Tree-ring Proxy Research Articles

Combining Ring Width, Density and Stable Carbon Isotope Proxies to Enhance the Climate Signal in Tree-Rings: An Example from the Southern French Alps

At present the most powerful tree-ring based climate reconstructions use high numbers of growth proxy series (ring width and density) to produce spatially smoothed estimates, such as average Northern Hemisphere summer temperatures. These single parameter reconstructions might be supplemented with regional climate reconstructions capable of capturing variability in more than one climate variable without lower replication compromising statistical quality, if multiple tree ring proxies were used. Pinus sylvestris and Pinus uncinata latewood density, width and δ13C series are presented from two sites in the French subalpine zone, east of Briancon. Where two proxies have the same dominant climate control their combination enhances that signal. Where proxies differ in dominant controlling climate variable, combining series allows access to bi-variable calibrations. Using this approach, multi-proxy reconstructions of both temperature and precipitation would better reflect complex synoptic variability in climate on spatially useful scales.

Climatic influences on fire in Araucaria araucana–Nothofagus forests in the Andean cordillera of south-central Chile

:Tree-ring records of fires were used to examine the effects of inter-annual climatic variability on fire occurrence in forests dominated by the fire-adapted Araucaria araucana in the Andes of south-central Chile. Instrumental as well as tree-ring proxy records of climate indicate that low moisture availability is the main factor influencing fire occurrence. Years of widespread fire are strongly associated with warmer and drier summers. Years of extensive fire also tend to be favoured by one or two preceding years of dry climatic conditions. The El Niño–Southern Oscillation (ENSO) and its strong influence over large-scale climatic features is an important factor promoting fire activity. Years of high fire activity coincide with warm and dry summers following El Niño events. Fire in the Araucarian region is strongly related to inter-annual climatic variation associated primarily with the coupled effect of ENSO events and variations in the intensity and latitudinal position of the southeast Pacific anticyclone.

An exploration of the controls of pre-instrumental streamflow using multiple tree-ring proxies

Previous studies have used tree-ring chronologies from several species to develop reconstructions of precipitation, temperature, streamflow and glacier mass balance for sites in Banff National Park, Alberta. This study examines the variability in a >300-year summer streamflow reconstruction for the Bow River at Banff in conjunction with changes in the major contributors to streamflow (glacier melt, winter and summer pecipitation). Reconstructed winter mass balance for Peyto Glacier is used as a surrogate for winter precipitation and April–August precipitation is reconstructed for Banff. Streamflow variability correlates most highly with winter precipitation and periods of high flow follow above average snowfall in the previous winter (high winter balance) and in some cases also with above normal summer precipitation. A clear response to changes in summer mass balance at Peyto Glacier (i.e. summer glacier melting) cannot be identified in this summer discharge record. Problems developing physically realistic flow reconstructions for snowmelt dominated rivers from summer sensitive tree-ring chronologies are also discussed.

A 1052-year tree-ring proxy for Alpine summer temperatures

A June–August Alpine temperature proxy series is developed back to AD 951 using 1,527 ring-width measurements from living trees and relict wood. The reconstruction is composed of larch data from four Alpine valleys in Switzerland and pine data from the western Austrian Alps. These regions are situated in high elevation Alpine environments where a spatially homogenous summer temperature signal exists. In an attempt to capture the full frequency range of summer temperatures over the past millennium, from inter-annual to multi-centennial scales, the regional curve standardization technique is applied to the ring width measurements. Correlations of 0.65 and 0.86 after decadal smoothing, with high elevation meteorological stations since 1864 indicate an optimal response of the RCS chronology to June–August mean temperatures. The proxy record reveals warm conditions from before AD 1000 into the thirteenth century, followed by a prolonged cool period, reaching minimum values in the 1820s, and a warming trend into the twentieth century. This latter trend and the higher frequency variations compare well with the actual high elevation temperature record. The new central Alpine proxy suggests that summer temperatures during the last decade are unprecedented over the past millennium. It also reveals significant similarities at inter-decadal to multi-centennial frequencies with large-scale temperature reconstructions, however, deviating during certain periods from H.H. Lamb‘s European/North Atlantic temperature history.

