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Dendrochronologia 20/1-2 (2002) 175±189 ã Urban & Fischer Verlag http://www.urbanfischer.de/journals/dendro

Using dendrochronology to reconstruct disturbance and forest dynamics around Lake Duparquet, northwestern Quebec Yves Bergeron, Bernhard Denneler, Danielle Charron, and Martin-Philippe Girardin Groupe de recherche en eÂcologie forestieÁre interuniversitaire Universite du QueÂbec aÁ MontreÂal CP 8888, Succursale Centre-ville MontreÂal, QueÂbec, Canada

Abstract This paper presents a synthesis of the dendroecological work conducted in the area of Lake Duparquet in the southern boreal forest of northwestern Quebec (Canada) during the last 15 years. The topics of these syn- and autecological studies encompassed forest dynamics and tree growth related to natural disturbances such as forest fires, insect outbreaks, and flooding, as well as the effects of climate change. Seven major fire events occurred around Lake Duparquet since 1720: 1760, 1797, 1823, 1847, 1870, 1916, and 1944. Post-fire stand dynamics, established by a chronosequence of over 200 years, are characterized by the gradual transition from broadleaf dominated stands towards mixed and finally almost pure conifer stands. After fire, insect outbreaks are the second most important disturbance type in the southern boreal forest. Spruce budworm, the predominating defoliating insect, but also forest tent caterpillar and larch sawfly have major impacts on growth and stand dynamics of their respective host species. Global warming since the end of Little Ice Age around 1850 coincided with increasing precipitation and, hence, decreasing droughts in the southeastern boreal area of North America. The accelerated radial growth of eastern white-cedar and black ash at Lake Duparquet is a direct effect of these wetter climatic conditions. Population dynamics and forest composition, however, are rather indirectly affected by climate change through the alteration of the natural disturbance regimes, i. e., the decreased frequency and size of the forest fires and the increased frequency and amplitude of the spring floods. Potential consequences of future global warming on disturbance dynamics and forest composition are briefly discussed. The results of the dendroecological studies contributed to the elaboration of a natural-disturbance based forest management model for the southern boreal forest of Quebec. Keywords: Boreal forest, climate change, dendroecology, natural disturbance, forest dynamics, forest fire, forest management, insect outbreak, ring width Address for correspondence: Yves Bergeron, Groupe de recherche en eÂcologie forestieÁre interuniversitaire Universite du QueÂbec aÁ MontreÂal CP 8888, Succursale Centre-ville MontreÂal, QueÂbec, Canada Tel.: ++1/8 19/7 62-09 71 #23 47; Fax: ++1/8 19/7 97-47 27 [email protected]

1125-7865/02/20/1-2-175 $ 15.00/0

176 Y. Bergeron et al.

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Cet article preÂsente une syntheÁse des travaux reÂaliseÂs en dendrochronologie dans la reÂgion du Lac Duparquet, dans le sud de la foreÃt boreÂale au QueÂbec (Canada) au cours des 15 dernieÁres anneÂes. Ces eÂtudes abordent une varieÂte de sujets syn- et auteÂcologiques sur la dynamique forestieÁre et la croissance radiale des arbres relieÂs aux perturbations naturelles telles les feux de foreÃt, les eÂpideÂmies d'insectes et les inondations de meÃme que les effets du changement climatique. Sept anneÂes avec des feux majeurs furent reconstitueÂes pour la reÂgion du Lac Duparquet depuis 1720: 1760, 1797, 1823, 1847, 1870, 1916 et 1944. Une chronoseÂquence de plus de 200 ans montre que la dynamique des populations apreÁs feu peut eÃtre caracteÂriseÂe par une transition graduelle de peuplements de feuillus aux peuplements mixtes et, finalement, aux peuplements reÂsineux. Les eÂpideÂmies d'insectes comptent parmi les perturbations les plus importantes dans la foreÃt boreÂale meÂridionale. La tordeuse des bourgeons de l'eÂpinette, l'insecte deÂfoliant preÂdominant, la livreÂe des foreÃts et la tenthreÁde du meÂleÁze ont un impacte majeur sur la croissance et la dynamique des populations de leurs hoÃtes respectifs. Le reÂchauffement atmospheÂrique depuis à ge Glaciaire aÁ environ 1850 coõÈncila fin du Petit A da avec des preÂcipitations plus importantes et, par conseÂquent, des peÂriodes de seÂcheresse moins seÂveÁres dans le sud-est de la reÂgion boreÂale de l'AmeÂrique du Nord. L'augmentation de la croissance radiale du ceÁdre blanc et du freÃne noir au Lac Duparquet est un effet direct de ces conditions plus humides. Cependant, le changement climatique influence la dynamique des populations et la composition forestieÁre plutoÃt indirectement par la diminution de la freÂquence et de la superficie des feux ainsi que par des crues printanieÁres plus importantes. Les conseÂquences potentielles du changement climatique sur les reÂgimes des perturbations naturelles et la composition forestieÁre sont discuteÂes brieÁvement. Les reÂsultats des eÂtudes dendroeÂcologiques ont contribue aÁ l'eÂlaboration d'un modeÁle d'ameÂnagement forestier durable base sur les perturbations naturelles pour la foreÃt boreÂale meÂridionale du QueÂbec.

