Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.

The calibration of radiocarbon dates by means of a master calibration curve has been invaluable to Earth, environmental and archeological sciences, but the fundamental reason for calibration is that atmospheric radiocarbon content varies because of changes in upper atmosphere production and global carbon cycling. Improved instrumentation has contributed to high-resolution (interannual) radiocarbon activity measurements, which have revealed sudden and anomalous activity shifts previously not observed at the common resolution of 5-10 years of most of the calibration scale. One such spike has been recently reported from tree rings from Japan and then again in Europe at A.D. 774-775, for which we report here our efforts to both replicate its existence and determine its spatial extent using tree rings from larch at high latitude (northern Siberia) and bristlecone pine from lower latitude (the White Mountains of California). Our results confirm an abrupt~15‰ 14 C activity increase from A.D. 774 to 776, the size and now the hemispheric extent of which suggest that an extraterrestrial influence on radiocarbon production is most likely responsible.

SummaryIn conifers tracheids fulfill most of the main wood functions (mechanical support, water transport). Earlywood tracheids account for most hydraulic conductivity within the annual tree ring. Therefore, dry conditions during the early growing season, when earlywood is formed, should lead to the formation of narrow tracheid lumens and a dense earlywood. Here we test if there is an inverse relationship between minimum wood density and early growing-season (spring) precipitation. We study growth and density data of three Pinaceae species (Pinus sylvestris, Pinus nigra, and Larix sibirica) widely distributed in three cool-dry Eurasian regions from the forest-steppe (Russia, Mongolia) and Mediterranean (Spain) biomes. Using dendrochronology, we measured for each annual tree ring and the common 1950-2002 period the following variables: earlywood (EW hereafter) and latewood widths (LW hereafter), and minimum (MN hereafter) and maximum wood density (MX hereafter). As expected, dry early-growing season (spring) conditions were associated to low EW values but, most importantly, to high MN values in the three study species. The associations between MN and spring precipitation were stronger than those observed with EW. We interpret the relationship between spring water availability and high minimum density as a drought-induced reduction in lumen diameter, hydraulic conductivity and growth. Consequently, forecasted growing-season drier conditions would translate into increased minimum wood density and reflect a reduction in hydraulic conductivity, radial growth and wood formation. Increased aridity would diminish the ability of Eurasian conifer forests subjected to coldness and drought to fix and store carbon as durable woody pools.

The long-term influence of climate change on spatio-temporal dynamics of the Polar mycobiota was analyzed on the eastern macro slope of the Polar Urals (Sob River valley and Mountain Slantsevaya) over a period of 60 years. The anthropogenic impact is minimal in the study area. Effects of environmental warming were addressed as changes in treeline and forest communities (greening of the vegetation). With warming, permafrost is beginning to thaw, and as it thaws, it decomposes. Therefore, we also included depth of soil thawing and litter decomposition in our study. Particular attention was paid to the reaction of aphyllophoroid fungal communities concerning these factors. Our results provide evidence for drastic changes in the mycobiota due to global warming. Fungal community composition followed changes of the vegetation, which was transforming from forest-tundra to northern boreal type forests during the last 60 years. Key fungal groups of the ongoing borealization and important indicator species are discussed. Increased economic activity in the area may lead to deforestation, destruction of swamps, and meadows. However, this special environment provides important services such as carbon sequestration, soil formation, protecting against flood risks, and filtering of air. In this regard, we propose to include the studied territory in the Polarnouralsky Natural Park.

Background: Recent warming is affecting species composition and species areal distribution of many regions. However, although most treeline studies have estimated the rates of forest expansion into tundra, still little is known about the long-term dynamic of stand productivity at the forest-tundra intersection. Here, we make use of tree-ring data from 350 larch (Larix sibirica Ledeb.) and spruce (Picea obovata Ledeb.) sampled along the singular altitudinal treeline ecotone at the Polar Urals to assess the dynamic of stand establishment and productivity, and link the results with meteorological observations to identify the main environmental drivers. Results: The analysis of stand instalment indicated that more than 90% of the living trees appeared after 1900. During this period, the stand became denser and moved 50 m upward, while in recent decades the trees of both species grew faster. The maximum afforestation occurred in the last decades of the twentieth century, and the large number of encountered saplings indicates that the forest is still expanding. The upward shift coincided with a slight increase of May-August and nearly doubling of September-April precipitation while the increase in growth matched with an early growth season warming (June + 0.27°C per decade since 1901). This increase in radial growth combined with the stand densification led to a 6-90 times increase of biomass since 1950. Conclusion: Tree-ring based twentieth century reconstruction at the treeline ecotone shows an ongoing forest densification and expansion accompanied by an increased growth. These changes are driven by climate change mechanism, whereby the leading factors are the significant increase in May-June temperatures and precipitation during the dormant period. Exploring of phytomass accumulation mechanisms within treeline ecotone is valuable for improving our understanding of carbon dynamics and the overall climate balance in current treeline ecosystems and for predicting how these will be altered by global change.