to still exist in the region.

El sabino, Taxodium, es el taxon dominante en bosques de galería del río San Pedro-Mezquital (RSPM), RH1. En México, no existe información de la respuesta de esta especie con volúmenes de caudales modificados. En el RSPM, el flujo original ha sido almacenado en represas construidas de 1950 a 1989; por lo tanto, los datos hidrométricos son artificiales. El objetivo del presente estudio fue analizar la asociación entre el crecimiento anual de especímenes colectados en un transecto de 20 km en el RSPM y el flujo artificial registrado. Los resultados indican que para la estación “El Saltito”, los sabinos responden significativamente (r = 0.72, n= 16, 1967-1986, p

The Amazon basin is a global center of hydroclimatic variability and biodiversity, but there are only eight instrumental rainfall stations with continuous records longer than 80 years in the entire basin, an area nearly the size of the coterminous US. The first long moisture-sensitive tree-ring chronology has been developed in the eastern equatorial Amazon of Brazil based on dendrochronological analysis of Cedrela cross sections cut during sustainable logging operations near the Rio Paru. The Rio Paru chronology dates from 1786 to 2016 and is significantly correlated with instrumental precipitation observations from 1939 to 2016. The strength and spatial scale of the precipitation signal vary during the instrumental period, but the Rio Paru chronology has been used to develop a preliminary reconstruction of February to November rainfall totals from 1786 to 2016. The reconstruction is related to SSTs in the Atlantic and especially the tropical Pacific, similar to the stronger pattern of association computed for the instrumental rainfall data from the eastern Amazon. The tree-ring data estimate extended drought and wet episodes in the mid- to late-nineteenth century, providing a valuable, long-term perspective on the moisture changes expected to emerge over the Amazon in the coming century due to deforestation and anthropogenic climate change.

Moisture sensitive tree-ring chronologies of Centrolobium microchaete have been developed from seasonally dry forests in the southern Amazon basin and used to reconstruct wet season rainfall totals from 1799-2012, adding over 150-years of rainfall estimates to the short instrumental record for the region. The reconstruction is correlated with the same atmospheric variables that influence the instrumental measurements of wet season rainfall. Anticyclonic circulation over mid-latitude South America promotes equatorward surges of cold and relatively dry extratropical air that converge with warm moist air to form deep convection and heavy rainfall over this sector of the southern Amazon basin. Interesting droughts and pluvials are reconstructed during the pre-instrumental 19th and early 20th centuries, but the tree-ring reconstruction suggests that the strong multi-decadal variability in instrumental and reconstructed wet season rainfall after 1950 may have been unmatched since 1799.

Human-driven climate warming worsens an otherwise moderate drought

Mexico has suffered a long history and prehistory of severe sustained drought. Drought over Mexico is modulated by ocean-atmospheric variability in the Atlantic and Pacific, raising the possibility for long-range seasonal climate forecasting, which could help mediate the economic and social impacts of future dry spells. The instrumental record of Mexican climate is very limited before 1920, but tree-ring chronologies developed from old-growth forests in Mexico can provide an excellent proxy representation of the spatial pattern and intensity of past moisture regimes useful for the analysis of climate dynamics and climate impacts. The Mexican Drought Atlas (MXDA) has been developed from an extensive network of 252 climate sensitive tree-ring chronologies in and near Mexico. The MXDA reconstructions extend from 1400 CE–2012 and were calibrated with the instrumental summer (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) on a 0.5° latitude/longitude grid extending over land areas from 14 to 34°N and 75–120°W using Ensemble Point-by-Point Regression (EPPR) for the 1944–1984 period. The grid point reconstructions were validated for the period 1920–1943 against instrumental gridded scPDSI values based on the fewer weather station observations available during that interval. The MXDA provides a new spatial perspective on the historical impacts of moisture extremes over Mexico during the past 600-years, including the Aztec Drought of One Rabbit in 1454, the drought of El Ano de Hambre in 1785–1786, and the drought that preceded the Mexican Revolution of 1909–1910. The El Nino/Southern Oscillation (ENSO) is the most important ocean-atmospheric forcing of moisture variability detected with the MXDA. In fact, the reconstructions suggest that the strongest central equatorial Pacific sea surface temperature (SST) teleconnection to the soil moisture balance over North America may reside in northern Mexico. This ENSO signal has stronger and more time-stable correlations than computed for either the Atlantic Multidecadal Oscillation or Pacific Decadal Oscillation. The extended Multivariate ENSO Index is most highly correlated with reconstructed scPDSI over northern Mexico, where warm events favor moist conditions during the winter, spring, and early summer. This ENSO teleconnection to northern Mexico has been strong over the past 150 years, but it has been comparatively weak and non-stationary in the MXDA over central and southern Mexico where eastern tropical Pacific and Caribbean/tropical Atlantic SSTs seem to be more important. The ENSO teleconnection to northern Mexico is weaker in the available instrumental PDSI, but analyses based on the millennium climate simulations with the Community Earth System Model suggest that the moisture balance during the winter, spring, and early summer over northern Mexico may indeed be particularly sensitive to ENSO forcing. Nationwide drought is predicted to become more common with anthropogenic climate change, but the MXDA reconstructions indicate that intense “All Mexico” droughts have been rare over the past 600 years and their frequency does not appear to have increased substantially in recent decades.

