Hans Asnong

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

Effects of fire severity and initial tree composition on understorey vegetation dynamics in a boreal landscape inferred from chronosequence and paleoecological data

Abstract Question and Location: How does soil burn severity and early post-fire tree composition affect long-term understorey vegetation dynamics in the coniferous forests of eastern Canada? Method: Vegetation dynamics were assessed using paleoecological methods and a chronosequence analysis of extant stands. The relation between environmental factors and succession was evaluated using ordination techniques on the chronosequence data. Understorey succession was studied by regression analysis on the chronosequence data and through within-site Markovian transition probabilities between successive 1-cm layers of plant macroremains from soil organic matter profiles. Results: Initial tree composition (Picea mariana and Pinus banksiana) had little effect on understorey composition. Soil burn severity (measured as the thickness of the residual forest floor humus) significantly affected temporal changes in understorey species. Following fires of high severity, stands underwent a gradual paludification with a net increase in Sphagnum and ericaceous shrubs (Ledum groenlandicum), and a decrease in feathermosses. Paludification was accelerated after low severity fires, which led to the dominance of Sphagnum less than 200 years after fire, and of L. groenlandicum shortly after fire. In situ paleoecological work confirmed results obtained with the chrono-sequence analysis. Conclusions: One vegetation gradient related to time after disturbance is insufficient to account for the full complexity of long-term changes in understorey composition following fire. Current forestry practices that protect the forest floor humus may induce a premature paludification. Abbreviations: AMS = Accelerated mass spectrometry; GCC = Global climate change; HS = High severity; LS = Low severity; TSF = Time since last fire. Nomenclature: Marie-Victorin (1995) and Montgomery (1977) for vascular plants; Anderson et al. (1990) for bryophytes and Lévesque et al. (1988) for macrofossils.

Recent peat accumulation rates in minerotrophic peatlands of the Bay James region, Eastern Canada, inferred by 210Pb and 137Cs radiometric techniques.

(210)Pb and (137)Cs dating techniques are used to characterise recent peat accumulation rates of two minerotrophic peatlands located in the La Grande Rivière hydrological watershed, in the James Bay region (Canada). Several cores were collected during the summer 2005 in different parts of the two selected peatlands. These minerotrophic patterned peatlands are presently affected by erosion processes, expressed by progressive mechanical destruction of their pools borders. This erosion process is related to a water table rise induced by a regional increase of humidity since the last century. The main objective of the present paper is to (1) evaluate if (210)Pb and (137)Cs dating techniques can be applied to build accurate chronologies in these environments and (2) detect changes in the peat accumulation rates in regard to this amplification of humidity. In both sites, unsupported (210)Pb shows an exponential decreasing according to the depth. Chronologies inferred from (210)Pb allow to reconstruct peat accumulation rates since ca. 1855 AD. The (137)Cs data displayed evident mobility and diffusion, preventing the establishment of any sustained chronology based on these measurements. In the two sites, peat accumulation rates inferred from (210)Pb chronologies fluctuate between 0.005 and 0.038 g cm(-2) yr(-1). As a result, the rise of the water table during the last decade has not yet affected peat accumulation rates.

Holocene carbon dynamics of boreal and subarctic peatlands from Québec, Canada

Peatlands constitute major sinks of organic carbon (C) and play a key role in the global C cycle. Here, we present a synthesis of peat records from six ecoclimatic regions in Québec, Canada, in order to quantify Holocene patterns of C accumulation and relationships with contemporary climate data. Average long-term apparent rates of C accumulation (LORCA) were calculated for 21 peat cores and range from 10 to 70 g C/m2/yr with a mean of 26.1 (standard error of mean (SEM) = 3.6) g C/m2/yr, which is slightly higher than the mean value for northern peatlands as a whole (Loisel et al., 2014). We found that regional climate has been a major factor controlling long-term peatland C accumulation and that site-specific factors may explain some variability between sites. Our data show that LORCA tend to decrease with latitude. The lowest LORCA are found in the northernmost peatlands located at the boreal forest/forest-tundra ecotone, whereas the highest values are recorded in the peatlands along the St. Lawrence Estuary, characterized by the highest mean summer temperature, number of growing degree-days above 0°C and mean annual precipitation. Temporal variations in Holocene C accumulations rates were synthesized for 16 peat cores, which show high values during the mid-Holocene (6000–4000 cal. yr BP) followed by a decline during the Neoglacial cooling, especially between 2000 and 1200 cal. yr BP. Our study contributes to a better understanding of sensitivity of peatland C balance to climate change in a poorly documented part of the circumboreal region.

Holocene palaeoecological reconstruction of three boreal peatlands in the La Grande Rivière region, Quebec, Canada

Pollen and macrofossil analyses from central peat cores along with 23 radiocarbon dates were used in palaeoecological reconstructions for three peatlands (LG1, LG2 and LG3) within the lower La Grande Rivière watershed in northern boreal Québec. Basal ages from LG3 and LG2 indicate up to an 1100 years later and possibly more abrupt Tyrrell Sea retreat in the LG3 area compared with the timeline for the region. Both autogenic and allogenic factors were found to have influenced local vegetation succession and rates of peat accumulation. Internal autogenic factors such as peat accumulation were key elements for the general peatland developmental pathway that followed the classic hydrosere sequence (pond-fen-bog). Regional climate and hydrography are the main external factors associated with changes in vegetation assemblages, surface wetness and consequently rates of peat accumulation. The LG2 and LG3 peatlands began developing shortly after 7000 cal. BP as shallow ponds with herbaceous freshwater aquatic and emergent taxa. Both sites rapidly evolved into fens with brown mosses. The autogenic transition from fen to bog occurred at both sites between 6000 and 5500 cal. BP. A long-term decrease in peat accumulation rates corresponding to a gradual densification of the local tree and shrub cover occurred at the LG2 and LG3 sites between 5000 and 1500 cal. BP. These changes were simultaneous at the two sites and therefore suggest the influence of external factors such as a shift to cooler and drier climatic conditions from the middle to late Holocene (Neoglacial period). Development of the LG1 site was delayed by a much later Tyrrell Sea retreat and started with a relatively long eutrophic aquatic phase. Both internal factors (minerotrophic conditions) and external factors (local topography and climate) contributed to the somewhat slower pace of vegetation succession. Synchronous increased peat accumulation rates in the last 1500 years at the three sites are attributable to regional vegetational shifts possibly due to the influence of recent climatic conditions and lower peat compaction of the acrotelm.