Great Basin bristlecone pine (Pinus longaeva) at 3 sites in western North America near the upper elevation limit of tree growth showed ring growth in the second half of the 20th century that was ...greater than during any other 50-year period in the last 3,700 years. The accelerated growth is suggestive of an environmental change unprecedented in millennia. The high growth is not overestimated because of standardization techniques, and it is unlikely that it is a result of a change in tree growth form or that it is predominantly caused by CO₂ fertilization. The growth surge has occurred only in a limited elevational band within almost equal to150 m of upper treeline, regardless of treeline elevation. Both an independent proxy record of temperature and high-elevation meteorological temperature data are positively and significantly correlated with upper-treeline ring width both before and during the high-growth interval. Increasing temperature at high elevations is likely a prominent factor in the modern unprecedented level of growth for Pinus longaeva at these sites.
The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been ...directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as
C,
Be and
Cl have been found. Analyzing annual
C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric
C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric
C recorded for both events exceed all previously known
C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure.
Calendar-dated tree-ring sequences offer an unparalleled resource for high-resolution paleoenvironmental reconstruction. Where such records exist for a few limited geographic regions over the last ...8,000 to 12,000 years, they have proved invaluable for creating precise and accurate timelines for past human and environmental interactions. To expand such records across new geographic territory or extend data for certain regions further backward in time, new applications must be developed to secure “floating” (not yet absolutely dated) tree-ring sequences, which cannot be assigned single-calendar year dates by standard dendrochronological techniques. This study develops two approaches to this problem for a critical floating tree-ring chronology from the East Mediterranean Bronze–Iron Age. The chronology is more closely fixed in time using annually resolved patterns of 14C, modulated by cosmic radiation, between 1700 and 1480 BC. This placement is then tested using an anticorrelation between calendardated tree-ring growth responses to climatically effective volcanism in North American bristlecone pine and the Mediterranean trees. Examination of the newly dated Mediterranean tree-ring sequence between 1630 and 1500 BC using X-ray fluorescence revealed an unusual calcium anomaly around 1560 BC. While requiring further replication and analysis, this anomaly merits exploration as a potential marker for the eruption of Thera.
In the White Mountains of California, eight bristlecone pine (Pinus longaeva) tree-ring width chronologies were developed from trees at upper treeline and just below upper treeline along North- and ...South-facing elevational transects from treeline to ~90 m below. There is evidence for a climate-response threshold between approximately 60-80 vertical m below treeline, above which trees have shown a positive growth-response to temperature and below which they do not. Chronologies from 80 m or more below treeline show a change in climate response and do not correlate strongly with temperature-sensitive chronologies developed from trees growing at upper treeline. Rather, they more closely resemble lower elevation precipitation-sensitive chronologies. At the highest sites, trees on South-facing slopes grow faster than trees on North-facing slopes. High growth rates in the treeline South-facing trees have declined since the mid-1990s. This suggests the possibility that the climate-response of the highest South-facing trees may have changed and that temperature may no longer be the main limiting factor for growth on the South aspect. These results indicate that increasing warmth may lead to a divergence between tree growth and temperature at previously temperature-limited sites.
New tree‐ring records of ring‐width from remnant preserved wood are analyzed to extend the record of reconstructed annual flows of the Colorado River at Lee Ferry into the Medieval Climate Anomaly, ...when epic droughts are hypothesized from other paleoclimatic evidence to have affected various parts of western North America. The most extreme low‐frequency feature of the new reconstruction, covering A.D. 762‐2005, is a hydrologic drought in the mid‐1100s. The drought is characterized by a decrease of more than 15% in mean annual flow averaged over 25 years, and by the absence of high annual flows over a longer period of about six decades. The drought is consistent in timing with dry conditions inferred from tree‐ring data in the Great Basin and Colorado Plateau, but regional differences in intensity emphasize the importance of basin‐specific paleoclimatic data in quantifying likely effects of drought on water supply.
The instrumental temperature record is of insufficient length to fully express the natural variability of past temperature. High elevation tree-ring widths from Great Basin bristlecone pine (
Pinus ...longaeva
) are a particularly useful proxy to infer temperatures prior to the instrumental record in that the tree-rings are annually dated and extend for millennia. From ring-width measurements integrated with past treeline elevation data we infer decadal- to millennial-scale temperature variability over the past 4,500 years for the Great Basin, USA. We find that twentieth century treeline advances are greater than in at least 4,000 years. There is also evidence for substantial volcanic forcing of climate in the preindustrial record and considerable covariation between high elevation tree-ring widths and temperature estimates from an atmosphere–ocean general circulation model over much of the last millennium. A long-term temperature decline of ~−1.1 °C since the mid-Holocene underlies substantial volcanic forcing of climate in the preindustrial record.
