Regional variability in peatland burning at mid- to high-latitudes during the Holocene

Acknowledgements This work developed from the PAGES (Past Global Changes) C-PEAT (Carbon in Peat on EArth through Time) working group. PAGES has been supported by the US National Science Foundation, Swiss National Science Foundation, Swiss Academy of Sciences and Chinese Academy of Sciences. We acknowledge the following financial support: UK Natural Environment Research Council Training Grants NE/L002574/1 (T.G.S.) and NE/S007458/1 (R.E.F.); Dutch Foundation for the Conservation of Irish Bogs, Quaternary Research Association and Leverhulme Trust RPG-2021-354 (G.T.S); the Academy of Finland (M.V); PAI/SIA 80002 and FONDECYT Iniciación 11220705 - ANID, Chile (C.A.M.); R20F0002 (PATSER) ANID Chile (R.D.M.); Swedish Strategic Research Area (SRA) MERGE (ModElling the Regional and Global Earth system) (M.J.G.); Polish National Science Centre Grant number NCN 2018/29/B/ST10/00120 (K.A.); Russian Science Foundation Grant No. 19-14-00102 (Y.A.M.); University of Latvia Grant No. AAp2016/B041/Zd2016/AZ03 and the Estonian Science Council grant PRG323 (TrackLag) (N.S. and A.M.); U.S. Geological Survey Land Change Science/Climate Research & Development Program (M.J., L.A., and D.W.); German Research Foundation (DFG), grant MA 8083/2-1 (P.M.) and grant BL 563/19-1 (K.H.K.); German Academic Exchange Service (DAAD), grant no. 57044554, Faculty of Geosciences, University of Münster, and Bavarian University Centre for Latin America (BAYLAT) (K.H.K). Records from the Global Charcoal Database supplemented this work and therefore we would like to thank the contributors and managers of this open-source resource. We also thank Annica Greisman, Jennifer Shiller, Fredrik Olsson and Simon van Bellen for contributing charcoal data to our analyses. Any use of trade, firm, or product name is for descriptive purposes only and does not imply endorsement by the U.S. Government.Peer reviewedPostprin

Widespread drying of European peatlands in recent centuries

This is the author accepted manuscript. The final version is available from Nature Research via the DOI in this record Climate warming and human impacts are thought to be causing peatlands to dry,potentially converting them from sinks to sources of carbon. However, it is unclear whether the hydrological status of peatlands has moved beyond their natural envelope. Here we show that European peatlands have undergone substantial, widespread drying during the last ~300 years. We analyse testate amoeba-derived hydrological reconstructions from 31 peatlands across Britain, Ireland, Scandinavia and continental Europe to examine changes in peatland surface wetness during the last 2000 years. 60% of our study sites were drier during the period CE 1800-2000 than they have been for the last 600 years; 40% of sites were drier than they have been for 1000 years; and 24% of sites were drier than they have been for 2000 years. This marked recent transition in the hydrology of European peatlands is concurrent with compound pressures including climatic drying, warming and direct human impacts on peatlands, although these factors vary between regions and individual sites. Our results suggest that the wetness of many European peatlands may now be moving away from natural baselines. Our findings highlight the need for effective management and restoration of European peatlands.Natural Environment Research Council (NERC

Climate drivers for peatland palaeoclimate records

Reconstruction of hydroclimate variability is an important part of understanding natural climate change on decadal to millennial timescales. Peatland records reconstruct ‘bog surface wetness’ (BSW) changes, but it is unclear whether it is a relative dominance of precipitation or temperature that has driven these variations over Holocene timescales. Previously, correlations with instrumental climate data implied that precipitation is the dominant control. However, a recent chironomid inferred July temperature record suggested temperature changes were synchronous with BSW over the mid-late Holocene. This paper provides new analyses of these data to test competing hypotheses of climate controls on bog surface wetness and discusses some of the distal drivers of large-scale spatial patterns of BSW change. Using statistically based estimates of uncertainty in chronologies and proxy records, we show a correlation between Holocene summer temperature and BSW is plausible, but that chronologies are insufficiently precise to demonstrate this conclusively. Simulated summer moisture deficit changes for the last 6000 years forced by temperature alone are relatively small compared with observations over the 20th century. Instrumental records show that summer moisture deficit provides the best explanatory variable for measured water table changes and is more strongly correlated with precipitation than with temperature in both Estonia and the UK. We conclude that BSW is driven primarily by precipitation, reinforced by temperature, which is negatively correlated with precipitation and therefore usually forces summer moisture deficit in the same direction. In western Europe, BSW records are likely to be forced by changes in the strength and location of westerlies, linked to large-scale North Atlantic ocean and atmospheric circulation

Relationship between plot-LAI and mesocosm evapotranspiration (ET) for the summers of 2008, 2009 and 2010.

