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Tree rings preserve important records of past flooding. We present the results of an examination of inter-annual tree-ring anatomical variability and vessel width in overcup oak (Quercus lyrata) an...

Millettia stuhlmannii (Taub.) is a highly desirable timber species throughout its range in southeastern Africa and one of the three most important timber trees in Mozambique where it is intensely harvested. Relatively little is known with regard to the species' long-term radial growth rates, leaving open the question of whether current harvest schemes are sustainable. Tree-ring analysis is a useful method for obtaining accurate estimations of age and growth rate of tropical tree species. This paper presents results of tree-ring analyses of M. stuhlmannii from a large timber concession in the Sofala Province of central Mozambique. The analyses include estimates of: (1) tree age, (2) radial growth dynamics, and (3) the relationship between tree diameter and the percentage of heartwood relative to diameter. Our findings indicate that annual radial growth can be highly variable among and within individual M. stuhlmannii trees at the site. We also find that the relationship between stem diameter and percentage heartwood is significantly positive, suggesting that optimal harvesting diameters for this characteristic can be reliably determined. Given the lack of information on growth history and the potential for unsustainable exploitation, these findings are highly relevant to the sustainable management of this important timber species.

Tree-ring reconstructions of streamflow can provide long-term perspectives of flow variability that may be useful in management strategies for water resources. Relative to other portions of the United States, there is a dearth of tree-ring based streamflow reconstructions in the Midwest. The Wabash River is an important water resource in Indiana and Illinois where many sectors rely on it. Here, to better understand the historic hydroclimate within the Wabash River Basin, we reconstructed June–September streamflow for a tributary of the Wabash River using three statistical approaches: the traditional basin approach, and the recently-developed climate-footprint, and drought-reconstruction approaches. Our reconstructions explained 53–71% of the variance for the common period (1941–1984). We find that in our three reconstructions, there are extreme droughts and pluvials that exceed those during the instrumental period, including a sustained five-decade low-flow event in the early 19th century. Overall, we find that the traditional basin approach produced the most robust model but had the shortest reconstruction length of the three approaches, as it requires the predictor chronologies to be derived from within the basin. Our results indicate that there are merits and drawbacks to each approach with the general pattern of sacrificing variance explained for reconstruction length.

Seasonal reconstructions of streamflow are valuable because they provide water planners, policy makers, and stakeholders with information on the range and variability of water resources before the observational period. In this study, we used streamflow data from five gages near the Alabama-Florida border and centuries-long tree-ring chronologies to create and analyze seasonal flow reconstructions. Prescreening methods included correlation and temporal stability analysis of predictors to ensure practical and reliable reconstructions. Seasonal correlation analysis revealed that several regional tree-ring chronologies were significantly correlated (p ≤ 0.05) with March–October streamflow, and stepwise linear regression was used to create the reconstructions. Reconstructions spanned 1203–1985, 1652–1983, 1725–1993, 1867–2011, and 1238–1985 for the Choctawhatchee, Conecuh, Escambia, Perdido, and Pascagoula Rivers, respectively, all of which were statistically skillful (R2 ≥ 0.50). The reconstructions were statistically validated using the following parameters: R2 predicted validation, the sign test, the variance inflation factor (VIF), and the Durbin–Watson (D–W) statistic. The long-term streamflow variability was analyzed for the Choctawhatchee, Conecuh, Escambia, and Perdido Rivers, and the recent (2000s) drought was identified as being the most severe in the instrumental record. The 2000s drought was also identified as being one of the most severe droughts throughout the entire reconstructed paleo-record developed for all five rivers. This information is vital for the consideration of present and future conditions within the system.

Despite growing in wet lowland and riparian settings, Taxodium distichum (L.) Rich. (bald cypress) has a strong response to hydroclimate variability, and tree ring chronologies derived from bald cypress have been used extensively to reconstruct drought, precipitation and streamflow. Previous studies have also demonstrated that false rings in bald cypress appear to be the result of variations in water availability during the growing season. In this study 28 trees from two sites located adjacent to the Choctawhatchee River in Northwestern Florida, USA were used to develop a false ring record extending from 1881 to 2014. Twenty false ring events were recorded during the available instrumental era (1931&ndash, 2014). This record was compared with daily and monthly streamflow data from a nearby gage. All 20 of the false-ring events recorded during the instrumental period occurred during years in which greatly increased streamflow occurred late in the growing season. Many of these wet events appear to be the result of rainfall resulting from landfalling tropical cyclones. We also found that the intra-annual position of false rings within growth rings reflects streamflow variability and combining the false-ring record with tree ring width chronologies improves the estimation of overall summer streamflow by 14%. Future work using these and other quantitative approaches for the identification and measurement of false ring variables in tree rings may improve tree-ring reconstructions of streamflow and potentially the record of tropical cyclone rainfall events.