Centennial-to-millennial fluctuations in July temperatures in North Finland as recorded by timberline tree rings of Scots pine

A tree-ring proxy of summer temperature anomalies for northern Finland for the past 7500 yr was analyzed using Fourier spectrum and wavelet approaches. Multicentennial (250–450 yr) variability is present in the proxy record during most of the time range. This variability is suggested to reflect low-frequency variability in the North Atlantic Oscillation. Century-scale (90–130 yr) variation is another important feature of the tree-ring proxy data during the Holocene and may be attributed to Glessberg solar activity variations. In addition, an approximately 2000-yr quasi-period is found in this temperature proxy data, similar to the millennial-scale variability, present in many climate records from the North Atlantic region. The results point to the importance of multiple forcings underlying significant Holocene climatic fluctuations.

Dendroclimatological potential of the Australian red cedar

We examined Toona ciliata M.Roemer (Australian red cedar) for its potential to deliver annually resolved tree-ring proxy data. Such proxies are valuable and sought-after sources for reconstructing climate beyond instrumental records, especially in Australia. T. ciliata was chosen because it is one of the few deciduous tree species in Australia experiencing a seasonally dormant period of the cambium. This was confirmed by a preliminary tree-ring analysis which revealed distinct growth rings. Because of initial uncertainties regarding reliable annual growth rings in the wood of T. ciliata, a dendrometer-band study was conducted in addition to the tree-ring analysis. Stem increments revealed a common period of dormancy during winter and the measurements were found to correlate with both precipitation and temperature, depending on the site. For the first time, our tree-ring analysis demonstrated that samples from different individuals can be cross-dated and the resulting site index from Upper Kangaroo Valley (New South Wales) has the potential to reconstruct early season temperatures and late-season rainfall.

Solar Variability and Holocene Climate: Evidence from Radiocarbon, Tree-Ring Proxies and Climate System Modeling

Solar Variability and Holocene Climate: Evidence from Radiocarbon, Tree-Ring Proxies and Climate System Modeling

Reconstruction of Summer Temperatures in Interior Alaska from Tree-Ring Proxies: Evidence for Changing Synoptic Climate Regimes

Maximum latewood density and δ 13C discrimination of Interior Alaska white spruce were used to reconstruct summer (May through August) temperature at Fairbanks for the period 1800–1996, one of the first high-resolution reconstructions for this region. This combination of latewood density and δ 13C discrimination explains 59.9% of the variance in summer temperature during the period of record 1906–1996. The 200-yr. reconstruction is characterized by 7 decadal-scale regimes. Regime changes are indicated at 1816, 1834, 1879, 1916, 1937, and 1974, are abrupt, and appear to be the result of synoptic scale climate changes. The mean of summer temperature for the period of reconstruction (1800–1996) was 13.49 °C. During the period of instrument record (1903–1996) the mean of summer temperature was 13.31 °C for both the reconstruction and the recorded data. The coldest interval was 1916–1937 (12.62 ° C) and the warmest was 1974–1996 (14.23 °C) for the recorded data. The reconstruction differs from records of northern hemisphere temperatures over this period, especially because of Interior Alaska warm periods reconstructed from 1834 to 1851 (14.24 °C) and from 1862 to 1879 (14.19 °C) and because of the cool period in the early part of the 20th century (1917–1974). We show additional tree ring data that support our reconstruction of these warm periods. Alternate hypotheses involving autogenic effect of tree growth on the site, altered tree sensitivity, or novel combinations of temperature and precipitation were explored and while they cannot be ruled out as contributors to the anomalously warm 19th century reconstruction, they were not supported by available data. White spruce radial growth is highly correlated with reconstructed summer temperature, and temperature appears to be a reliable index of carbon uptake in this system.

