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 growth responses of mature Norway spruce (Picea abies (L.) Karst.) trees exposed to elevated [CO(2)] (CE; 670-700 ppm) and long-term optimized nutrient availability or elevated air temperature (TE; ±3.9 °C) were studied in situ in northern Sweden in two 3 year field experiments using 12 whole-tree chambers in ca. 40-year-old forest. The first experiment (Exp. I) studied the interactions between CE and nutrient availability and the second (Exp. II) between CE and TE. It should be noted that only air temperature was elevated in Exp. II, while soil temperature was maintained close to ambient. In Exp. I, CE significantly increased the mean annual height increment, stem volume and biomass increment during the treatment period (25, 28, and 22%, respectively) when nutrients were supplied. There was, however, no significant positive CE effect found at the low natural nutrient availability. In Exp. II, which was conducted at the natural site fertility, neither CE nor TE significantly affected height or stem increment. It is concluded that the low nutrient availability (mainly nitrogen) in the boreal forests is likely to restrict their response to the continuous rise in [CO(2)] and/or TE.

-• A network of oak (Quercus robur L.) chronologies containing 49 sites and 635 single trees was analysed to identify weather variables affecting annual tree-ring increment dynamics in southern Sweden during 1860-2000.• We analysed (1) the growth response of oak to non-extreme weather, and (2) the temporal and spatial patterns of regional growth anomalies (pointer years) and associated climatic extremes resolved on a monthly scale.• Growth was controlled by precipitation in the current (June-July) and the previous growing season (August) in 48% and 22% of all sites, respectively. Temperature during July of the current year and August of the previous year was negatively correlated with growth in 29% and 43% of the sites, respectively. Growth was positively correlated with temperature in October of the previous season in 72% of the sites. The most extensive growth anomaly occurred in 1965 and was probably caused by intrusion of cold Arctic air masses into the region at the end of March that year.• During climatically non-extreme years, oak growth is driven mostly by the dynamics of summer precipitation. Many of the negative growth anomalies, however, were associated with temperature extremes. Southern Swedish oak pointer years tend not to coincide with the pan-European oak pointer years.climatic variability / dendroecology / extreme events / hardwood / Scandinavia Résumé -Influence de la variabilité interannuelle de climat sur la croissance du chêne pédonculé dans le Sud de la Suède.• Une série de chronologies de chênes (Quercus robur L.) comprenant 49 stations et 635 arbres individuels a été analysée pour identifier les variables climatiques affectant la dynamique de croissance annuelle des cernes dans le Sud de la Suède, pendant la période 1860-2000.• Nous avons analysé (1) la réponse de croissance des chênes aux évènements météorologiques non extrêmes et (2) les patrons temporels et spatiaux des anomalies régionales de croissance (années indices) et les extrêmes climatiques associés au pas de temps mensuel.• La croissance était contrôlée par les précipitations de la saison de végétation (juin-juillet) et les précipitations de la saison de végétation précédente (août), respectivement dans 48 % et 22 % de toutes les stations. Les températures du mois de juillet de la saison de végétation et du mois d'août de l'année précédente étaient corrélées négativement avec la croissance, respectivement dans 29 % et 43 % des stations. La croissance était corrélée positivement avec la température du mois d'octobre de l'année précédente dans 72 % des stations. L'anomalie de croissance la plus considérable est arrivée en 1965 et a été causée probablement par l'intrusion de masses d'air froid arctique dans la région, à la fin du mois de mars.• Pendant les années climatiquement non-extrêmes, la croissance du chêne est principalement commandée par la dynamique des précipitations estivales. Cependant, beaucoup d'anomalies négatives de croissance ont été associées avec des extrêmes de température. Les années indices du Sud ...

