Vegetation, hydrology and fire effects on Holocene carbon accumulation
in boreal peatlands of the James Bay region, Quebec
1 INTRODUCTION
Besides important long-term net sinks of carbon (C), northern ombrotrophic
peatlands form sensitive records of climate-related ecohydrological change, as
they depend on precipitation as a source of water and nutrients. Low
decomposition rates are primarily the result of cold, anoxic and acidic conditions
and decay-resistant biomass. Climatic fluctuations or recurrent fires may change
the balance of production and decay, resulting in a temporary net C loss. The
interactions between climate, vegetation and fire on millennial timescales are
complex, especially considering high spatial variability in peatland vegetation.
The aim of the project is to identify the relative influence of vegetation, hydrology
and peat fire regimes on C accumulation under Holocene climate variations.
Analyses of multiple cores from several peatlands are essential to be able to
distinguish regional and local factors. Therefore, we present here the results of
nine cores that were extracted from three peatlands.
Simon van Bellen1, Michelle Garneau2 and Yves Bergeron3
1Chaire DÉCLIQUE / GEOTOP / Institut des Sciences de l’Environnement, Université du Québec à Montréal, C.P. 8888, Succ. Centre-Ville, Montreal, Qc, H3C 3P8, Canada,
[email protected] 2Chaire DÉCLIQUE / GEOTOP / Département de Géographie, Université du Québec à Montréal, C.P. 8888, Succ. Centre-Ville, Montreal, Qc, H3C 3P8, Canada
3Chaire industrielle CRSNG-UQAT-UQAM en aménagement forestier durable, Université du Québec en Abitibi-Témiscamingue, 445 boul. de l'Université, Rouyn-Noranda, Qc, J9X 5E4, Canada
REFERENCES
Booth, R.K. et al., 2008. J Quaternary Sci 23, 43-57
Haslett, J. and Parnell, A., 2008. J Roy Stat Soc C-App 57, 399-418
Higuera, P. et al., 2009. Ecol Monogr 79, 201-219
Mansuy, N. et al., 2010. Int J Wildland Fire 19, 1083-1098
Thibault, S. and Payette, S., 2009. Permaforst Periglac 20, 383-389
Turunen, J. et al., 2002. Holocene 12, 69-80
3 MATERIAL AND METHODS
Stratigraphic analyses were performed on cores
extracted from central/deep and lateral sections of
each peatland. Central, deep sections form long and
high-resolution records for vegetation and water
table reconstructions, whereas lateral sections are
more useful for fire reconstructions.
Vegetation reconstructions were obtained by plant
macrofossil analysis and past water tables were
quantified by testate amoeba assemblages from the
deep core in each peatland. Past amoeba
assemblages were converted to a water table
record using a transfer function (Booth, 2008).
Macroscopic charcoal fragments (>355 µm) were
analyzed for local fire reconstructions from 6 lateral
cores (12-132 m from upland forest). Significant
charcoal peaks were identified using CharAnalysis
(Higuera et al., 2009). Total charcoal accumulation
is referred to as CHAR and expressed as fragments
cm-2 per unit of time.
Chronologies were obtained from a total of 73
radiocarbon dates and piecewise linear interpolation
using Bchron (Haslett and Parnell, 2008). Peat C
content was quantified by high-resolution bulk
density and loss-on-ignition analyses, assuming C
represents 50% of the organic matter mass
(Turunen et al., 2002). C accumulation rates were
defined by the ratio of total amount of C
accumulated during a period and the duration of that
period, expressed as g m-2 yr-1.
VEGETATION EFFECTS
Low accumulation rates between 3000
and 1200 cal BP have been
reconstructed in the deep core from LLC
(Fig. 5).
Forced by fluctuations in water table
level, important shifts in Sphagnum
sections were reconstructed from the
same core after 3000 cal BP (Fig. 6).
Whereas the core is generally
composed of well-preserved Sphagnum
section Acutifolia during the Holocene,
the Neoglacial period shows important
episodes of the wet-tolerant section
Cuspidata. The latter is generally poorly
resistant to decomposition, which may
partly explain the decrease in C
accumulation rates.
CARBON ACCUMULATION AND FIRE
RECORDS
Regional area-weighted mean Holocene
C accumulation rate is 16.2 g m-2 yr-1. The
increase in rates towards present-day is
the result of incomplete decay in the
upper horizons.
Eastmain region peatlands show
miminum C accumulation rates between
2000 and 1200 cal BP (Fig. 1). During this
period, charcoal peak height generally
increases, indicating increased biomass
burning (Fig. 2). Although fires have likely
influenced C accumulation rates, they
have not been a driving factor as
regression analysis failed to identify a
significant negative relationship between
C accumulation rates and CHAR (Fig. 3).
4 RESULTS
2 STUDY REGION
5 DISCUSSION AND CONCLUSION
Decreasing late-Holocene C accumulation rates are associated to fluctuating water tables, decreasing Sphagnum section
Acutifolia presence, and increasing peat fire regimes. Nevertheless, Neoglacial cooling itself may have been the principal
cause for declining rates, as cooling generally shortens growing season length while diminishing primary production. Climate-
driven changes in temperature and snow cover may have allowed the formation of decadally persistent frozen peat horizons,
causing important fluctuations in water table and local vegetation. However, occasional varying trends in accumulation and
water table dynamics between cores shows that local, geomorphological or autogenic factors have mediated the climatic
influence.
ACKNOWLEDGMENTS
Thanks to A.A. Ali, H. Asnong, H. Asselin, R.K. Booth, P-
L. Dallaire, lab and field assistants and les Tourbeux for
discussion and inspiration.
Start of peat
accumulation
Mineral
sediments
NEOGLACIAL INFLUENCE
The combined water table records show important fluctuations
since 3000 cal BP (Fig. 4). As this period is associated with cold
and dry conditions and the southern limit of sporadic peatland
palsas is located ~140 km north of the Eastmain region (Thibault
and Payette, 2009), we hypothesize that dry shifts observed may
have been amplified by decadal persistence of frozen peat
horizons.
Individual fires (Fig. 3) may have coincided with regionally
recorded dry shifts, however, high-resolution dating is essential
for confirmation
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Sphagnum fuscum stem leaf (×100)
Eastmain region is located near
52ºN/75-76ºW. Mean annual
precipitation and temperature is
735 mm and -2.1ºC. Mean forest
fire frequency is one event every
~90 years (Mansuy et al., 2010).
The three peatlands (1.7-2.7 km2)
show important hummock-hollow
patterns, with Picea mariana and
Ericaceae at the transition to the
upland forest. Maximum peat
thickness attains ~5 m.
10
20
30
0 200 400 600 800
C a
ccum
ula
tion r
ate
(g
m-2
yr-1
)
CHAR (pieces cm-2
1000 yrs-1
)