Clanwilliam cedar (Widdringtonia cedarbergensis; WICE), a long-lived conifer with distinct tree rings in Cape Province, South Africa, has potential to provide a unique high-resolution climate proxy for southern Africa. However, the climate signal in WICE tree-ring width (TRW) is weak and the dendroclimatic potential of other WICE tree-ring parameters therefore needs to be explored. Here, we investigate the climatic signal in various tree-ring parameters, including TRW, Minimum Density (MND), Maximum Latewood Density (MXD), Maximum Latewood Blue Intensity (MXBI), and stable carbon and oxygen isotopes (δ18O and δ13C) measured in WICE samples collected in 1978. MND was negatively influenced by early spring (October-November) precipitation whereas TRW was positively influenced by spring November-December precipitation. MXD was negatively influenced by autumn (April-May) temperature whereas MXBI was not influenced by temperature. Both MXD and MXBI were negatively influenced by January-March and January-May precipitation respectively. We did not find a significant climate signal in either of the stable isotope time series, which were measured on a limited number of samples. WICE can live to be at least 356 years old and the current TRW chronology extends back to 1564 CE. The development of full-length chronologies of alternative tree-ring parameters, particularly MND, would allow for an annually resolved, multi-century spring precipitation reconstruction for this region in southern Africa, where vulnerability to future climate change is high.

Tree-ring dating has been applied to three historical log buildings in northeastern Alabama to determine if any of the structures could have been associated with historic Fort Armstrong. Fort Armstrong was constructed near Cedar Bluff, Alabama by militia members under the ultimate command of Andrew Jackson to support actions against the “Red Stick” faction of Creeks during the First Creek War in 1813. Comparison of tree-ring samples from the structures with reference chronologies from nearby living trees strongly indicates that none of the three buildings was constructed prior to about 1849 and therefore could not have been contemporaneous with Fort Armstrong. However, the dating does indicate that the buildings are Antebellum in age and were very likely constructed as part of the early development of Snow Hill Plantation. As such these structures provide insight into the physical and cultural patterns of an early Alabama plantation. We believe that this project represents the first dendroarchaeological research reported from Alabama.

Summary Widespread destructive flooding is a common phenomenon along the Lower Mississippi River, and river managers have long sought to understand the temporal variability and relevant climatic factors of the system. One of the important drawbacks to better understanding the flood regime of this and similar large river systems is the relatively short instrumental record of flooding. In this study, we present a novel, annually-resolved tree-ring record of spring flooding based on anatomically anomalous “flood rings” preserved in trees growing about 60 km downstream of the Mississippi and Ohio River confluence. Our chronology records 39 flood-ring years between 1770 and 2009 including nearly all of the observed significant floods of the 20th century as well as severe floods documented in prior centuries. Comparison of the flood ring record with stream gage observations suggests that large-magnitude floods lasting for more than 10 days, during the spring flood season, are most likely to cause a flood ring in sampled trees. Instrumental and paleo-proxy records of atmospheric circulation features relevant to spring flooding on the Lower Mississippi were also examined. Results of this research suggest that similar flood-ring records could provide important insight into flood history elsewhere in the Mississippi River system and perhaps climate variability over North America.

Old Rocky Mountain Douglas-fir (Pseudotsuga menziesii) trees and remnant “subfossil” logs have been found on the outcrop of a mafic igneous intrusion above the Mancos River Valley near Mesa Verde National Park. These trees and logs have been used to develop earlywood (EW), latewood (LW), and total ring width (TRW) chronologies dating from AD 722–2011. The new chronologies include good series replication during the former chronological “gap” from AD 1250 to 1400, which was so problematic for the initial development of the “Central Pueblo” chronology by A. E. Douglass. Discrete reconstructions of the cool-season (September-May) and early warm-season (June-July) moisture balance for Mesa Verde have been derived from the EW and adjusted LW width chronologies from the Mancos Valley. Cool-season drought is estimated to have been more severe and sustained than early warm-season conditions during the “Great Drought” of the late-13th Century when southwestern Colorado was depopulated. The combined archaeo...

