Improving estimates of tropical peatland area, carbon storage, and greenhouse gas fluxes

The workshops that led to this article were supported financially by the Universities of Leicester and Nottingham, and the Natural Environment Research Council-funded ‘Earth Observation Technology Cluster’ knowledge exchange initiativeOur limited knowledge of the size of the carbon pool and exchange fluxes in forested lowland tropical peatlands represents a major gap in our understanding of the global carbon cycle. Peat deposits in several regions (e.g. the Congo Basin, much of Amazonia) are only just beginning to be mapped and characterised. Here we consider the extent to which methodological improvements and improved coordination between researchers could help to fill this gap. We review the literature on measurement of the key parameters required to calculate carbon pools and fluxes, including peatland area, peat bulk density, carbon concentration, above-ground carbon stocks, litter inputs to the peat, gaseous carbon exchange, and waterborne carbon fluxes. We identify areas where further research and better coordination are particularly needed in order to reduce the uncertainties in estimates of tropical peatland carbon pools and fluxes, thereby facilitating better-informed management of these exceptionally carbon-rich ecosystems.PostprintPeer reviewe

Vegetation responses to abrupt climatic changes during the Last Interglacial Complex (Marine Isotope Stage 5) at Tenaghi Philippon, NE Greece

The discovery that climate variability during the Last Glacial shifted rapidly between climate states has intensified efforts to understand the distribution, timing and impact of abrupt climate change under a wide range of boundary conditions. In contribution to this, we investigate the nature of abrupt environmental changes in terrestrial settings of the Mediterranean region during the Last Interglacial Complex (Marine Isotope Stage [MIS] 5) and explore the relationships of these changes to high-latitude climate events. We present a new, temporally highly resolved (mean: 170 years) pollen record for the Last Interglacial Complex from Tenaghi Philippon, north-east Greece. The new pollen record, which spans the interval from 130,000 to 65,000 years ago, forms part of an exceptionally long polleniferous sediment archive covering the last 1.35 million years. The pollen data reveal an interglacial followed by alternating forest and steppe phases representing the interstadials and stadials of the Early Glacial. Superimposed on these millennial-scale changes is evidence of persistent sub-millennial-scale variability. We identify ten high-amplitude abrupt events in the pollen record, characterised by rapid contractions of closed forest to open steppe environment and interpreted to indicate major changes in moisture availability and temperature. The contractions in forest cover on millennial timescales appear associated with cooling events in the Mediterranean Sea, North Atlantic and Greenland regions, linked to the Dansgaard-Oeschger (DO) cycles of the Early Glacial. On sub-millennial timescales, the pattern of changes in forest cover at Tenaghi Philippon display a structure similar to the pattern of short-lived precursor and rebound-type events detected in the Greenland ice-core record. Our findings indicate that persistent, high-amplitude environmental variability occurred throughout the Early Glacial, on both millennial and submillennial timescales. Furthermore, the similarity of the pattern of change between Tenaghi Philippon and Greenland on sub-millennial timescales suggests that teleconnections between the high-latitudes and the Mediterranean region operate on sub-millennial timescales and that some terrestrial archives, such as Tenaghi Philippon, are particularly sensitive recorders of these abrupt climate changes

Net primary productivity and litter decomposition rates in two distinct Amazonian peatlands

Measurements of net primary productivity (NPP) and litter decomposition from tropical peatlands are severely lacking, limiting our ability to parameterise and validate models of tropical peatland development and thereby make robust predictions of how these systems will respond to future environmental and climatic change. Here, we present total NPP (i.e., above- and below-ground) and decomposition data from two floristically and structurally distinct forested peatland sites within the Pastaza Marañón Foreland Basin, northern Peru, the largest tropical peatland area in Amazonia: (1) a palm (largely Mauritia flexuosa) dominated swamp forest and (2) a hardwood dominated swamp forest (known as ‘pole forest’, due to the abundance of thin-stemmed trees). Total NPP in the palm forest and hardwood-dominated forest (9.83 ± 1.43 and 7.34 ± 0.84 Mg C ha⁻¹ year⁻¹, respectively) was low compared with values reported for terra firme forest in the region (14.21–15.01 Mg C ha⁻¹ year⁻¹) and for tropical peatlands elsewhere (11.06 and 13.20 Mg C ha⁻¹ year⁻¹). Despite the similar total NPP of the two forest types, there were considerable differences in the distribution of NPP. Fine root NPP was seven times higher in the palm forest (4.56 ± 1.05 Mg C ha⁻¹ year⁻¹) than in the hardwood forest (0.61 ± 0.22 Mg C ha⁻¹ year⁻¹). Above-ground palm NPP, a frequently overlooked component, made large contributions to total NPP in the palm-dominated forest, accounting for 41% (14% in the hardwood-dominated forest). Conversely, Mauritia flexuosa litter decomposition rates were the same in both plots: highest for leaf material, followed by root and then stem material (21%, 77% and 86% of mass remaining after 1 year respectively for both plots). Our results suggest potential differences in these two peatland types' responses to climate and other environmental changes and will assist in future modelling studies of these systems