Millennial to multi‐decadal variability in the climate of southern Africa

AbstractWavelet analysis has been used to determine changes in the variability of southern Africa climate over the last three millennia. The structure of the variability in a 3000‐year δ18O record has been compared with that in tree ring data for the last 600 years and both to the variability in annual rainfall and runoff during the 20th century. Preferred modes of variability are shown to have occurred in all the data sets. For overlap periods, the variability modes are in phase where the data are for a common seasonal rainfall regime.From the combined proxy and instrumental data it appears that an ∼1500‐year oscillation in rainfall variability may have been present during the late Holocene. Considerable variability in South African climate has been shown to reside in the bands 500–800 and 60–120 years. The latter is manifest in stable isotope and tree ring proxy data and in measured rainfall and runoff, and shows a distinctive latitudinal variation over southern Africa. The ∼80‐year variability has been present for at least 600 years in the tree ring data and at least 3000 years in the stable isotope data. During the 20th century, the variability in the 60–120‐year range was confined south of 20° S in southern Africa.The existence of considerable multi‐decadal climate variability in the range 16–20 years has been confirmed south of 15° S. Tree ring data show that the ∼18‐year oscillation in climate has been present for at least 600 years. The 500–800‐year, ∼80‐year and ∼18‐year variability has been both frequency‐ and amplitude‐modulated over time, but never to the extent that the variability bands have lost their integrity. Their persistence is a striking feature of southern African climate.The millennial, centennial and multi‐decadal variability in climate over southern Africa finds counterparts with temporal variability in similar time ranges reported elsewhere. Spatial teleconnections in variability likewise have been shown to occur. The changes in climate variability over the subcontinent are part of variability changes affecting the Earth system in many other regions. Copyright © 2002 Royal Meteorological Society.

Inter-hemispheric comparison of fire history: The Colorado Front Range, U.S.A., and the Northern Patagonian Andes, Argentina

Fire history was compared between the Colorado Front Range (U.S.A.) and northern Patagonia (Argentina) by dating fire-scars on 525 Pinus ponderosa and 418 Austrocedrus chilensis, respectively, and determining fire weather on the basis of instrumental and tree-ring proxy records of climatic variation. Years of above average moisture availability preceding fire years, rather than drought alone, is conducive to years of widespread fire in the Colorado Front Range and the northern Patagonian study areas. Above-average precipitation promotes fire by enhancing the growth of herbaceous plants which increases the quantity of fine fuels during the fire season a few years later. The short-term variability in moisture availability that is conducive to widespread burning is strongly related to El Nino Southern Oscillation (ENSO) activity. The warm (El Nino) phase of ENSO is associated with greater moisture availability during the spring in both regions which leads to peaks in fire occurrence several years after El Nino events. The warmer and drier springs associated with la Nina events exacerbate the drying of fuels so that fire years commonly coincide with La Nina events. In both regions, there was a dramatic decline in fire occurrence after the early 1900s due to a decline in intentionally set fires by Native Americans and European settlers, fuel reduction by livestock grazing, and increasingly effective organized fire suppression activities after the 1920s. In both regions there was a marked increase in fire frequency during the mid-and late-19th centuries which coincides with increased ignitions by Native Americans and/or European settlers. However, year-to-year variability in ring widths of Pinus ponderosa and Austrocedrus chilensis also increased from relatively low values in the late 1700s and early 1800s to peaks in the 1850s and 1860s. This implies frequent alternation of years of above and below average moisture availability during the mid-19th century when the frequencies of major fire years rise. The high correlation of tree-growth variability betweem the two regions implies a strong inter-hemispheric variation in climatic variability at a centennial time scale which closely parallels a variety of proxy records of ENSO activity. Based on the relationship of fire and ENSO events documented in the current study, this long-term trend in ENSO activity probably contributed to the mid- and late-19th century increase in fire spread in both regions. These similar trends in fire occurrence have contributed to similar patterns of forest structures, forest health, and current hazard of catastrophic wildfire in the Colorado Front Range and northern Patagonia.

Climatic and human influences on fire history in Pike National Forest, central Colorado

We investigated interannual and multidecadal variability in fire regimes, as related to climate and human land-use in Pike National Forest, central Colorado. Short and long-term trends in fire-scar records were related to tree-ring proxy records of moisture availability and to variability in El Niño – Southern Oscillation (ENSO). Fire occurrence is strongly tied to interannual drought conditions and is associated with cycles of ENSO. Fire events tend to occur in years of reduced moisture availability (La Niña years) and are often preceded by 2–4 years of increased moisture availability (El Niño years). A period of reduced variability in the tree-ring record from 1760 to 1820 AD, roughly corresponds to a period of reduced fire occurrence from approximately 1792 to 1842. Coincident with increased fire occurrence, variability in the climate proxies was high in the middle to late 1800s until the early 1900s. Multidecadal impacts through land use are also evident in the fire record with sharp increases during Euro-American settlement in ca. 1850 and abrupt declines with the start of active fire suppression after ca. 1920. Both humans and climatic variation appear to have interacted synergistically to create long-term trends in fire occurrence over the past two centuries.