This study demonstrates that Millettia stuhlmannii produces annual growth rings responsive to seasonal climate and should be useful for dendrochronology. Millettia stuhlmannii is a highly valuable and potentially overexploited timber species indigenous to southeastern Africa. It is of particular economic importance in Mozambique though relatively little is known about its growth rate or response to climate. This study investigates whether M. stuhlmannii is potentially useful for dendrochronology—that is whether this species forms annual growth rings that are responsive to external forcing such as climate. Five methods were used to determine whether M. stuhlmannii growth rings are indeed annual in nature, including analysis of ring anatomy, dating trees of known age, cambial wounding, classical cross-dating, and comparison of annual growth to climate variables. Growth rings of Millettia stuhlmannii are distinct and well formed, young trees from plantations of known age formed an appropriate number of distinct annual rings, trees showed distinct wood reaction to cambial wounding, adding exactly one complete ring in one calendar year, cross-dating within and between trees was somewhat successful, and annual growth is significantly correlated with wet season precipitation. Results of this study indicate that M. stuhlmannii is a potentially useful species for dendrochronology. These findings should allow a better understanding of this species’ growth dynamics and ecology, as well as its response to climate variability in the past and potentially to future climate change.

We surveyed high quality, remnant black oak sand savannas across four sites in northeastern Illinois to compare characteristics of stand structure and tree vigor with fire history. Dendrochronological methods were applied to 289 dated fire scars identified on 60 Quercus velutina trees. Stand structure was characterized using 30 circular plots (0.04 ha each) per stand during summer 2007. Tree recruitment dynamics differed among the four stands, suggesting that canopy decline dynamics among them is likely to differ in coming decades. Frequent fire intervals (less than two years) were associated with canopy openness, but also a paucity of future canopy trees. Under these frequent fire regimes, we predict a loss of canopy cover, as no smaller trees were present to assume dominance. Fire intervals longer than two years were associated with transition to closed canopy forests. These results suggest that savanna managers should consider other disturbances, such as selective cutting and or grazing, alon...

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...

Pterocarpus angolensis D.C. is a highly valuable and sought after timber species across much of its range in southern Africa. Attempts at plantation cultivation of this species have not been successful, suggesting that sustainable management of natural populations is necessary for the continued utilization of this resource. Such management will require reliable age and long-term growth data. Despite the value of P. angolensis, few long-term studies of the species’ growth rate have been conducted, and some of these studies indicate that current harvest schemes may be inappropriate. This paper presents results of tree-ring analyses of P. angolensis from nine locations across southern Africa. The analyses include estimates of: (1) tree age, (2) diameter increment, (3) the relationship between tree diameter and the percentage of heartwood, and (4) the relationship between mean diameter increment and local rainfall. The mean annual diameter increment averaged over all nine sites was generally lower than other published values, though the range of values was within that of published figures. Cumulative diameter as a function of age indicates that radial growth can be quite variable, both within and among stands. These variable age–diameter relationships within and across the nine sites suggest that diameter may be a reasonable indicator of tree age at the stand level but not across the species’ range. The relationship between stem diameter and percentage heartwood is significantly positive, suggesting that optimal harvesting diameters may be reliably determined. Average annual diameter increment is significantly positively correlated with mean seasonal rainfall, which explains about half of the variance in mean diameter increment. These findings should be helpful for the sustainable management of this important species.

We present the first tree-ring reconstruction of rainfall in tropical Africa using a 200-year regional chronology based on samples of Pterocarpus angolensis from Zimbabwe. The regional chronology is significantly correlated with summer rainfall (November–February) from 1901 to 1948, and the derived reconstruction explains 46% of the instrumental rainfall variance during this period. The reconstruction is well correlated with indices of the El Nino-southern oscillation (ENSO), and national maize yields. An aridity trend in instrumental rainfall beginning in about 1960 is partially reproduced in the reconstruction, and similar trends are evident in the nineteenth century. A decadal-scale drought reconstructed from 1882 to 1896 matches the most severe sustained drought during the instrumental period (1989–1995), and is confirmed in part by documentary evidence. An even more severe drought is indicated from 1859 to 1868 in both the tree-ring and documentary data, but its true magnitude is uncertain. A 6-year wet period at the turn of the nineteenth century (1897–1902) exceeds any wet episode during the instrumental era. The reconstruction exhibits spectral power at ENSO, decadal and multi-decadal frequencies. Composite analysis of global sea surface temperature during unusually wet and dry years also suggests a linkage between reconstructed rainfall and ENSO.

