1. Introduction
Boreal forest landscapes develop under the combined influence of climate, natural (fires, insect outbreaks) and anthropogenic (logging, fires of human origin) disturbances, as well as physical environment [
1,
2]. This combination of factors generated the contemporary landscape diversity defined, at different spatial scales, through hierarchies of ecological classification [
3,
4]. These classifications consider the physical features, such as the abundance of lakes, the area covered by peatlands or sandy soils and the relief. All these physical features have a strong influence on the long-term history of fire and vegetation [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. Knowing the long-term natural variability of landscapes, it will become easier to define forest strategies in regard to ecosystem management and climatic changes [
16,
17,
18,
19,
20].
In each boreal landscape, the species are distributed along a toposequence of surficial deposits, slope and drainage conditions, from bedrock and well-drained soils (till, clay or sand) to poorly drained organic soils. In northeastern North America, jack pine (
Pinus banksiana Lambert) and black spruce (
Picea mariana (Miller) Britton, Sterns & Poggenburgh) are well adapted to sites characterized by bedrock or sandy soil [
21,
22]. Thick, moderately well-drained soils support a vegetation more demanding with respect to the nutrient regime, such as trembling aspen (
Populus tremuloides Michaux), white birch (
Betula papyrifera Marshall) and balsam fir (
Abies balsamea (Linnaeus) Miller), whereas poorly-drained soils are mainly colonized by
P. mariana and
Larix laricina (Du Roi) K. Koch. Under the influence of natural disturbances, mainly fires, the vegetation of each portion of the toposequence changes with time. For example, after fires on well-drained rich soils, the light-demanding early successional species (
P. tremuloides, B. papyrifera) give way to late successional ones (
P. mariana,
A. balsamea), thereby defining a successional forest dynamic [
23]. However, in other parts of the landscape, as on low altitude and undulated or flat relief where
P. mariana and
P. banksiana are dominant, fires can occur so frequently spaced in time that cohorts of early successional species succeed one another, creating a recurrence dynamic of stands dominated by these species [
24,
25]. Under such circumstances, stands dominated by early successional species can be considered in equilibrium with climate and disturbance regimes. These examples show that, in the context of the eastern Canadian boreal forest,
P. mariana can be considered as both an early and a late successional species.
In an integrative study of paleoecological data covering the major biomes of Québec (Canada) initiated in order to demonstrate the specificity of each biome, Blarquez et al. [
26] suggest that in the boreal coniferous forest dominated by
Picea mariana, biomass burning was higher during the mid-Holocene period (~6000 to 4000 cal. Years BP) than during the late Holocene (over ~4000 years). Some sites show a brief increase of fire frequency around 1000 cal. Years BP, possibly related to the Medieval climatic optimum [
10,
27,
28]. The majority of the study lakes considered by Blarquez et al. [
26] are located in the western part of the
Picea mariana domains (moss and lichen). If fires were less frequent during the late Holocene, they were, however, larger in extent [
27]. The widespread general decline of fire frequency during the late Holocene in the
P. mariana domains would be explained by climatic factors, i.e., an increase in annual precipitation, as well as a decrease of temperature (mainly during July), a shorter growing season and lower summer insolation [
10,
26,
27,
29,
30,
31,
32,
33,
34,
35]. Blarquez et al. [
26] also suggest that fire frequency increased throughout the Holocene in the boreal mixedwood. Numerous sites studied in this biome are, however, located in eastern Québec, where hydroclimatic conditions differ greatly from those prevailing in the western part of the province. Indeed, annual precipitation is more than 200 mm higher in eastern Québec [
36]. Moreover, the mixedwood includes three bioclimatic domains according to Québec classification; these are, from north to south,
Abies balsamea-Betula papyrifera,
Abies balsamea-Betula alleghaniensis and
Acer saccharum-Betula alleghaniensis [
4]. Specific fire reconstruction studies in the
Abies balsamea-Betula papyrifera domain show a gradual increase of fire events during the late Holocene, and some of these studies were undertaken in the Clay Belt of Québec [
37,
38] and Ontario [
12] (
Figure 1).