Genesis and development of an interfluvial peatland in the central Congo Basin since the Late Pleistocene

The central Congo Basin contains the largest known peatland complex in the tropics. Here we present a detailed multi-proxy record from a peat core, CEN-17.4, from the centre of a 45 km wide interfluvial peatland (Ekolongouma), the first record of its kind from the central Congo peatlands. We use pollen, charcoal, sedimentological and geochemical data to reconstruct the site's history from the late Pleistocene to the present day. Peat began accumulating at the centre of the peatland ∼19,600 cal BP (∼17,500–20,400 cal BP, 95% confidence interval), and between ∼9500 (9430–9535 cal BP) and 10,500 (10,310–10,660 cal BP) cal BP towards the margins. Pollen data from the peatland centre show that an initial grass- and sedge-dominated vegetation, which burned frequently, was replaced by a Manilkara-type dominated flooded forest at ∼12,640 cal BP, replaced in turn by a more mixed swamp forest at ∼9670 cal BP. Mixed swamp forest vegetation has persisted to the present day, with variations in composition and canopy openness likely caused at least in part by changes in palaeo-precipitation. Stable isotope data (δDn-C29-v&icecorr) indicate a large reduction in precipitation beginning ∼5000 and peaking ∼2000 cal BP, associated with the near-complete mineralization of several metres of previously accumulated peat and with a transition to a drier, more heliophilic swamp forest assemblage, likely with a more open canopy. Although the peatland and associated vegetation recovered from this perturbation, the strong response to this climatic event underlines the ecosystem's sensitivity to changes in precipitation. We find no conclusive evidence for anthropogenic activity in our record; charcoal is abundant only in the Pleistocene part of the record and may reflect natural rather than anthropogenic fires. We conclude that autogenic succession and variation in the amount and seasonality of precipitation have been the most important drivers of ecological change in this peatland since the late Pleistocene

Atmospheric methane, southern European vegetation and low-mid latitude links on orbital and millennial timescales

Pollen records from marine and terrestrial sequences in southern Europe reveal a strong coherence between changes in tree populations and atmospheric methane concentrations over the last 800 thousand years. Variations in the continental hydrological balance provide a link for the observed patterns, leading to concomitant changes in southern European vegetation, and low-latitude wetland extent and methane/ volatile organic compound emissions, although additional contributions to the methane budget from extratropical sources are not excluded. Here we propose that the close coupling between low- and mid- latitude hydrological changes reflects shifts in the mean latitudinal position of the Intertropical Convergence Zone, which determines the extent to which southern Europe is dominated by subtropical or mid/high- latitude influences. This provides a conceptual framework within which to view vegetation variability in southern Europe on orbital and millennial timescales

The geochemistry of Amazonian peats

The chemical, physical and palaeobotanical composition of peat can be used to infer the history of a peatland and the processes presently operating within it. Here we present new data on the geochemistry of a peat sequence from a lowland palm swamp, Quistococha, in Peruvian Amazonia. We show, through comparison with subfossil pollen data from the same sequence, that changes in the depositional environment cause changes in peat properties including lignin content, C/N ratios, and the abundance of several metal cations, but that these properties are altered by post-depositional processes to a large extent. An upward trend in the top 1.5 m of the sequence in the concentrations of N, K, Ca, Mg and Na probably reflects nutrient uptake and cycling by the standing biomass. Upward trends in Mn and Fe concentrations suggest that limited oxygenation of the peat may occur to a similar depth. Comparison with other published records suggests that such deep biological alteration may be characteristic of tropical forested peats

