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

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Dendrochronological analysis was applied to subfossil remains of Scots pine (Pinus sylvestris L.) buried in a South Swedish peat deposit. In combination with peat stratigraphy, this approach was explored for its potential to provide information on the local hydrological and depositional history at the site, forming the basis for a regional palaeohydrological analysis. A 726-year ring-width chronology was developed and assigned an absolute age of 7233-6508 cal a BP (5284-4559 BC) through cross-dating with German bog-pine chronologies, whereas two short additional records of older ages were radiocarbon dated. Registration of growth positions of individual trees allowed assessment of the spatial dynamics of the pine population in response to hydrological changes and peatland ontogeny. Annually resolved growth variability patterns in the pine population reveal several establishment and degeneration phases, probably reflecting fluctuations in bog-surface wetness. A major establishment phase at 7200-6900 cal a BP reflects the onset of a period of lowered groundwater level, also indicated by increased peat humification, and a development consistent with regional temperature and lake level reconstructions revealed from other proxies. This study demonstrates that subfossil bog-pine populations may provide annually to decadally resolved reconstructions of local groundwater variability, which are highly relevant in a long-term palaeoclimatic context.

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

In this study, a reconstruction of the long-term development and lateral expansion of a south Swedish peat bog was performed using a multi-proxy approach, including dendrochronology, peat stratigraphy and macrofossil and pollen analyses. By combining mapping of cross-dated subfossil trees with radiocarbon-dated peat sequences, an improved approach to reconstruction of lateral peat expansion was applied. Apart from providing approximate ages of tree burial episodes, the ring-width records offer information on hydrological variations prior to the bog expansion. New bog oak, pine and alder chronologies are presented and their potential as a dating tool for peatland expansion as well as for local to regional environmental interpretations is examined. Our tree-replication records show that increased amounts of bog trees in the central parts can be linked to drier bog-surface conditions, whereas an increase in wood remains in the marginal zone is related to enhanced preservation due to lateral bog expansion. Our reconstructions of the development of the peat deposit and associated changes in the distribution of vegetation communities provide new insight into peatland responses to climate change at the end of the ‘Holocene Thermal Maximum’ (5000–4000 cal. yr BP).