<p>ET of the mesocosms with trees (LT and HT) were averaged over the summer (period V) for each year separately and standardized by dividing by the ET from mesocosms without trees (NT mesocosms). Symbols above the dashed line indicate a higher evapotranspiration than NT mesocosms, whereas symbols below this line indicate lower evapotranspiration than NT mesocosms. The solid line indicates a weak, but significant, (<i>P</i><0.05), linear relationship (linear regression, R² = 0.25, y = 0.1x+1.1).</p

Seasonal changes in tree density effects on mesocosm water table.

<p>Bars represent mean water tables ±1 SE (n = 5) in cm relative to a fixed point per week, from week 38 in 2007 (38) until week 37 in 2008 (37). Positive values indicate a water table closer to the surface. Water table level was measured relative to a fixed point (the overflow outlet), which was 10–15 cm below the moss surface. Birch density treatments are identified by differently shaded bullets. NT = control without trees, LT = low tree density, HT = high tree density. Arrows indicate onsets of leaf senescence in 2007 and leaf emergence in 2008. * = week during which storage coefficient has been determined, ** week in which demineralized water has been added to each mesocosm. ETp periods indicates periods (I-V) differing in solar radiation and potential evapotranspiration, over which evapotranspiration has been averaged for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g001" target="_blank">Figures 1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g003" target="_blank">3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-g004" target="_blank">4</a>. Note: mesocosm trees were planted in December 2007.</p

Water balance components.

<p>Water balance components (means, SE, n = 5) were calculated over the summers of 2008–2010 (period V, calendar weeks 19–32). Water balance components (P, I, R and ΔW are in mm summer⁻¹, S is in mm mm⁻¹ and ET in mm day⁻¹. Positive values of water storage refer to changes in water volume as a result of a net rise in the water table between the first and last date of period V, whereas negative values refer to changes in water volume as a result of a net decrease in the water table between the first and last date of period V. Different letters denote statistically significant differences between tree density treatments for the same year based on 2-way ANOVAs with treatment as factor and block as random factor. Ns = P<0.10, (*) = 0.10≥P<0.05, * = P≤0.05, ** = 0.05>P≤0.01, *** = P<0.01.</p

Vegetation composition understory.

<p>Abundances (means, SE, n = 5) of vascular plants, litter and moss in the mesocosms over the experiment (2007–2010). Data on 2008 are missing, due to a malfunctioning voice recorder. 100% of frame = species hit in all 150 points of the point-quadrat frame (see methods). For plants with horizontal (planar) leaf orientation 100% frame roughly corresponds to LAI = 1. Values over 100 indicate multiple hits per point. Statistics give results of repeated measures ANOVAs with treatment as within subject factor and year as between subject factor. Ns = P<0.10, (*) = 0.10≥P<0.05, ** = 0.05>P≤0.01, *** = P<0.01.</p

Seasonal changes in tree density effects on mesocosm evapotranspiration.

<p>Bars represent means +1 SE (n = 5) per birch density treatment averaged over periods (I–V) in order of increasing atmospheric demand for water. Periods I and II cover late autumn -early spring, whereas periods III–V represent mid spring-mid autumn (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0091748#pone-0091748-t002" target="_blank">Table 2</a>). Measurements spanned 1 year from week 38 in 2007 until week 37 in 2008, the only year for which we had water table data for all seasons. NT = control without trees, LT = low tree density, HT = high tree density. Different letters above the bars denote statistically significant (<i>P</i><0.05) differences between tree density treatments within a period based on five separate 2-way ANOVAs with treatment as factor and block as random factor, one ANOVA for each period.</p