Tree‐ring chronologies from bristlecone pine (Pinus longaeva) are a unique proxy used to understand climate variability over the middle to late Holocene. The annual rings from trees growing toward ...the species' lower elevational range are sensitive to precipitation variability. Interpretation of the ring‐width signal at the upper forest border has been more difficult. We evaluate differences in climate induced by topography (topoclimate) to better understand the dual signals of temperature and moisture. We unmix signals from trees growing at and near the upper forest border based on the seasonal mean temperature (SMT) experienced by each tree. We find that trees growing in exposures with SMT <7.5 ∘C are limited by temperature, while trees with SMT > 7.5 ∘C are limited by moisture. We demonstrate this independently through analysis of growth in the frequency and time domains and using a process model of xylogenesis. Furthermore, we identify increasing moisture sensitivity in trees formerly limited by temperature.
Plain Language Summary
Tree rings from ancient bristlecone pines are a remarkable proxy for climate, extending back thousands of years. However, interpreting this record has been difficult because some trees record information on both past temperature and past soil moisture variability. These dual growth responses can be hard to separate. We show that differentiating trees based on relatively small differences in location, determined by their topographical setting and therefore the “local climate” individual trees have experienced, can help unravel these mixed signals and provide a better basis for understanding growth and past climate. We find that even near the alpine treeline, only trees in the coldest settings show ring‐width growth that is sensitive to temperature. However, we also show that recent warming might have weakened this temperature response by causing bristlecone pine to switch to a moisture‐sensitive growth response.
Key Points
High elevation bristlecone pine growth patterns vary with small changes in topoclimate
Growth of high elevation trees in warmer exposures is limited by moisture, rather than temperature
The highest, coldest trees show weakened sensitivity to temperature in recent years
On the southern Colorado Plateau, three lower forest border tree-ring chronologies were used to reconstruct the temperature and precipitation regime over the past 1470 yr. The chronologies were ...derived from ponderosa pine Pinus ponderosa, pinon P. edulis, and Douglas-fir Pseudotsuga menziesii from elevations of approximately 1890 2290 m, and correlation and response function analyses were applied to investigate the climate tree growth relationship and the precipitation data. From the precipitation reconstruction calibration, the tree-ring chronologies successfully simulated both high- and low-frequency trends in the data, but the temperature reconstruction calibration showed that only decadal and multi-decadal scale variability in temperature could be captured. Indications were found of some correspondence between cool/dry and warm/wet intervals on the southern Colorado Plateau and extreme cool and warm periods reported in other regions.
The mid-second millennium BCE eruption of Thera (Santorini) offers a critically important marker horizon to synchronize archaeological chronologies of the Aegean, Egypt, and the Near East and to ...anchor paleoenvironmental records from ice cores, speleothems, and lake sediments. Precise and accurate dating for the event has been the subject of many decades of research. Using calendar-dated tree rings, we created an annual resolution radiocarbon time series 1700-1500 BCE to validate, improve, or more clearly define the limitations for radiocarbon calibration of materials from key eruption contexts. Results show an offset from the international radiocarbon calibration curve, which indicates a shift in the calibrated age range for Thera toward the 16th century BCE. This finding sheds new light on the long-running debate focused on a discrepancy between radiocarbon (late 17th-early 16th century BCE) and archaeological (mid 16th-early 15th century BCE) dating evidence for Thera.
In 2018 Pearson et al. published a new sequence of annual radiocarbon (14C) data derived from oak (Quercus sp.) trees from Northern Ireland and bristlecone pine (Pinus longaeva) from North America ...across the period 1700–1500 BC. The study indicated that the more highly resolved shape of an annually based calibration dataset could improve the accuracy of 14C calibration during this period. This finding had implications for the controversial dating of the eruption of Thera in the Eastern Mediterranean. To test for interlaboratory variation and improve the robustness of the annual dataset for calibration purposes, we have generated a replicate sequence from the same Irish oaks at ETH Zürich. These data are compatible with the Irish oak 14C dataset previously produced at the University of Arizona and are used (along with additional data) to examine inter-tree and interlaboratory variation in multiyear annual 14C time-series. The results raise questions about regional 14C offsets at different scales and demonstrate the potential of annually resolved 14C for refining subdecadal and larger scale features for calibration, solar reconstruction, and multiproxy synchronization.
PDF