Persistent millennial-scale climate variability in Southern Europe during Marine Isotope Stage 6

Exploring the mode and tempo of millennial-scale climate variability under evolving boundary conditions can provide insights into tipping points in different parts of the Earth system, and can facilitate a more detailed understanding of climate teleconnections and phase relationships between different Earth system components. Here we use fossil diatom and stable carbon and oxygen isotope analysis of lake sediment deposits (core I-284) from the Ioannina basin, NW Greece, to explore in further detail millennial-scale climate instability in southern Europe during Marine Isotope Stage 6 (MIS 6; ca. 185‒130 ka). This interval correlates with the Vlasian Stage in Greece and the Late Saalian Substage in northern Europe, which were both characterised by extensive glaciations. The new dataset resolves at least 18 discrete warmer/wetter intervals, many of which were associated with strong Asian Monsoon events and North Atlantic interstadials. A number of cooler/drier intervals are also identified in the I-284 record, which are typically associated with weaker Asian Monsoon events and North Atlantic stadials, consistent with a variable Atlantic Meridional Overturning Circulation. Unlike the subdued changes in tree populations that are observed at Ioannina during mid-to-late MIS 6, the diatom record contains frequent high-amplitude oscillations in species assemblages, pointing to its sensitivity at a time when the lake system must have been close to environmental thresholds. Millennial-scale variability in diatom species assemblages continues into late MIS 6 at Ioannina, contributing important evidence for an emerging picture of frequent and persistent climate instability even at times of high global ice volume

Campanian Ignimbrite tephra reveals asynchronous vegetation responses to abrupt climate change in the eastern Mediterranean region

The timing and rate of ecosystem response to abrupt climate change is a product of numerous complex interactions between biotic and abiotic drivers. Palaeoecological studies from long sedimentary records, particularly those that span periods of dynamic climate such as the last glacial cycle, can help to contextualise ecosystem responses to climate variability through time. Detailed studies that compare proxy data from multiple sites, with high chronological precision, have the potential to ascribe mutual climate drivers, and, therefore, track spatiotemporal variability in ecosystem responses. Here, we interrogate the vegetation impact of past climate change in the eastern Mediterranean, using three sub-centennially resolved pollen archives from Greece. The widespread Campanian Ignimbrite (CI/Y-5; ca. 39.85 ka BP) tephra marker is used as an isochron to directly correlate pollen records from Ioannina (NW Greece), Tenaghi Philippon (NE Greece), and Megali Limni (NE Aegean). Our results reveal spatiotemporal variability in the timing of vegetation response in the Mediterranean to climate forcing across Heinrich Stadial 4 (40.2–38.3 ka BP), a period of known abrupt climatic change. We identify a decline in tree pollen in all three sites, likely related to the onset of enhanced regional aridity, with vegetation at Tenaghi Philippon responding prior to the CI/Y-5, in contrast to at Megali Limni and Ioannina, where much of the vegetation change occurs following tephra deposition

Deforestation and human agency in the North Atlantic region: Archaeological and palaeoenvironmental evidence from the Western Isles of Scotland

This paper considers the timing and mechanisms of deforestation in the Western Isles of Scotland, focusing in particular on the landscape around the Calanais stone circles, one of the best preserved late Neolithic/early Bronze Age monumental landscapes in north-west Europe. We present new archaeological and palaeoenvironmental evidence from a soil and peat sequence at the site of Aird Calanais, which spans the main period of use of the Calanais circles. We then draw on a new synthesis of archaeobotanical and palynological evidence from across the Western Isles and a review of comparable data from the wider North Atlantic zone, before assessing the role of early farming communities in clearing the wooded landscapes of the region. Pollen and radiocarbon dating at the site of Aird Calanais reveal that a layer of birch branches, dating to the late Neolithic (2912–2881 cal bc), was contemporaneous with a decline in woodland at the site, as well as with the major phase of Neolithic activity at the Calanais stone circle complex. However, our synthesis of the pollen and plant macrofossil evidence from across the Western Isles suggests that the picture across these islands was altogether more complex: woodlands declined both before, as well as during, the Neolithic and deciduous woodlands remained sufficiently abundant for Neolithic fuel procurement. Finally, we consider the implications of the results for understanding the interactions between first farmers and woodlands in the wider North Atlantic region