Mexico has a wealth of historical information regarding extreme climate, famine and disease in the colonial period and even in prehispanic times. The recent development of long tree-ring chronologies throughout Mexico now allows comparison between records of past climate and socioeconomic conditions. North American tree-ring data indicate that, for the 3 consecutive years from 1785 to 1787, a major drought occurred across Mexico and the central United States with the most intense conditions extending north from Zacatecas into Texas. This period included the infamous ‘El Ano del Hambre,’ one of greatest famines in Mexican history. The tree-ring data are also correlated with maize yield in central Mexico and with the value of Church tithes normally paid with agricultural goods. Drought and crop failure caused the dramatic inflation of crop value and Church tithes in 1786–1787 when tree-ring data indicate that the 1780s drought was most extreme.

Earlywood ring-width chronologies derived primarily from Douglas-fir trees were used to reconstruct winter–spring (November–May) precipitation and fall–spring (September–June) streamflow volumes for the period 1765–1993 in the forested upper Nazas watershed in Durango, Mexico.The tree-ring data were obtained from mixed conifer stands within or adjacent to the upper Nazas watershed.Precipitation data were derived from one of the longest regional records.The streamflow data were obtained from a guage located in the upper Nazas watershed.The Principal Component 1 (PC1) of nine residual earlywood chronologies accounted for 73% of the variance in November–May precipitation 1967–1993, and 64% for the total period with available data 1941–1993.The mean of three residual earlywood chronologies from Douglas-fir explained 51% of the normalized streamflow at Sardinas from 1971–1992. The 20th century was characterized by severe droughts, especially between 1950 and 1963 that also affected other regions of Mexico and the southwestern United States.Additional droughts of similar or greater magnitude occurred in the 1790s, 1810s, 1860–1870s and 1890–1910s.Similar periods of low flow occur in the September–June streamflow reconstruction between 1765 and 1993.These results indicate that tree-ring chronologies from this region document a high percentage of the precipitation and streamflow variance.Spectral analysis detected significant high periodicities in both records at peaks of 4 and 7 years that could be related to the ENSO frequency bands (approximately 4.0 and 6.25 years).Analysis of the reconstructed records show strong influence of ENSO on precipitation and streamflow amounts on an interannual basis.These results can provide significant inputs to decisions regarding management of water resources that are used to irrigate land in the Comarca Lagunera: specifically they indicate that water budgeting should be managed over longer time periods to account for this ENSO-related variability rather than on the year-to-year basis that is presently used. r 2005 Elsevier GmbH.All rights reserved.

The classical period in Mexico (AD 250-750) was an era of splendor. The city of Teotihuacan was one of the largest and most sophisticated human conglomerates of the pre-industrial world. The Mayan civilization in southeastern Mexico and the Yucatan peninsula reached an impressive degree of development at the same time. This time of prosperity came to an end during the Terminal Classic Period (AD 750-950) a time of massive population loss throughout Mesoamerica. A second episode of massive depopulation in the same area was experienced during the sixteenth century when, in less than one century, between 80% and 90% of the entire indigenous population was lost. The 16th century depopulation of Mexico constitutes one of the worst demographic catastrophes in human history. Although newly imported European and African diseases caused high mortality among the native population, the major 16th century population losses were caused by a series of epidemics of a hemorrhagic fever called Cocoliztli, a highly lethal disease unknown to both Aztec and European physicians during the colonial era. The cocoliztli epidemics occurred during the 16th century megadrought, when severe drought extended at times from central Mexico to the boreal forest of Canada, and from the Pacific to the Atlantic coast. The collapse of the cultures of the Classic Period seems also to have occurred during a time of severe drought. Tree ring and lake sediment records indicate that some of the most severe and prolonged droughts to impact North America-Mesoamerica in the past 1000-4000 years occurred between AD 650 and 1000, particularly during the 8th and 9th centuries, a period of time that coincides with the Terminal Classic Period. Based on the similarities of the climatic (severe drought) and demographic (massive population loss) events in Mesoamerica during the sixteenth century, we propose that drought-associated epidemics of hemorrhagic fever may have contributed to the massive population loss during the Terminal Classic Period.