We investigated an old-growth oak-shortleaf pine (Pinus echinata) stand of high ecological integrity in east-central Oklahoma located west of the continuous native distribution of shortleaf pine. With the exception of an abundance of shortleaf pine, the basal area (17.2 m2/ha), density (559.6 trees/ha), and species composition of this unmanaged stand is similar to other old hardwood forests in the Cross Timbers ecoregion. Tree-ring dating indicates that the oldest post oak (Quercus stellata) and shortleaf pine sampled in Shortleaf Canyon were 262 and 230 yr old, respectively. Dendrochronology indicated that drought-related disturbances contributed to several episodic pulses of natural regeneration of shortleaf pine at this site. Coupled with local historical documentation and regional paleoecological evidence, the absence of older pines at this isolated site suggests that P. echinata may have naturally colonized Shortleaf Canyon recently, perhaps during the last 250 yr.

High-resolution biogenic and geologic proxies in which one increment or layer is formed per year are crucial to describing natural ranges of environmental variability in Earth's physical and biological systems. However, dating controls are necessary to ensure temporal precision and accuracy; simple counts cannot ensure that all layers are placed correctly in time. Originally developed for tree-ring data, crossdating is the only such procedure that ensures all increments have been assigned the correct calendar year of formation. Here, we use growth-increment data from two tree species, two marine bivalve species, and a marine fish species to illustrate sensitivity of environmental signals to modest dating error rates. When falsely added or missed increments are induced at one and five percent rates, errors propagate back through time and eliminate high-frequency variability, climate signals, and evidence of extreme events while incorrectly dating and distorting major disturbances or other low-frequency processes. Our consecutive Monte Carlo experiments show that inaccuracies begin to accumulate in as little as two decades and can remove all but decadal-scale processes after as little as two centuries. Real-world scenarios may have even greater consequence in the absence of crossdating. Given this sensitivity to signal loss, the fundamental tenets of crossdating must be applied to fully resolve environmental signals, a point we underscore as the frontiers of growth-increment analysis continue to expand into tropical, freshwater, and marine environments.

During the Medieval Climate Anomaly (MCA), Western North America experienced episodes of intense aridity that persisted for multiple decades or longer. These megadroughts are well documented in many proxy records, but the causal mechanisms are poorly understood. General circulation models (GCMs) simulate megadroughts, but do not reproduce the temporal clustering of events during the MCA, suggesting they are not caused by the time history of volcanic or solar forcing. Instead, GCMs generate megadroughts through (1) internal atmospheric variability, (2) sea-surface temperatures, and (3) land surface and dust aerosol feedbacks. While no hypothesis has been definitively rejected, and no GCM has accurately reproduced all features (e.g., timing, duration, and extent) of any specific megadrought, their persistence suggests a role for processes that impart memory to the climate system (land surface and ocean dynamics). Over the 21st century, GCMs project an increase in the risk of megadrought occurrence through greenhouse gas forced reductions in precipitation and increases in evaporative demand. This drying is robust across models and multiple drought indicators, but major uncertainties still need to be resolved. These include the potential moderation of vegetation evaporative losses at higher atmospheric [CO2], variations in land surface model complexity, and decadal to multidecadal modes of natural climate variability that could delay or advance onset of aridification over the the next several decades. Because future droughts will arise from both natural variability and greenhouse gas forced trends in hydroclimate, improving our understanding of the natural drivers of persistent multidecadal megadroughts should be a major research priority.