CLIMATIC AND HUMAN INFLUENCES ON FIRE REGIMES IN PONDEROSA PINE FORESTS IN THE COLORADO FRONT RANGE

In the northern Colorado Front Range, fire suppression during the 20th century is believed to have created a high hazard of catastrophic fire in ponderosa pine (Pinus ponderosa) forests. Since the early 1990s, resource managers have increased the use of prescribed fires to re-create fire regimes and forest structures similar to those of the pre-Euro-American settlement period in order both to reduce fire hazard and to improve forest health. To improve understanding of historical fire regimes, we conducted a study of fire history along an elevational gradient from ∼1830 to 2800 m in ponderosa pine forests in the northern Front Range. Fire-scar dates were determined from 525 partial cross sections from living and dead trees at 41 sample sites. Fire frequencies and fire intervals were analyzed in relation to changes in human activities and interannual climatic variability as recorded in instrumental climatic records and tree-ring proxy records. Prior to modern fire suppression, the low elevation, open ponderosa pine forests of the northern Front Range were characterized by frequent surface fires, similar in frequency to many other ponderosa pine ecosystems in the West. In contrast, in higher elevation forests (above ∼2400 m) where ponderosa pine is mixed with Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), the fire regime was characterized by a much lower fire frequency and included extensive stand-replacing fires as well as surface fires. In the mid-1800s there was a marked increase in fire occurrence that can be related both to Euro-American settlement and increased climatic variability. This episode of increased fire left a legacy of dense, even-aged stands in higher elevation ponderosa pine forests, whereas increased stand densities in low elevation forests are attributed mainly to fire exclusion during the 20th century. Warmer and drier spring–summers, indicated in instrumental climatic records (1873–1995) and in tree-ring proxy records of climate (1600–1983), are strongly associated with years of widespread fire. Years of widespread fire also tend to be preceded two to four years by wetter than average springs that increase the production of fine fuels. Alternation of wet and dry periods over time periods of 2–5 years is conducive to fire spread and is strongly linked to El Niño–Southern Oscillation (ENSO) events. The warm (El Niño) phase of ENSO is associated with greater moisture availability during spring that results in a peak of fire occurrence several years following El Niño events. Conversely, dry springs associated with La Niña events were followed by more widespread fire during the same year. The 1600–1920 fire-scar record indicates that individual years during which high percentages of the 41 sample sites synchronously recorded fire have occurred at least several times per century. The association of these years of widespread fire with very strong ENSO events demonstrates the importance of ENSO-related climatic variabililty in creating extreme fire hazard at a landscape scale.

Can origin of the 2400-year cycle of solar activity be caused by solar inertial motion?

Abstract. A solar activity cycle of about 2400 years has until now been of uncertain origin. Recent results indicate it is caused by solar inertial motion. First we describe the 178.7-year basic cycle of solar motion. The longer cycle, over an 8000 year interval, is found to average 2402.2 years. This corresponds to the Jupiter/Heliocentre/Barycentre alignments (9.8855 × 243). Within each cycle an exceptional segment of 370 years has been found characterized by a looping pattern by a trefoil or quasitrefoil geometry. Solar activity, evidenced by 14C tree-ring proxies, shows the same pattern. Solar motion is computable in advance, so this provides a basis for future predictive assessments. The next 370-year segment will occur between AD 2240 and 2610.Key words: Solar physics (celestial mechanics)

Can origin of the 2400-year cycle of solar activity be caused by solar inertial motion?