Earlywood width chronologies from Douglas-fir tree rings were used to reconstruct winter (November–March) precipitation for more than 600 years over Durango, Mexico. The tree-ring data were obtained from two sites of long-lived Douglas-fir in northern and southern Durango and the seasonal climatic precipitation data were regionally averaged from five weather stations well distributed across the state. The averaged earlywood chronology accounted for 56% of the variance in instrumental November–March precipitation 1942–1983. We validated the reconstruction against independent precipitation records. The worst winter drought of the 20th century in Durango occurred 1950–1965. However, the reconstruction indicates droughts more severe than any witnessed in the 20th century, e.g., the 1850s–1860s, and the megadrought in the mid- to late-16th century. Reconstructed winter precipitation 1540–1579 shows 33 of 40 years were dry. Persistent drought may be linked to extended La Nina episodes. The Tropical Rainfall Index (TRI) correlates well with instrumental and reconstructed winter precipitation (r = 0.49 and 0.55, respectively), reflecting the strong ENSO modulation of cool season climate over northern Mexico. The ENSO teleconnection varies through time, with TRI-reconstructed precipitation correlations ranging from 0.78 to 0.27 in five periods 1895–1993. The 1942–1983 winter observed and reconstructed Durango data correlate well with the corresponding seasonalization of the All-Mexico Rainfall Index (AMRI; r=0.68, P

This study presents two independent reconstructions of precipitation from James Madison's Montpelier plantation at the end of the eighteenth century. The first is transcribed directly from meteorological diaries recorded by the Madison family for 17 years and reflects the scientific interests of James Madison and Thomas Jefferson. In his most active period as a scientist, Madison assisted Jefferson by observing the climate and fauna in Virginia to counter the contemporary scientific view that the humid, cold climate of the New World decreased the size and number of its species. The second reconstruction is generated using tree rings from a forest in the Montpelier plantation and connects Madison's era to the modern instrumental precipitation record. These trees provide a significant reconstruction of both early summer and prior fall precipitation. Comparison of the dendroclimatic and diary reconstructions suggests a delay in the seasonality of precipitation from Madison's era to the mid-twentieth...

The state of Chihuahua lies in an arid to semiarid zone in the NW central plain of Mexico. Its agricultural economy is highly vulnerable to frequent droughts. In this study, we recon- struct winter-spring precipitation from 1647-1992 using 6 earlywood width chronologies of Douglas fir from around Chihuahua. The tree-ring data explain 56% of the winter-spring precipitation vari- ance in a linear regression for 1949-1992, and there is strong correlation between the tree-ring reconstructed precipitation data and the observed precipitation data not used for calibration (r = 0.74, p < 0.01). The 5 driest years in the reconstructed precipitation record were 1974, 1954, 1742, 1980, and 1820 in order of severity, and the longest dry period in the 346 yr record lasted about 17 yr (1948-1964) during the severe 1950s drought that also affected the SW United States. The recon- structed precipitation record has a statistically significant 4 yr spectral peak in the El Nino/Southern Oscillation (ENSO) frequency band and is significantly correlated with indices of ENSO (tropical rainfall index; r = 0.58, p = 0.001). The correlation between the ENSO and Chihuahuan reconstructed precipitation varied in strength when computed for non-overlapping 18 yr sub-periods (ranging from r = 0.43 to r = 0.68), which may reflect changes in the ENSO teleconnection to climate in northern Mexico.

The Cross Timbers are a mosaic of upland deciduous forest, savanna, and glade that typifies the broad ecotone between the eastern deciduous forest and the grasslands of the southern Great Plains. The pre-settlement Cross Timbers may have covered some 7,909,700 ha from central Texas, across Oklahama into eastern Kansas, and today may represent the least disturbed forest ecosystem of comparable size still left in the eastern United States. Extensive tree-ring research indicates that ancient forests dominated by 200 to 400 year old post-oaks (Quercus stellata Wang) survive throughout the Cross Timbers, particularly in Oklahoma. These ancient forests persist largely because the Cross Timbers formation is non-commercial for timber production, and has not experienced large-scale industrial logging. Because ancient forest relics are often found on stressful non-commercial sites in the Cross Timbers and elsewhere, it is possible to design predictive models to locate the specific terrain where undisturbed forests are likely to survive. A predictive model for southern Osage County, Oklahoma, was developed based on the steep, infertile soils of the Niotaze-Darnell complex. We tested the model with field inspection and tree-ring analysis of fifty randomly selected belt transects, and 74% of the sampled terrain is still old-growth Cross Timbers woodland. This translates into 8200 ha of ancient Cross Timbers on this single site type in southern Osage County. The abundance of ancient forest in the Cross Timbers is not widely appreciated. However, large contiguous tracts of ancient Cross Timbers up to 700 ha were identified with this predictive model, strongly supporting inferences concerning the relatively undisturbed nature of this ecosystem.