[1] Arctic environments, where surface temperatures increase and sea ice cover and permafrost depth decrease, are very sensitive to even slight climatic variations. Placing recent environmental change of the high-northern latitudes in a long-term context is, however, complicated by too short meteorological observations and too few proxy records. Driftwood may represent a unique cross-disciplinary archive at the interface of marine and terrestrial processes. Here, we introduce 1445 driftwood remains from coastal East Greenland and Svalbard. Macroscopy and microscopy were applied for wood anatomical classification; a multi-species subset was used for detecting fungi; and information on boreal vegetation patterns, circumpolar river systems, and ocean current dynamics was reviewed and evaluated. Four conifer (Pinus, Larix, Picea, and Abies) and three deciduous (Populus, Salix, and Betula) genera were differentiated. Species-specific identification also separated Pinus sylvestris and Pinus sibirica, which account for~40% of all driftwood and predominantly originate from western and central Siberia. Larch and spruce from Siberia or North America represents~26% and~18% of all materials, respectively. Fungal colonization caused different levels of driftwood staining and/or decay. Our results demonstrate the importance of combining wood anatomical knowledge with insight on boreal forest composition for successfully tracing the origin of Arctic driftwood. To ultimately reconstruct spatiotemporal variations in ocean currents, and to better quantify postglacial uplift rates, we recommend consideration of dendrochronologically dated material from many more circumpolar sites.

As part of a geqeral study of arctic driftwood, 206 samples of driftwood logs from the Mackenzie delta area were analyzed by dendrochronological methods (tree-ring studies). The aim was to detect the origin of the wood. Three forest stands in the delta were also sampled, and tree-ring chronologies were constructed. The Mackenzie driftwood can be divided into four groups: 1) driftwood originating from the upper Mackenzie delta area with individual logs having up to 600 tree rings, 2) driftwood originating near the southern limit of the delta, 3) Wood with relatively few tree rings with possible origin in the Liard River drainage area, and 4) driftwood samples not datable with any available chronologies.Three driftwood samples from the Coast of Greenland could be correlated with tree-ring chronologies from the Mackenzie delta area and another three were correlated with chronologies from Alaska. American driftwood has not been detected in collections from Svalbard and Iceland, although more than 200 samples have been analyzed from each area. This indicates that some American driftwood is transported from the Beaufort and Bering seas to the North Atlantic Ocean, probably via the western part of the East Greenland Current,. This wood is deposited on the Coast of Greenland. American driftwood probably does not reach the islands in the central and eastern North Atlantic.RÉSUMÉ. Dans le cadre d'une ttude gtntrale du bois flottt dans l'Arctique, on a analyst 206 tchantillons de billes de bois flottt provenant du delta du Mackenzie grâce B des methodes dendrochronologiques (btudes des faisceaux de croissance de l'arbre). Le but ttait de dtterminer l'origine du bois. On a aussi fait un tchantillonnage de trois peuplements de fortts dans le delta et on en a ttabli la chronologie des faisceaux de croissance. On peut diviser le bois flottt du Mackenzie en quatre groupes : 1) le bois provenant de la partie amont du delta du Mackenzie, avec des billes possddant chacune jusqu'à 600 cernes concentriques, 2) le bois flottt provenant de la limite rnbridionale du delta, 3) le bois ayant relativement peu de cernes, provenant peut-ttre du bassin hydrographique de la rivitre au Liard, et 4) des tchantillons de bois flottt pour la datation desquels aucune chronologie n'ttait disponible.On a pu corrtler par dendrochronologie trois bhantillons de bois flottt provenant de la cbte du Groenland avec des &hantillons provenant de la rtgion du delta du Mackenzie, et trois autres avec des kchantillons provenant de l'Alaska. Dans des collections provenant du Svalbard et d'Islande, on n'a pas dttectt de bois flottt amtricain, bien qu'on ait analyst plus de 200 tchantillons de chaque rtgion. Cela rtvtle qu'une petite quantitt de bois flottt amtricain est transportt de la mer de Beaufort et de la mer de Btring jusqu'h l'octan Atlantique nord, probablement par la partie occidentale du courant est-groenlandais. Ce bois est dtpost sur la c8te du Groenland. Le bois flottt amtricain n'atteint probablement pas les îles du centre et de l'est de l'Atlantique...