Rainfall and river levels in the Amazon are associated with significant precipitation anomalies of opposite sign in temperate North and South America, which is the dominant mode of precipitation variability in the Americas that often arises during extremes of the El Niño/Southern Oscillation (ENSO). This co-variability of precipitation extremes across the Americas is imprinted on tree growth and is detected when new tree-ring chronologies from the eastern equatorial Amazon are compared with hundreds of moisture-sensitive tree-ring chronologies in mid-latitude North and South America from 1759 to 2016. Pan-American co-variability exists even though the seasonality of precipitation and tree growth only partially overlaps between the Amazon and mid-latitudes because ENSO forcing of climate can persist for multiple seasons and can orchestrate a coherent response, even where the growing seasons are not fully synchronized. The tree-ring data indicate that the El Niño influence on inter-hemispheric precipitation and tree growth extremes has been strong and stable over the past 258-years, but the La Niña influence has been subject to large multi-decadal changes. These changes have implications for the dynamics and forecasting of hydroclimatic variability over the Americas and are supported by analyses of the available instrumental data and selected climate model simulations. Fil: Stahle, D.W.. University of Arkansas for Medical Sciences; Estados Unidos Fil: Torbenson, Max Carl Arne. Ohio State University; Estados Unidos Fil: Howard, I. M.. University of Arkansas for Medical Sciences; Estados Unidos Fil: Granato Souza, D.. University of Arkansas for Medical Sciences; Estados Unidos Fil: Barbosa, A. C.. Universidad Federal de Lavras; Brasil Fil: Feng, S.. University of Arkansas for Medical Sciences; Estados Unidos Fil: Schöngart, J.. National Institute For Amazon Research; Brasil Fil: Lopez Callejas, Lidio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Villanueva, J.. Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias; México Fil: Fernandes, K.. Columbia University; Estados Unidos

This study characterized associations between climate variables and radial growth of four oak species at sites distributed across central and eastern North America. Tree-ring data were obtained from 24, 29, 33, and 55 sites for Quercus prinus L., Quercus velutina Lam., Quercus macrocarpa Michx., and Quercus stellate Wangenh., respectively. Pearson’s correlation coefficients were computed between radial growth and monthly and seasonal temperature and precipitation. Growth was most strongly and consistently correlated with precipitation and temperature during the early growing season (May to July). Coincident positive correlations with precipitation and negative correlations with temperature indicate that this relationship is mediated by site water balance. The combination of this plausible cause–effect mechanism and extensive spatial replication of these correlations suggest that they reflect cause–effect relationships. Growth of Q. stellata was correlated with precipitation during the dormant season, suggesting that stored soil water is important for growth of this species in the southern Great Plains. Despite substantial spatial variation in temperature and growing-season initiation between sites in Texas and Manitoba, Canada, there was little variation in the phenology of growth–climate associations; growth–climate correlations were strongest during the same May–July period at all sites. Results of this study support the hypothesis that temperate zone ring-porous oak species have similar phenology of growth–climate correlations and can be treated as a biologically meaningful functional group in forest simulation models.

