| 1. |
- Duethorn, Elisabeth, et al.
(författare)
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Influence of micro-site conditions on tree-ring climate signals and trends in central and northern Sweden
- 2013
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Ingår i: Trees. - : Springer Science and Business Media LLC. - 0931-1890 .- 1432-2285. ; 27:5, s. 1395-1404
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Tidskriftsartikel (refereegranskat)abstract
- Tree-ring chronologies are important indicators of pre-instrumental, natural climate variability. Some of the longest chronologies are from northern Fennoscandia, where ring width measurement series from living trees are combined with series from sub-fossil trees, preserved in shallow lakes, to form millennial-length records. We here assess the recent ends of such timeseries by comparing climate signals and growth characteristics in central and northern Sweden, of (1) trees growing at lakeshore micro-sites (representing the source of sub-fossil material of supra-long chronologies), with (2) trees collected in dryer micro-sites several meters inland. Calibration trials reveal a predominating June-September temperature signal in N-Sweden and a weaker but significant May-September precipitation signal in C-Sweden. At the micro-site level, the temperature signal in N-Sweden is stronger in the lakeshore trees compared to the inland trees, whereas the precipitation signal in C-Sweden remains unchanged among the lakeshore and inland trees. Tree-rings at cambial ages > 40 years are also substantially wider in the lakeshore micro-site in C-Sweden, and juvenile rings are more variable (and wider) in the dryer micro-site in N-Sweden (compared to the adjacent micro-sites). By combining the data of the various micro-sites with relict samples spanning the past 1,000 years, we demonstrate that growth rate differences at the micro-site scale can affect the low frequency trends of millennial-length chronologies. For the supra-long chronologies from northern Fennoscandia, that are derived from sub-fossil lake material, it is recommended to combine these data with measurement series from only lakeshore trees.
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| 2. |
- Esper, Jan, et al.
(författare)
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Northern European summer temperature variations over the Common Era from integrated tree-ring density records
- 2014
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 29:5, s. 487-494
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Tidskriftsartikel (refereegranskat)abstract
- Tree-ring chronologies of maximum latewood density are most suitable to reconstruct annually resolved summer temperature variations of the late Holocene. The two longest such chronologies have been developed in northern Europe stretching back to the 2nd century BC, and the 5th century AD. We show where similarities between the two chronologies exist, and combine portions of both into a new summer temperature reconstruction for the Common Era. To minimize the transfer of potential biases, we assess the contribution of the candidate reconstructions' measurements, and exclude data (i) from exceptionally young and old trees, and (ii) produced by different laboratory technologies. Our new composite reconstruction reveals warmer conditions during Roman, Medieval and recent times, separated by prolonged cooling during the Migration period and Little Ice Age. Twentieth century warmth, as indicated in one of the existing density records, is reduced in the new reconstruction, also affecting the overall, millennial-scale, cooling trend over the late Holocene (-0.30 degrees C per 1000 years). Due to the reduced biological memory, typical for tree-ring density measurements, the new reconstruction is most suitable for evaluating the rate and speed of abrupt summer cooling following large volcanic eruptions.
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| 3. |
- Hellmann, Lena, et al.
(författare)
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Diverse growth trends and climate responses across Eurasia's boreal forest
- 2016
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 11:7
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Tidskriftsartikel (refereegranskat)abstract
- The area covered by boreal forests accounts for similar to 16% of the global and 22% of the Northern Hemisphere landmass. Changes in the productivity and functioning of this circumpolar biome not only have strong effects on species composition and diversity at regional to larger scales, but also on the Earth's carbon cycle. Although temporal inconsistency in the response of tree growth to temperature has been reported from some locations at the higher northern latitudes, a systematic dendroecological network assessment is still missing for most of the boreal zone. Here, we analyze the geographical patterns of changes in summer temperature and precipitation across northern Eurasia >60 degrees N since 1951 AD, as well as the growth trends and climate responses of 445 Pinus, Larix and Picea ring width chronologies in the same area and period. In contrast to widespread summer warming, fluctuations in precipitation and tree growth are spatially more diverse and overall less distinct. Although the influence of summer temperature on ring formation is increasing with latitude and distinct moisture effects are restricted to a few southern locations, growth sensitivity to June-July temperature variability is only significant at 16.6% of all sites (p <= 0.01). By revealing complex climate constraints on the productivity of Eurasia's northern forests, our results question the a priori suitability of boreal tree-ring width chronologies for reconstructing summer temperatures. This study further emphasizes regional climate differences and their role on the dynamics of boreal ecosystems, and also underlines the importance of free data access to facilitate the compilation and evaluation of massively replicated and updated dendroecological networks.
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