Tree-ring reconstruction of March-June precipitation from the Atlas cedar forest of Mount Takoucht, Béjaïa (northern Algeria)

Aim of study: A March-June precipitation has been reconstructed for the period 1830-2015 using Atlas cedar (Cedrus atlantica Manetti) tree-ring records. Area of study: Atlas cedar forest of Mount Takoucht (Béjaïa, northern Algeria). Material and methods: Seasonal correlations were computed in order to identify the best period of the year for the climate reconstruction. The temporal stability of the tree-ring signal for precipitation was checked using the split-sample calibration-verification procedure. The reconstruction was performed using the transfer function method. Main results: The reconstructed data revealed high interannual to decadal variation in late winter to early summer precipitation. Wet conditions dominated during the 1830s and 1840s and were followed by sustained dry conditions during the mid-19th century, which registered two of the most severe droughts (1858 and 1869) over the period of reconstruction. Relatively moderate climate conditions marked the late 19th and early 20th centuries. A gradual return towards drier conditions was observed from the 1920s and reached high frequencies of drought around mid-20th century. After an exceptional prolonged wet period of 24 years (1966-1989), the reconstruction registered its highest frequency in extreme dry/wet events: the decade 1993-2002 recorded the highest drought frequency of the reconstruction, with the third most severe dry event (1999), while the last years were marked by a clear shift toward wet conditions. Research highlights: These findings provide relevant records on past climate variability in one of the rainiest areas in Algeria and constitute valuable knowledge for specific drought and wet periods monitoring in the region.

Megadroughts and pluvials in southwest Australia: 1350–2017 CE

Declining winter rainfall coupled with recent prolonged drought poses significant risks to water resources and agriculture across southern Australia. While rainfall declines over recent decades are largely consistent with modelled climate change scenarios, particularly for southwest Australia, the significance of these declines is yet to be assessed within the context of long-term hydroclimatic variability. Here, we present a new 668-year (1350–2017 CE) tree-ring reconstruction of autumn–winter rainfall over inland southwest Australia. This record reveals that a recent decline in rainfall over inland southwest Australia (since 2000 CE) is not unusual in terms of either magnitude or duration relative to rainfall variability over the last seven centuries. Drought periods of greater magnitude and duration than those in the instrumental record occurred prior to 1900 CE, including two ‘megadroughts’ of > 30 years duration in the eighteenth and nineteenth centuries. By contrast, the wettest > decadal periods of the last seven centuries occurred after 1900 CE, making the twentieth century the wettest of the last seven centuries. We conclude that the instrumental rainfall record (since ~ 1900 CE) does not capture the full scale of natural hydroclimatic variability for inland southwest Australia and that the risk of prolonged droughts in the region is likely much higher than currently estimated.

Tree-ring reconstruction of snow avalanches in Şureanu Mountains (Southern Carpathians, Romania)

<p>Snow avalanches (SAs) are a widespread natural hazard in the Carpathians, damaging forests and threatening properties, tourism infrastructures and people. In Şureanu Mountains (Southern Carpathians), SA activity is not documented in the historical archives and consequently information regarding the SA frequency and their spatial extent is lacking. Along the forested avalanche paths, disturbed trees record selectively in their annual rings evidence of past events. Tree rings represent therefore a natural archive which can provide valuable information about the past SA activity. The aim of the present study is to reconstruct the occurrence and spatial extent of past SA activity with tree rings in Şureanu Mts. For this purpose, two avalanche paths adjacent to a ski area located in the central part of Şureanu Mts., have been investigated. Samples (cores and discs) collected from 121 and 141 Norway spruce (Picea abies (L.) Karst.) trees damaged by SAs along both paths have been analyzed. Tree-growth anomalies (e.g. scars, callus tissues, onset sequences of tangential rows of traumatic resin ducts, compression wood and growth suppression sequences) associated with the mechanical impact produced by SAs on trees were identified and used to reconstruct the SA history. Within the investigated paths, the reconstructed SA chronology spans the period of the last century. The minimum SA frequency and maximum extent reconstructed served to define the return periods within the two paths investigated. Tree-ring derived records provided the most consistent SA chronology in the study area, and can further be integrated in the avalanche hazard zoning assessment.</p>

Seven centuries of reconstructed Brahmaputra River discharge demonstrate underestimated high discharge and flood hazard frequency

The lower Brahmaputra River in Bangladesh and Northeast India often floods during the monsoon season, with catastrophic consequences for people throughout the region. While most climate models predict an intensified monsoon and increase in flood risk with warming, robust baseline estimates of natural climate variability in the basin are limited by the short observational record. Here we use a new seven-century (1309–2004 C.E) tree-ring reconstruction of monsoon season Brahmaputra discharge to demonstrate that the early instrumental period (1956–1986 C.E.) ranks amongst the driest of the past seven centuries (13th percentile). Further, flood hazard inferred from the recurrence frequency of high discharge years is severely underestimated by 24–38% in the instrumental record compared to previous centuries and climate model projections. A focus on only recent observations will therefore be insufficient to accurately characterise flood hazard risk in the region, both in the context of natural variability and climate change.

Tree-Ring Reconstruction of Single-Day Precipitation Totals over Eastern Colorado

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.

Tree-ring Reconstruction of Bark Beetle Disturbances in the Picea schrenkiana Fisch. et Mey. Forests of Southeast Kazakhstan

Ips hauseri Reitter is the most important bark beetle on Picea schrenkiana in southeast Kazakhstan, but its biology, ecology, and outbreak dynamics are poorly known. We dendrochronologically reconstructed a 200-year history of disturbances in the Kazakh Tien Shan P. schrenkiana forests. Only localized, low-severity bark beetle events occurred during the reconstructed period, indicating that extensive high-severity bark beetle outbreaks have not occurred historically in the Tien Shan spruce forest, unlike bark beetle outbreaks in spruce forests in North America, Europe, and Russia. Disturbance frequency doubled after about 1965, probably due to warming climate. Results, combined with the failure of an outbreak to fully develop after blowdown events associated with hurricane-force windstorms in 2011, indicate that prolonged drought may be necessary to sustain I. hauseri outbreaks, or that year-to-year variation in the Tien Shan weather prevents outbreak development. I. hauseri is probably less aggressive than I. typographus, at least on their natural hosts within their natural ranges.