The hypothesis that a prolonged drought across southwestern North America in the late thirteenth century contributed to the abandonment of the region by Ancestral Pueblo populations, ultimately including the depopulation of the Mesa Verde region, continues to be a focus of archaeological research in the Pueblo region. We address the hypothesis through the re-measurement of tree-ring specimens from living trees and archaeological wood at Mesa Verde, Colorado, to derive chronologies of earlywood, latewood, and total ring width. The three chronology types all date from AD 480 to 2008 and were used to separately reconstruct cool and early warm season effective moisture and total water-year precipitation for Chapin Mesa near many of the major prehistoric archaeological sites. The new reconstructions indicate three simultaneous cool and early growing season droughts during the twelfth and thirteenth centuries that may have contributed to the environmental and social factors behind Ancestral Pueblo migrations over this sector of the Colorado Plateau. These sustained inter-seasonal droughts included the “Great Drought” of the late-thirteenth century, which is estimated to have been one of the most severe regimes of cool and early summer drought in the last 1,500-years and coincided with the end of Puebloan occupations at Mesa Verde. The elevation of the 30 cm isohyet of water-year precipitation reconstructed for southwestern Colorado from the new ring-width data is mapped from AD 1276–1280 and identifies areas where dry-land cultivation of maize may not have been practical during the driest years of the Great Drought. There is no doubt about the exact dating of the tree-ring chronologies, but the low sample size of dated specimens from Mesa Verde during the late-thirteenth and fourteenth centuries contributes uncertainty to these environmental reconstructions at the time of abandonment.

Rings of ocean upwelling Coastal upwelling along the coast of California has become more variable than during nearly any period in the past 600 years. Black et al. used a 576-year tree ring record to construct a record of wintertime climate along the California coast. Because wintertime climate and coastal upwelling are so closely linked there, they were able to determine that upwelling variability has increased more over the past 60 years than for all but two intervals during that time. The apparent causes of the recent trend appear to be unique, resulting in reduced marine productivity and negative impacts on fish, seabirds, and mammals. Science , this issue p. 1498

Epidemic typhus is a serious infectious disease caused by the obligate, intracellular, gram-negative bacterium Rickettsia prowazekii and transmitted by body lice (Pediculus humanus corporis). This disease is recognized for its high mortality rate throughout human history, particularly before modern sanitary practices and the availability of antimicrobial drugs (1). Typhus spreads where conditions are crowded and unsanitary. Historical epidemics often followed war, climate extremes, famine, and social upheaval. Zinsser (2) noted that throughout history, epidemic typhus might have claimed more human lives during war than combat. Epidemic typhus could reemerge as a serious infectious disease in areas of the world where social strife and underdeveloped public health programs persist (3). Typhus was first recorded in Mexico in 1655, and the most recent major epidemic began in 1915 during the Mexican revolution. Mexico’s rich historical record of epidemic disease is documented in archives of demographic data that include census records, health records, death certificates, and accounts of physicians. Mexico City and the high, densely populated valleys of central Mexico were particularly susceptible to smallpox, cholera, and typhus epidemics because of crowding and poor sanitation (4). Numerous epidemics, some identified as typhus, occurred during the colonial and early modern eras. We have compiled a record of 22 typhus epidemics in Mexico during 1655–1918. We compared the timing of these typhus epidemics with tree-ring reconstructions of growing-season moisture conditions to assess the relationship between climate and typhus during this period.

The meteorological factors involved in the formation of earlywood frost rings in Rocky Mountain bristlecone pine (Pinus aristata) have not been described in detail. This study used 51 tree-ring dated Rocky Mountain bristlecone pine trees growing at ca. 3500 m a.s.l. on Mt. Goliath, Colorado, to develop earlywood and latewood frost ring chronologies dating from 1930 to 2010 for investigation of the regional and large-scale weather anomalies responsible for these unusual growing season freeze events. The high-elevation meteorological station at Niwot Ridge, Colorado, was used to document the daily temperature anomalies most likely associated with these frost-damaged rings. NCEP-NCAR Reanalysis data were used to examine the synoptic meteorological conditions that tend to prevail during these unusual growing season temperature conditions. Earlywood frost rings occur during anomalous late-May and June freeze events in the Colorado Rockies associated with unseasonal mid-latitude circulation, including the penetration of a deep upper-level low pressure system and cold surface air temperatures into the west-central United States. The three latewood frost rings all occurred during September freeze events also associated with unseasonal and highly amplified mid-latitude circulation. The chronology of these early and late growing season freeze events may provide a useful independent check on daily temperature minima estimated with reanalysis techniques, and they can be extended into the pre-instrumental era thanks to the great age of Rocky Mountain bristlecone pine. Frost damage in Mt. Goliath bristlecone pine appears to be most frequent and severe in young trees found in the depressed tree line below a large cirque subject to intense cold air drainage. The development of the most detailed tree-ring records of past freeze events may therefore benefit from site selection in these cold air drainages, along with age-stratified tree sampling to ensure that the young and most frost susceptible age classes are well represented throughout the chronology.

