Journal of
Ecology

2001

89

, 930–946

© 2001 British
Ecological Society

Blackwell Science LtdChange of fire frequency in the eastern Canadian boreal
forests during the Holocene: does vegetation composition
or climate trigger the fire regime?

CHRISTOPHER CARCAILLET*†, YVES BERGERON†,
PIERRE J. H. RICHARD*, BIANCA FRÉCHETTE*, SYLVIE GAUTHIER‡†
and YVES T. PRAIRIE§

*

Département de Géographie, Université de Montréal, CP 6128 Centre-Ville, Montréal (Qc) H3C 3J7, Canada;

†

Groupe de Recherche en Écologie Forestière interuniversitaire, Université du Québec à Montréal, CP 8888 Centre
Ville, Montréal (Qc) H3C 3P8, Canada;

‡

Canadian Forest Service, Laurentian Forestry Center, 1055 rue du PEPS,
PO Box 3800, Ste Foy (Qc) G1V 4C7, Canada;

§

Département des Sciences Biologiques, Université du Québec à
Montréal, CP 8888 Centre Ville, Montréal (Qc) H3C 3P8, Canada

Summary
1

Studies on the variability of natural fire regimes are needed to understand plant
responses in a changing environment. Since vegetation changes might follow or trigger
changes in fire frequency, climate models suggest that changes in water balance will
accompany current global warming, and the response of fire regimes to Holocene
hydro-climate changes and vegetation switches may thus serve as a useful analogue for
current change.

2

We present high-resolution charcoal records from laminated cores from three small
kettle lakes located in mixed-boreal and coniferous-boreal forest. Comparison with
some pollen diagrams from the lakes is used to evaluate the role of the local vegetation
in the fire history. Fire frequency was reconstructed by measuring the separation of
peaks after detrending the charcoal accumulation rate from any background.

3

Several distinct periods of fire regime were detected with fire intervals. Between

c.

7000–3000 cal. year

bp

, fire intervals were double those in the last 2000 years. Fire fre-
quency changed 1000 years earlier in the coniferous-boreal forest than in the mixed-
boreal forest to the south. The absence of changes in combustibility species in the pollen
data that could explain the fire frequency transition suggests that the vegetation does
not control the long-term fire regime in the boreal forest.

4

Climate appears to be the main process triggering fire. The increased frequency may
be the result of more frequent drought due to the increasing influence of cool dry west-
erly Pacific air-masses from mid to late Holocene, and thus of conditions conducive to
ignition and fire spread. In east Canada, this change matches other long-term climate
proxies and suggests that a switch in atmospheric circulation 2–3000 years ago trig-
gered a less stable climate with more dry summers. Future warming is moreover likely
to reduce fire frequency.

Key-words

:



boreal forest, climate, charcoal, fire frequency, Holocene, laminated sedi-
ments, pollen, Quebec

Journal of Ecology

(2001)

89

, 930–946

Introduction

Fire is the main natural disturbance in the North
American boreal forest (Rowe & Scotter 1973). Sus-
tainable forest harvesting could mimic the effects of
current natural fire regimes (Delong & Tanner 1996),
because effects of natural fire are similar to those of
industrial forest harvesting (Johnson

et al

. 1998).
There is, however, little doubt that the current and pre-
dicted global warming could cause changes in forest
disturbances. While some studies in northern America
anticipate an increase in fire frequency (Clark 1988;

Correspondence and present address: C. Carcaillet, Institut
de Botanique, Ecole Pratique des Hautes Etudes, 163 rue A.
Broussonet, F-34090 Montpellier, France.

JEC_614.fm Page 930 Wednesday, November 21, 2001 8:48 AM

931

Holocene fire
frequency changes
in Canadian boreal
forests

© 2001 British
Ecological Society,

Journal of Ecology

,

89

, 930–946

Stocks

et al

. 1998) or size of burns (Flannigan & Van
Wagner 1991; Wotton & Flannigan 1993), others sug-
gest a decrease (Bergeron & Flannigan 1995; Bergeron

et al

. 2001). Simulations based on mathematical
models predict that the Canadian Fire Weather Index
(CFWI) will increase in central and eastern North
America but will be lower in the north-east (Flannigan

et al

. 1998, 2001). Understanding the long-term rela-
tionships between climate and fire regimes is thus
essential for the sustainable management of the boreal
forest in a changing climatic environment (Bergeron

et al

. 1998).
Dendroecological studies show that both frequency
and size of fire decreased during the 20th century in
both west (e.g. Van Wagner 1978; Johnson

et al

. 1990;
Larsen 1997; Weir

et al

. 2000) and east Canadian
coniferous forests (e.g. Cwynar 1977; Foster 1983;
Bergeron 1991; Bergeron

