Subfossil peatland trees as proxies for Holocene palaeohydrology and palaeoclimate

Abstract

1Due to the scarcity of reliable and highly resolved moisture proxies covering much of the 2 Holocene, there has been increased interest in the study of living and subfossil peatland trees 3 sensitive to gradual and extreme changes in hydrology, precipitation, and related environmental 4 processes. Peatland development and the associated carbon accumulation, which are strongly 5 influenced by hydrological fluctuations, are also of prime importance as peatlands represent long-6 term sinks of atmospheric carbon. Improved knowledge of peatland development and soil 7 moisture variability during the Holocene is therefore essential to our understanding of long-term 8 hydroclimate changes, the terrestrial carbon cycle, and to enable more robust predictions of 9 peatland response to future climate changes. 10Here, we review the existing mid-to late Holocene peatland tree-ring chronologies that 11 have been used to study climate variability on (sub-)annual to centennial scales with a primary 12 focus on northern Europe. Since the 1970s, absolutely dated tree-ring chronologies covering 13 substantial parts of the Holocene have been developed from excavated remains of oak (Quercus 14 spp.) and pine (Pinus sylvestris L.). The annual tree-ring patterns of these trees are often 15 characterized by periods of depressed growth reflecting annual to decadal hydroclimatic changes. 16In addition, changes in the spatio-temporal distribution of trees throughout the Holocene are often 17 found to reflect decadal to centennial climate and hydrological changes. Moreover, synchronicity 18 between tree-ring chronologies and tree-population dynamics over larger geographical areas 19 show periods of coherent regional climate forcing, especially during the mid-Holocene. 20This review (i) provides an overview of pioneering and recent studies presenting tree-ring 21 chronologies developed from subfossil peatland trees, and (ii) presents recent developments in the 22 fields of dendroecology (i.e. the response of tree growth and changes in vitality as a result of 23 changes in climatic variables) and dendroclimatology (i.e. the reconstruction of climate 24 4 fluctuations based on tree-ring analyses) in peatland regions. Moreover, we (iii) use long-term 25 climate reconstructions based on alternative proxies for comparison, and (iv) present different 26 ways to analyse tree-ring records to generate novel information on annual to centennial 27 timescales. This analysis is based on an unprecedented network of tree-ring chronologies from 28 Denmark, Finland, Germany, Great Britain, Ireland, Lithuania, the Netherlands, Poland, Sweden, 29and Canada, as well as a wealth of old and previously (un) published literature from Scandinavia 30 and Germany, which has not been accessible to a wider audience in the past due to inaccessibility 31 or linguistic barriers. Finally, a map of possible hotspots for the assessment of continuous 32 peatland-tree studies is presented, along with suggestions for new research directions in the field. 33 34

Absence of Age‐Related Trends in Stable Oxygen Isotope Ratios From Oak Tree Rings

“…are widespread, their timber is a common component of both historic buildings and archaeological remains, and oak stems are also abundant in river gravels and preserved in peat (Edvardsson et al, 2016;Haneca et al, 2009). In NW Europe the two native oak species (Q. robur L. and Q. petraea Liebl.)…”

Dendrochronologically dated pine stumps document phase-wise bog expansion at a northwest German site between ca. 6700 and ca. 3400 BC

“…The replication of the northwest German dendrochronological pine record, even though it is influenced by sampling patterns and the accessibility of trees depending on peat mining, also reflects climate. This shows in the European similarities of chronology replication (Edvardsson et al, 2016). In addition, it is also illustrated by the comparison with other climate-related records (Fig.…”

“…The water table rise observed in mires can be lagged behind the climatic shifts by months or even years (Edvardsson et al, 2016).…”

“…The trees dated absolutely by dendrochronology deliver the finest temporal reconstruction of bog expansion and local bog formation, particularly by their dying-off dates (Eckstein et al, 2009(Eckstein et al, , 2010Edvardsson et al, 2014Edvardsson et al, , 2016Krąpiec et al, 2016;Leuschner et al, 2002Leuschner et al, , 2007. Eckstein et al (2009Eckstein et al ( , 2010Eckstein et al ( , 2011 have shown that the tree dying-off occurs in phases, which are often synchronous in different mires.…”

Arctic hydroclimate variability during the last 2000 years – current understanding and research challenges

“…replication curves) were obtained for the two environments during the Common Era. High accumulation of peatland pines during the MCA indicated dry surface conditions beneficial for pine colonization (Torbenson et al, 2015;Edvardsson et al, 2016). This phase overlapped with a phase of low accumulation (low preservation) of riparian pine trees.…”