Bald cypress trees over 2,000-years old have been discovered in the forested wetlands along Black River using dendrochronology and radiocarbon dating. The oldest bald cypress yet documented is at least 2,624-years old, making Taxodium distichum the oldest-known wetland tree species, the oldest living trees in eastern North America, and the fifth oldest known non-clonal tree species on earth. The annual ring-width chronology developed from the ancient Black River bald cypress trees is positively correlated with growing season precipitation totals over the southeastern United States and with atmospheric circulation over the Northern Hemisphere, providing the longest exactly-dated climate proxy yet developed in eastern North America. The Nature Conservancy owns 6,400 ha in their Black River Preserve and the North Carolina legislature is considering establishment of a Black River State Park, but ancient forested wetlands are found along most of this 106 km stream and remain threatened by logging, water pollution, and sea level rise.

Freshwater discharge into Albemarle Sound and many other large estuaries can have a major impact on the physical and biological properties of the estuary. Earlywood (EW) and latewood (LW) width tree ring chronologies recently developed from ancient trees and sub-fossil logs found at Devil’s Gut of the Roanoke River, North Carolina, were used with new EW and LW width data from Blackwater River, Virginia, to reconstruct the July Palmer Hydrological Drought Index (PHDI) over the drainage basin of Albemarle Sound from 934 to 2005. This reconstruction is based on a principal component regression using millennium-long chronologies of both EW and LW widths, calibrated and verified against instrumental July PHDI from 1895 to 1985. The reconstruction provides a lengthy representation of inter-annual to multi-decadal growing season moisture variability that directly affects water quality in the Albemarle Sound estuarine ecosystem. Single-year drought and wetness extremes were more common in the modern instrumental period (1895–2005), but prolonged droughts reconstructed during the Medieval and Early Colonial eras were more severe and sustained than the reconstructed or observed droughts during the twentieth century. Several abrupt transitions between decadal drought and wetness regimes are reconstructed, especially during the Medieval Era. Composite analysis of lower-magnitude sub-decadal regime shifts in the instrumental record during the twentieth century indicates that major changes in mid-latitude circulation over the Northern Hemisphere may have been involved in these longer decadal transitions between moisture regimes over Albemarle Sound in prehistory.

A computer program, Historical Observation Tools (HOB Tools), has been developed to facilitate many of the calculations used by historical climatologists to develop instrumental and documentary temperature and precipitation datasets and makes them readily accessible to other researchers. The primitive methodology used by the early weather observers makes the application of standard techniques difficult. HOB Tools provides a step-by-step framework to visually and statistically assess, adjust, and reconstruct historical temperature and precipitation datasets. These routines include the ability to check for undocumented discontinuities, adjust temperature data for poor thermometer exposures and diurnal averaging, and assess and adjust daily precipitation data for undercount. This paper provides an overview of the Visual Basic.NET program and a demonstration of how it can assist in the development of extended temperature and precipitation datasets using modern and early instrumental measurements from the United States.

[1] The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June–August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October–April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century.

Three new 159-year long reconstructions of spring, summer, and growing season precipitation totals were developed for northeastern Kansas and northwestern Missouri from five station clusters (Lawrence, Leavenworth, and Manhattan, Kansas; Miami and Oregon, Missouri). Nonstandard observation practices are inherent in the early meteorological data, which can induce an undercount in precipitation measurements, particularly during the cool season. Threshold analyses of these five station clusters indicated undercount can be lessened for daily precipitation totals of 0.50 in. and greater during the warm season (“half-inch threshold”). Therefore, “adjusted reconstructions” of total precipitation for the spring (AMJ), summer (JA), and growing season (AMJJA) were derived using the “half-inch threshold” totals and an estimate of the missing amount between 0.00 and 0.50 in. based on an average of the modern observations at each station (or the nearest available station). The new precipitation reconstructions suggest that the most severe spring drought may have occurred during the mid-19th century, although the potential for undercount is likely highest during the spring season. The most severe summer precipitation deficit is estimated during the 1930s Dust Bowl drought, followed by the summer drought of the 1910s. When precipitation is totaled for the entire growing season, the mid-19th century and Dust Bowl droughts were of approximately equal magnitude and duration in this reconstruction. However, the integration of precipitation and temperature into seasonal measures of effective moisture, using a new 19th century temperature reconstruction for northeastern Kansas, indicates that the 1930s growing season moisture deficit was the most severe and sustained since 1855, highlighting the extraordinarily high temperatures recorded during the 1930s Dust Bowl drought.