et al

. 2001), possibly due to a
drop in drought frequency and an increase in long-
term annual precipitation (Bergeron & Archambault
1993). From a longer-term perspective, high-resolution
studies of fire regime are necessary to decipher the rela-
tionship between climate and fire. Unfortunately, no
high-resolution data covering the entire post-glacial
period are available for the Canadian boreal forest but
charcoal analyses at eastern sites have shown that fire
activity has increased during the late Holocene climatic
cooling (Filion

et al

. 1991; Carcaillet & Richard 2000).
It is generally speculated that long-term vegetation
dynamics in boreal and temperate forests are control-
led by change in fire (Mehringer

et al

. 1977; Anderson

et al

. 1986; Rhodes & Davis 1995; Hörnberg

et al

.
1999; Tinner

et al

. 1999). However, charcoal and pol-
len analyses at the southern limit of the mixed-boreal
forest in Ontario do not disclose any such relationship
(Cwynar 1978; Fuller 1997) and vegetation composi-
tion also appears independent of changes of fire fre-
quency in north-west Wyoming (Millspaugh

et al

.
2000). Nevertheless, there is some evidence for a rela-
tionship between vegetation and fire in the Atlantic
Canadian boreal forest (Green 1982). In the extreme
northern boreal forest, it has been suggested that fire
has triggered landscape fragmentation over several
millennia, without any change in the location of the
northern tree-line (Payette & Lavoie 1994). Changes in
flammability may explain the occurrence of different
fire regimes through time, both in conifer and broad-
leaved forests (Clark 1988; Clark

et al

. 1996a,b). Vege-
tation and fire change almost in synchrony (Clark

et al

.
1996a), and either could be the driving force. An influ-
ence of fuel on the fire regime is difficult to demonstrate
and remains unclear during the Holocene but would
require significant change in vegetation proxy to be
shown to precede a change in fire regime.
We assessed whether any vegetation change could
explain the fire frequency changes during the Holocene
and, if not, whether climate change could be responsi-
ble. We compared pollen data with reconstructed fire
frequency history at the same sites. If the fire frequency
changes were due to differences in vegetation compo-
sition, we predict differences in the relative abundance
of low flammable/combustible trees (e.g.

Populus

,

Betula

,

Salix

,

Alnus

,

Acer

), and species such as

Pinus

,

Picea

and

Abies

. At the stand level, broad-leaved spe-
cies, particularly

Populus

and

Betula

, limit fire propa-
gation owing to their low fuel quality (Hély

et al

.
2000a) but changes in abundance will take a few dec-
ades to centuries before they have an effect on fire
regime. We also compared fire history with hydro-
climatic reconstructions independent of fire and vegeta-
tion, e.g. using

d

18

O or lake-levels as proxy data.

Study site

lake basin characteristics

Lac Francis, Lac Pas-de-Fond and Lac à la Pessières
are head lakes located in western Québec, south of
James Bay near the boundary with Ontario (Fig. 1). In
this area, till outcrops are scattered within a more or
less uniform and flat landscape. The lakes lie on eskers
covered by clay sediment from the proglacial lake Ojib-
way that deposited the ‘Northern Clay Belt’, covering a
large area south of James Bay (Veillette 1994). Lake
Ojibway disappeared abruptly when the residual Lau-
rentide ice-sheet collapsed, allowing the northward
outflow of proglacial fresh water to Hudson bay

c.

8400
cal. year

bp

(Barber

et al

. 1999). Subsequently, accu-
mulated sediment is expected to be predominantly
organic.
The three lakes are located along a vegetation gra-
dient, with the southernmost, L. Francis, in mixed-
boreal forest, L. Pessière in the coniferous-boreal forest
and L. Pas-de-Fond at the northern edge of the
present-day limit of mixed-boreal forests (Fig. 1). The
sites are separated by

c.

100 km and therefore differ
in climate as well as vegetation.
All three lakes are small and highly coloured, and
except for Francis are dimictic closed-basins with a
small outflowing brooklet (properties given in Table 1).
The water columns show strong thermal stratification,
with the thermocline occurring at 1.5 m to 4 m depth
depending on lake size, and the hypolimnion strata
becoming rapidly anoxic.

regional climate history

At the retreat of the post-glacial lake Ojibway,
8400 years ago, orbital forcing led to the climate being
warmer in summer and colder in winter than at present
(Kutzbach

et al

. 1998). Mean summer temperatures
have progressively decreased with some oscillations,
e.g. during the Little Ice Age and the Warm Medieval
Period. Lake levels have also changed several times,
suggesting changes in the precipitation regime, being
low between 6200 and 3800 years ago (northern
Québec; Payette & Filion 1993), or between 11 000 and
4400 years, apart from intermediate levels at 7000–

JEC_614.fm Page 931 Wednesday, November 21, 2001 8:48 AM