The aim of this study is to explain the long-term natural variability of the vegetation and fire in a Québec Clay Belt landscape dominated by early successional species (
Populus tremuloides,
Pinus banksiana and
Picea mariana) (
Figure 1). These stands are abundant throughout the Clay Belt. Considering the type of landscape (vegetation, surficial deposit) and the increase of fires during the last millennium in some areas dominated by clay, we hypothesize that the mesic-subhydric clay of our studied landscape was also affected by increased fire activity during the late Holocene, which promoted the maintenance until now of early successional species through a recurrent forest dynamic. To test this hypothesis, we first analyzed the contemporary vegetation of a 4000 km
2 study area with the objective of understanding the current forest dynamics. We then reconstructed the long-term vegetation and fire history through pollen and macroscopic charcoal analyses of two selected peatlands in order to obtain insight into the forest dynamics at the margin of the peatlands.
4. Discussion
This study presents the long-term natural variability (fire and vegetation) of a portion of the
Picea mariana-moss bioclimatic domain belonging to Québec’s Clay Belt. This variability has been described through paleoecological analysis of sediment cores from two bogs 10.5 km apart, located in a forested landscape dominated by early successional species (
Populus tremuloides,
Pinus banksiana,
Picea mariana). The sediment core from Aspen (100 m from the forest) provides a continuous history (vegetation and fire events) of 8000 years, whereas that from Shadow (25 m from the forest) covers a period of 4000 years. The results increase our knowledge about the Holocene history (fire and vegetation) of western Québec boreal forests, mainly based on paleoecological studies of lake sediments. The results from the two peatlands support our initial hypothesis specifying that increased fire activity during the late Holocene on mesic-subhydric clay promoted the development and maintenance of early successional species through a fire recurrence dynamic. Four main periods were recognized in the long-term vegetation and fire history of the forests surrounding our study sites. For each, we compare our results with those of lakes studied in western Québec
Picea mariana-moss and
Abies balsamea-Betula papyrifera domains (
Figure S1;
Table S2). These lakes are located in low-altitude landscapes dominated by peatlands (Pessière, Geais, Profond, Raynald, Loutre, Garot), in mid-altitude landscapes with undulated clay (Cèdres, Schön, Francis, Pas de Fond) and relatively high-altitude landscapes with undulated till (Twin, Richard, Aurélie, Nans). The latter are located to the east of our study area, near Lake Mistassini.
4.1. The Afforestation Phase (ca. 8000 to ca. 6800 BP)
At Aspen, only a few isolated charcoal pieces were found in the sediments that represent the period between the beginning of vegetation colonization following the draining of the waters of proglacial Lake Ojibway (ca. 8000 BP) and the densification of the forest cover (forest phase) around 6800 BP. The rarity of charcoal during this first period corresponding to afforestation (PAZs 1 and 2) can be linked to the low initial density of the forest, which may have been too sparse to support recurrent fire activity. At Aspen, the presence of brown mosses and seeds of
Menyanthes trifoliata Linnaeus indicates minerotrophic and water-saturated soil at the time and supports the hypothesis that the very humid local conditions present initially could have prevented fires from propagating toward the center of the peatland. The reconstructions of fire history from lake sediments in western Québec
Picea mariana-moss domain also suggest a low fire frequency during the afforestation phase [
10].
4.2. A Landscape Dominated by Late and Early Successional Forest Stands (ca. 6800 to ca. 2500 BP)
The beginning of the forest phase at Aspen (ca. 6800 BP; PAZ 3a) is marked by an increase in the pollen representation of
Abies balsamea, a late successional species that is generally under-represented in pollen assemblages [
54]. Until 2500 BP, at least four fires occurred in the surrounding forests (5370, 4910, 3880 and 2330 BP). The presence of smaller quantities of charcoal fragments at other levels of the core indicates that it may be possible that other fires occurred during this period. In the forests bordering Shadow, fires occurred at dates different from those at Aspen (4110, 3680, 2900 BP). Since the two sites are only 10.5 km apart, this asynchronism suggests that the fires covered small areas and were not severe. We hypothesize that this type of fire regime promoted the development of
Picea mariana-Abies balsamea stands. Early successional species (
Betula papyrifera, Pinus banksiana,
Alnus spp.) were also well represented in the pollen diagrams, suggesting that the landscapes were composed of stands belonging to late and early successional stages. Our results concerning the fires differ from those based on lake sediments which show a relatively high fire frequency (expressed by a number of fires per millenium). However, there is a wide variation in the maximum frequency of fires and the period during which this rate was maintained [
10,
15,
27,
31,
34].