The Rio Grande is a major source of water for parts of Mexico and the USA. The 2 main source regions for the Rio Grande system are the San Juan Mountains of the southern Rocky Mountains and the Sierra Madre Occidental in Mexico, which is the headwaters for the Rio Con- chos, the largest tributary of the Rio Grande. Precipitation and streamflow from these 2 source regions are largely independent of each other; winter snowpack is the dominant contributor to the annual streamflow north of the USAMexico border, and the North American monsoon is a key factor in the Rio Conchos basin. Reconstructions of water year (OctoberSeptember) streamflow for a gauge in the upper Rio Grande, 1508�2002, and of OctoberJuly precipitation in the Rio Con- chos watershed region, 1649�1993, also indicate a lack of correlation between the 2 basins over century time scales. Despite this lack of correlation, periods of concurrent multiyear drought have occurred over the past 4 centuries, most notably in the 1770s, 1890s and 1950s. These rare concur- rent droughts in the upper Rio Grande and Rio Conchos source regions may arise from large-scale forcing out of the Pacific Ocean and will be relevant to the binational planning of these water resources, which serve a large and growing population of users.

Ancient baldcypress trees found in wetland and riverine environments have been used to develop a network of exactly dated annual ring-width chronologies extending from the southeastern United States, across Mexico, and into western Guatemala. These chronologies are sensitive to growing season precipitation in spite of frequently flooded site conditions, and have been used to reconstruct moisture levels the southeastern United States and Mexico for over 1000 years. The El Nino/Southern Oscillation (ENSO) is a major influence on the climate reconstructions derived from these baldcypress chronologies, especially in Mexico where some of the most extreme reconstructed droughts occurred during El Nino events. In the Southeast, the ENSO influence on climate and tree growth changes sign from spring to summer, and this change in dynamical forcing is recorded by sub-seasonal chronologies of earlywood and latewood width. Most existing baldcypress chronologies have been extended with tree-ring data from “subfossil” wood recovered from surface and submerged deposits. Well-preserved subfossil logs have also been recovered in quantity from buried deposits of great age, and may permit development of long continuously dated Holocene chronologies and discontinuous “floating” Pleistocene chronologies. The extensive subfossil baldcypress swamp discovered 6 m below the streets of Washington D.C. was overrun by a transgression of the Potomac estuary, possibly during the previous super interglacial (marine OIS 5e), and provides direct evidence for one potential impact of unmitigated anthropogenic warming and sea level rise.

As part of the Lakeport Plantation Restoration Project conducted by Arkansas State University, we examined tree-ring samples of baldcypress (Taxodium distichum L. Rich.) timbers from the Lakeport Plantation house in Chicot County, Arkansas. Our objectives for the study were to: (1) determine cutting dates of timbers used in the construction of the plantation house and an ancillary log shed in order to support or refute available historical and archaeological evidence for the construction date of the structures, and (2) provide tree-ring data to improve the spatial and temporal tree-ring record for the region. We determined that virtually all the cutting dates for the plantation house were confined to the dormant season of 1858–1859 suggesting that cutting and construction occurred at approximately the same time. We positively dated a total of 25 samples against the exactly-dated master chronology based on living baldcypress trees at Black Swamp, Arkansas, and compiled a 346-year chronology extend...