https://doi.org/10.15447/sfews.2011v9iss1art4 Blue oak tree-ring chronologies correlate highly with winter–spring precipitation totals over California, with Sacramento and San Joaquin river stream flow, and with seasonal variations in the salinity gradient in San Francisco Bay. The convergence of fresh and saline currents can influence turbidity, sediment accumulation, and biological productivity in the estuary. Three selected blue oak chronologies were used to develop a 625-year-long reconstruction of the seasonal salinity gradient, or low salinity zone (LSZ), which provides a unique perspective on the interannual-to-decadal variability of this important estuarine habitat indicator. The reconstruction was calibrated with instrumental LSZ data for the winter–spring season, and explains 73% of the variance in the February–June position of the LSZ from 1956 to 2003. Because this calibration period post-dates the sweeping changes that have occurred to land cover, channel morphology, and natural streamflow regimes in California, the reconstruction provides an idealized estimate for how the LSZ might have fluctuated under the seasonal precipitation variations of the past 625 years, given the modern geometry and bathymetry of the estuary and land cover across the drainage basin. The February–June season integrates precipitation and runoff variability during the cool season, and does not extend into the late-summer dry season when LSZ extremes can negatively affect Sacramento–San Joaquin Delta (Delta) agriculture and some aquatic organisms. However, there is such strong inter-seasonal persistence in the instrumental LSZ data that precipitation totals during the cool season can strongly pre-condition LSZ position in late summer. The 625-year-long reconstruction indicates strong interannual and decadal variability, the frequent recurrence of consecutive 2-year LSZ maxima and minima, large-scale ocean atmospheric forcing, and an interesting asymmetrical influence of warm El Niño–Southern Oscillation (ENSO) events.

[1] Ancient Montezuma baldcypress (Taxodium mucronatum) trees found in Barranca de Amealco, Queretaro, have been used to develop a 1,238-year tree-ring chronology that is correlated with precipitation, temperature, drought indices, and crop yields in central Mexico. This chronology has been used to reconstruct the spring-early summer soil moisture balance over the heartland of the Mesoamerican cultural province, and is the first exactly dated, annually resolved paleoclimatic record for Mesoamerica spanning the Late Classic, Post Classic, Colonial, and modern eras. The reconstruction indicates that the Terminal Classic drought extended into central Mexico, supporting other sedimentary and speleothem evidence for this early 10th century drought in Mesoamerica. The reconstruction also documents severe and sustained drought during the decline of the Toltec state (1149–1167) and during the Spanish conquest of the Aztec state (1514–1539), providing a new precisely dated climate framework for Mesoamerican cultural change.

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.

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 Río 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.

[1] We have developed a network of 18 new tree ring chronologies to examine the history of warm season tree growth over Mexico from 1780 to 1992. The chronologies include Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and Montezuma pine (Pinus montezumae Lamb.) latewood width, and Montezuma bald cypress (Taxodium mucronatum Ten.) total ring width. They are located in southwestern Texas, the Sierra Madre Oriental, Sierra Madre Occidental, and southern Mexico as far south as Oaxaca. Seven of these chronologies are among the first precipitation sensitive tree ring records from the American tropics. Principal component analysis of the chronologies indicates that the primary modes of tree growth variability are divided north and south by the Tropic of Cancer. The tree ring data in northern Mexico (PC1) are most sensitive to June–August rainfall, while the data from southern Mexico (PC2) are sensitive to rainfall in April–June. We find that the mode of tree growth variability over southern Mexico is significantly correlated with the onset of the North American Monsoon. Anomalies in monsoon onset, spring precipitation, and tree growth in southern Mexico all tend to be followed by precipitation anomalies of opposite sign later in the summer over most of central Mexico.