4.3. Landscapes Dominated by Picea mariana and Abies balsamea (ca. 2500 to ca. 2100 BP)
Anthracomass values for the period corresponding to ca. 2500 to ca. 2100 BP (PAZ 3b) are low at Aspen, whereas no charcoal particles were found at Shadow. The pollen representation of
Abies balsamea is still relatively high, whereas that of
Pinus banksiana,
Betula and
Alnus spp. drops. It is estimated that this period corresponds to a dominance of
Picea mariana and
A. balsamea forests in the studied landscape, and perhaps in the majority of landscapes belonging to Québec’s Clay Belt. The reconstructions of fire history from lake sediments in the western
Picea mariana-moss domain also show a general decrease in fire frequency during this period, but with a certain variability among sites [
10,
28,
31,
34].
4.4. Increase of Early Successional Species (ca. 2100 to ca. 800 BP)
Anthracomass values for the period corresponding to ca. 2100 to ca. 800 BP are relatively low at Aspen (PAZ 3c). They are higher at Shadow, however, where three fires occurred in a short lapse of time (1960, 1670 and 1580–1550 BP). The pollen representation of
Abies balsamea remains constant, while an increase of
Pinus banksiana,
Betula and
Alnus is observed, especially at Shadow. An increase of
P. tremuloides is not evident in the two pollen diagrams because this species is strongly under-represented in pollen assemblages [
55]. The variability of fires and vegetation observed between the two peatlands also characterizes lake sites. Fire frequency is relatively high around ca. 1000 BP for some lakes (Profond, Raynald, Richard and Nans;
Figure S1) but low for others (Geais, Loutre, Twin, Aurélie). These differences could be associated with the short duration of the Medieval Warm Period, which caused a varied fire pattern in the western portion of
Picea mariana-moss domain [
28]. Differences in physical features probably do not explain this variability because lakes with high and low fire frequency are located both in peatlands (Profond, Raynald, Geais, Loutre) and till environments (Richard, Nans, Twin, Aurélie).
4.5. More Severe Fires Favored the Development of Landscapes Dominated by Early Successional Species (from 800 BP to the Present-Day)
In the last 1000 years, three of the four dated fires recorded at Aspen and Shadow appear to have affected the two sites simultaneously (840/880–835, 290–300, 50–100 BP). Another fire (180 BP) was only recorded at Aspen. The fact that the forests surrounding the two peatlands were affected by the same fires on three occasions strongly suggests that they once covered wider areas and were more severe than those of the previous periods. In this region, two fire periods that were both severe and extended over a wide area occurred recently, in the 19th (ca. 1820) and 20th (ca. 1910) centuries [
7,
40]. The fire dating from 180 BP (Aspen) would correspond to ca. 1820, and the fire dating from 50–100 BP to ca. 1910. The link between the recent fires that occurred in the forest bordering the two peatlands and the deposition of charcoal in organic sediments is therefore well-established, suggesting that the same process characterized all of the Holocene period [
56], at least in the case of large and severe fires. The fires occurring in our study area (4000 km
2) and in the ‘Plaine du lac Matagami’ during the last two centuries are considered natural because this region was sparsely inhabited until recently. The village of Matagami (
Figure 1) was founded only in 1963 to open access to the James Bay hydroelectric reservoirs. The fires of the 1910 period are so widely distributed in the western
Picea mariana moss domain that any possible role played by Aboriginal peoples living in the area during this fire period would have been minor [
2].
The fires of the 19th and 20th centuries may have played an important role in the decrease of
A. balsamea as shown on the two pollen diagrams of Aspen and Shadow (
Figure 6 and
Figure 7). The relative abundance of
Abies balsamea until recently is different from the proposed fragmentation of
Abies balsamea forests initiated as early as 3500 BP [
57]. During the last millennium, the pollen assemblages of Aspen and Shadow are dominated by
Pinus banksiana and are at their highest level on the scale of the last 8000 years. An increase of
Pinus banksiana in the two last centuries is also recorded elsewhere, notably at Schön Lake located close to the studied peatlands [
15]. This recent dominance of early successional species is summarized in
Figure 8, where positive scores of the first ordination axis are reported.