Comprised of largely non-commercial, xeric, oak-dominated forests, the Cross Timbers in Arkansas have been heavily altered over the last two centuries, and thus only scattered parcels of old-growth timber remain. We inventoried and mapped two such stands on Fort Chaffee Military Training Center in Sebastian County, Arkansas. The west-facing Christmas Knob site is located on an isolated hill, while the southerly-facing Big Creek Narrows site is on a long, narrow rocky outcrop called Devil's Backbone Ridge. These sites occupied rocky, south- to southwest-facing sandstone-dominated slopes, with primarily post oak (Quercus stellata) and blackjack oak (Q. marilandica) overstories. Post oak dominated the largest size classes at both sites. Increment cores indicated that some post oaks exceeded 200 y of age, and tree-ring dating also confirmed an uneven-aged structure to these stands. Both locations had irregular reverse-J shaped diameter distributions, with gaps, deficiencies, and excesses in larger size classes that often typify old-growth stands. On average, the post oaks at the Big Creek Narrows site were taller, larger in girth, and younger than those on the Christmas Knob site, suggestive of a better quality site at Big Creek. The application of neighborhood density functions on stem maps of both sites found random patterns in tree locations. These stands are very similar in their structure to old-growth examples in other parts of the Cross Timbers ecoregion.

Una serie dendrocronológica climáticamente sensitiva de anillo total se desarrolló en Isla Guadalupe con la especie endémica ciprés de Guadalupe (Hesperocyparis guadalupensis (S. Watson) Bartel. Var. guadalupensis). La cronología comprende los últimos 205 años (1804-2008) y mostró una respuesta significativa (r = 0.45 a 0.48, p

A new tree-ring reconstruction of the Palmer Drought Severity Index (PDSI) for Mesoamerica from AD 771 to 2008 identifies megadroughts more severe and sustained than any witnessed during the twentieth century. Correlation analyses indicate strong forcing of instrumental and reconstructed June PDSI over Mesoamerica from the El Nino/Southern Oscillation (ENSO). Spectral analyses of the 1,238-year reconstruction indicate significant concentrations of variance at ENSO, sub-decadal, bi-decadal, and multidecadal timescales. Instrumental and model-based analyses indicate that the Atlantic Multidecadal Oscillation is important to warm season climate variability over Mexico. Ocean-atmospheric variability in the Atlantic is not strongly correlated with the June PDSI reconstruction during the instrumental era, but may be responsible for the strong multidecadal variance detected in the reconstruction episodically over the past millennium. June drought indices in Mesoamerica are negatively correlated with gridded June PDSI over the United States from 1950 to 2005, based on both instrumental and reconstructed data. Interannual variability in this latitudinal moisture gradient is due in part to ENSO forcing, where warm events favor wet June PDSI conditions over the southern US and northern Mexico, but dryness over central and southern Mexico (Mesoamerica). Strong anti-phasing between multidecadal regimes of tree-ring reconstructed June PDSI over Mesoamerica and reconstructed summer (JJA) PDSI over the Southwest has also been detected episodically over the past millennium, including the 1950–1960s when La Nina and warm Atlantic SSTs prevailed, and the 1980–1990s when El Nino and cold Atlantic SSTs prevailed. Several Mesoamerican megadroughts are reconstructed when wetness prevailed over the Southwest, including the early tenth century Terminal Classic Drought, implicating El Nino and Atlantic SSTs in this intense and widespread drought that may have contributed to social changes in ancient Mexico.

Altered fire regimes and increased drought can lead to major vegetation changes, especially in ecotones. A decrease in fire can lead to woody species encroachment in prairies and increasing forest stand density. The threat of global climate change raises questions about potential increases in the length, severity, and incidence of droughts substantially altering species composition. Re-measured upland forests in south-central North America's midcontinent forest-prairie ecotone exhibited major changes in woody species composition and structure over fifty years and successional trajectories appeared to favor invasive Juniperus virginiana L. over the previous dominant Quercus species. The objective of this study was to determine whether climate and fire exclusion affected the recruitment history of dominant woody species in these upland forests located near the xeric western edge of the eastern deciduous forest biome of North America. We removed cores and cross-sections from 992 J. virginiana, Quercus marilandica Munchh. and Q. stellata Wangenh. trees from eleven forest stands located across central and northwest Oklahoma, and determined their ages using standard dendrochronological methods. Recruitment of all species increased following a severe mid-20th century drought, but a rapid increase in J. virginiana recruitment and decrease in Quercus recruitment appeared to be linked to a decrease in fire. Future fire regime changes and increased drought due to global climate change could lead to widespread shifts from Quercus- to Juniperus- dominated forests and cause substantial changes to ecosystem services.