A solar activity cycle of about 2400 years has until now been of uncertain origin. Recent results indicate it is caused by solar inertial motion. First we describe the 178.7-year basic cycle of solar motion. The longer cycle, over an 8000 year interval, is found to average 2402.2 years. This corresponds to the Jupiter/Heliocentre/Barycentre alignments (9.8855 × 243). Within each cycle an exceptional segment of 370 years has been found characterized by a looping pattern by a trefoil or quasitrefoil geometry. Solar activity, evidenced by 14C tree-ring proxies, shows the same pattern. Solar motion is computable in advance, so this provides a basis for future predictive assessments. The next 370-year segment will occur between AD 2240 and 2610.Key words: Solar physics (celestial mechanics)

FIRE HISTORY IN NORTHERN PATAGONIA: THE ROLES OF HUMANS AND CLIMATIC VARIATION

The effects of humans and climatic variation on fire history in northern Patagonia, Argentina, were examined by dating fire scars on 458 trees at 21 sites in rain forests of Fitzroya cupressoides and xeric woodlands of Austrocedrus chilensis from 39° to 43° S latitude. Climatic variation associated with fires was analyzed on the basis of 20th-century observational records and tree ring proxy records of climatic variation since approximately AD 1500. In the Austrocedrus woodlands, fire frequency increases after about 1850, coincident with greater use of the area by Native American hunters. Increased burning, particularly in the zone of more mesic forests, is also strongly associated with forest clearing by European settlers from about 1880 to the early 1900s. The marked decline in fire frequency during the 20th century coincides with both the demise of Native American hunters in the 1890s and increasingly effective fire exclusion. Strong synchroneity in the years of widespread fire at sample sites dispersed over a north–south distance of ∼400 km indicates a strong climatic influence on fire occurrence at an annual scale. Tree ring reconstructions of regional precipitation and temperature show a steeply declining influence of climatic variability on fire occurrence from annual to multidecadal scales. It is the interannual variability in climate, rather than variations in average climatic conditions over longer periods, that strongly influences fire regimes in northern Patagonia. Although climatic variability overrides human influences on fire regimes at an interannual scale, human activity is an equally important determinant of fire frequency at multidecadal scales. Climatic conditions conducive to widespread fire in both xeric Austrocedrus woodlands and Fitzroya rain forests are typical of the late stages of La Niña (cold phase of the Southern Oscillation) events, as indicated by trends in the Southern Oscillation Index and eastern tropical Pacific sea surface temperatures during the 1–2 years before and after fire event years. Years of extreme fire occurrence are associated both with dry winter–springs of La Niña events and with the warm summers following El Niňo events. Years in which the southeast Pacific subtropical anticyclone is intense and located farther south than normal are years of enhanced drought and fire. Similarly, years of widespread fire in northern Patagonia are associated with variations in mean sea level atmospheric pressure at about 50°–60° S latitude in the South American–Antarctic Peninsula sector of the Southern Ocean, as reconstructed from tree rings for AD 1746–1984. Precipitation and, hence, fire regimes in northern Patagonia are significantly influenced by high-latitude blocking events, which drive westerly cyclonic storms northward. Variations at decadal to centennial time scales in major circulation features, such as ENSO activity and the meridionality of regional air flow at high latitudes, as well as changes in the degree of coupling of these features, influence climate and fire regimes of northern Patagonia.

Temperature histories from tree rings and corals

Recent temperature trends in long tree-ring and coral proxy temperature histories are evaluated and compared in an effort to objectively determine how anomalous twentieth century temperature changes have been. These histories mostly reflect regional variations in summer warmth from the tree rings and annual warmth from the corals. In the Northern Hemisphere, the North American tree-ring temperature histories and those from the north Polar Urals, covering the past 1000 or more years, indicate that the twentieth century has been anomalously warm relative to the past. In contrast, the tree-ring history from northern Fennoscandia indicates that summer temperatures during the ‘Medieval Warm Period’ were probably warmer on average than those than during this century. In the Southern Hemisphere, the tree-ring temperature histories from South America show no indication of recent warming, which is in accordance with local instrumental records. In contrast, the tree-ring records from Tasmania and New Zealand indicate that the twentieth century has been unusually warm particularly since 1960. The coral temperature histories from the Galapagos Islands and the Great Barrier Reef are in broad agreement with the tree-ring temperature histories in those sectors, with the former showing recent cooling and the latter showing recent warming that may be unprecedented. Overall, the regional temperature histories evaluated here broadly support the larger-scale evidence for anomalous twentieth century warming based on instrumental records. However, this warming cannot be confirmed as an unprecedented event in all regions.