Most of the lake sediments studied by previous authors, and located in a peatland environment (Pessière, Geais, Profond, Raynald, Loutre,
Figure S1), show a decrease in fire frequency during the last or two last millennia [
10,
31,
34]. However, some lakes indicate an increase in fire frequency during this period in the western
Picea mariana-moss domain. This is the case of Schön Lake, located close to the studied peatlands, to the north, in an environment dominated by clay [
15,
34] (
Figure S1). Remy et al. [
15] suggest that fire frequency increased since 2000 BP in the western
Picea mariana moss (lakes Garot, Schön,
Figure S1) and
Picea mariana lichen (lakes Loup, Nano, Trèfle, Marie-Ève) domains. Most of these lakes are located in an environment with well-drained surficial deposits; Garot Lake, with an environment dominated by peatlands, is the only exception. Recent methodological advances in fire event reconstruction may lead to new results, and mainly at the extremities of the sediment core [
58]. For example, while Ali et al. [
10] show a decrease in fire frequency during the last 2000 years BP at Lake Geais, Oris et al. [
34] indicate an increase since 1000 years BP for the same lake. An increase of fire frequency has been observed at lakes Francis and Pas de Fond, located in the Clay Belt portion of the western
Abies balsamea-
Betula papyrifera bioclimatic domain [
37,
38] (
Figure S1). Both lakes are associated with a very marked cut ca. 2000 BP, characterized by the transition from a fire frequency higher than 200 years to less than 200 years. Thus, a higher fire incidence in the last millennia could characterize the entire Québec Clay Belt regardless of bioclimatic domain. All these fire activities and climate conditions led to landscapes dominated by early successional species, mainly
Pinus banksiana and
Populus tremuloides (
Figure 9) [
59,
60].
Finally, does the last millennium, particularly the last two centuries, correspond to a Holocene period characterized by a decrease in fire occurrence and severity, or does this period constitute the most important period of the entire Holocene in regard to large and severe fires? It may be that, during the recent millennia, landscapes dominated by peatlands show a fire decrease while those composed mainly of clay or other mineral soils an increase. The answer could lie in the type of surficial deposit (peatland vs. clay and other mineral soils) rather than the bioclimatic domain (
Picea mariana vs.
Abies domains) [
8,
11].
4.6. Contemporary Forest Dynamics
The forests bordering the two peatlands and the entire landscape studied (P4b,
Figure 3) were strongly affected by contemporary fires. We estimate that the fires of the last millennia favoured a recurrence dynamic of
Populus tremuloides,
Pinus banksiana and
Picea mariana stands. Today, forest regeneration is mainly composed of
P. mariana accompanied by some
P. tremuloides. Based on this type of regeneration, the forests should evolve towards mature
P. mariana-
P. tremuloides stands [
22].
Abies balsamea is presently rare in the landscapes dominated by early successional forests, and this is probably a response to the two close and severe fire periods (1820–1910) that occurred in less than 200 years. If sufficient time elapses before the next fire occurs (approximately 200–250 years) [
18],
Abies could reach maturity and regenerate abundantly. Late successional forests dominated by
Picea mariana and
Abies balsamea, such as those that characterized the landscape during the Holocene (ca. 6800 to ca. 800 BP), could thus form once again. However, this possibility remains unlikely in the context of climate change because an increase in frequency and severity of fires is predicted for the western Québec
Picea mariana-moss domain [
32,
33,
60].
5. Conclusions
This study carried out in Québec’s Clay Belt (Canada) provides new knowledge about the role of forest fires in landscapes dominated by early successional species (Populus tremuloides, Pinus banksiana, Picea mariana) during the last millennia. More severe and extensive fires stimulated the development and the continuity of these forests through a recurrence dynamic. Such a fire regime differs from that which prevailed for thousands of years during the Holocene, and allowed the growth of the late successional P. mariana and Abies balsamea forests in the western Picea-mariana-moss domain. This information on long-term natural variability provides insights that can guide the development and implementation of sound, ecosystem forest and fire management plans in the context of climate change.
Our study opens the door to the possibility that two types of fire regimes occurred during the Holocene in Québec’s Clay Belt: the first, defined by a decrease in fire frequency during the late Holocene, characterized landscapes dominated by peatlands. The second shows an increase of fires over the last millennium and is related to landscapes mainly formed by mesic-subhydric clays. This second regime could characterize the entire Québec Clay Belt, regardless of bioclimatic domain (Picea mariana-moss and Abies balsamea-Betula papyrifera). In order to demonstrate the presence of these two regimes, studies coupling analysis of lake sediments, peatlands and mineral soils should be conducted jointly in the future.