ForeÃt boreÂale, changement climatique, dendroeÂcologie, perturbation naturelle, dynamique forestieÁre, feu de foreÃt, ameÂnagement forestier, eÂpideÂmies d'insectes, largeur de cerne

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Introduction During the twentieth century dendrochronology has gained in popularity, and has been applied in many fields of research. Its general use as a tool to reconstruct natural disturbances as well as forest dynamics, however, is relatively recent. The extensive application of dendrochronology for ecological topics, generally called dendroecology (Fritts, Swetnam 1989; Schweingruber 1996), coincided with the recognition, among forest ecologists, that natural disturbances control forest dynamics in almost all ecosystems (Pickett, White 1985). Forest ecology has gradually evolved from static descriptions of forest community distribution along environmental gradients (Whittaker 1975) towards the reconstruction and understanding of forest dynamics in relation to external disturbances, be they biotic (e. g., insects) or abiotic (e. g., fire). In this context, dendrochronological dating of events such as specific disturbances, tree recruitment, or growth releases has proved useful for studying forest dynamics. There is now a considerable body of literature that illustrates the use of dendroecology to reconstruct forest dynamics. Most of these studies dealt with forests controlled by one major disturbance type such as fire (Johnson, Fryer 1989), insects (Swetnam et al. 1985; Morin 1994), flooding (BeÂgin, Payette 1988), windthrow (Morin 1990), or gap dynamics (Lorimer 1980; Payette et al. 1990). However, there are relatively few studies that have attempted to synthesize the cumulative effects of several disturbances in the same region. Over the last 15 years, disturbance regimes and forest dynamics have been extensively studied around Lake Duparquet, a medium-sized lake within the area of the southern boreal forest of northwestern Quebec. In this paper we present a synthesis of the dendroecological work conducted, and we briefly discuss their importance in the context of forest management.

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Fire regimes and post-fire dynamics Fire is the main natural disturbance type of the boreal forest (Johnson 1992; Payette 1992). It is generally assumed that the North American boreal forest is characterized by intense crown fires under relatively short fire cycles (i. e., the time passed until an area equivalent to the study area has burned), and by

Figure 1. Stand-origin map of, (A), the southern boreal forest of northwestern Quebec between 48 and 49°N, and, (B), the area surrounding Lake Duparquet. Homogeneous stands were delineated using aerial photographs and dendroecological analyses (data from Bergeron 1991, Dansereau and Bergeron 1993, and Bergeron et al. 2001).

a forest mosaic mainly composed of post-fire evenaged stands (Johnson 1992). On the other hand, long fire intervals allowing for changes in canopy dominance and development of uneven-aged stands have also been reported particularly in the eastern boreal forests (Frelich, Reich 1995; Bergeron et al. 2001). Fire history was reconstructed for a large sector around Lake Duparquet using field and archives data (Bergeron et al. 2001; Fig. 1 A). For the recent past, the years of the last forest fire were determined using available historical documents and air photographs taken during the 1920s and 1930s. For the remaining parts of the study area, fire years were determined using fires scars or assumed to coincide with the age of the oldest trees on each site. Standard dendroecological techniques were applied to establish dates of fire scars and tree ages (Stokes, Smiley 1968; Arno, Sneck 1977). Limited by tree longevity, the time interval covered by the analyses encompasses the last 400 years. Mean forest age, as evaluated by time since fire, is estimated at about 140 years (Bergeron et al. 2001). The fire cycle extended significantly with time from about 85 years (before 1850) to 145 years (1850±1920) and 325 years (1920±1999), respectively, and stand ageclass distribution shows a distinct decrease in area originating from fire during the 20th century (Fig. 2). Possible causes of this change will be discussed in the chapter about climate change.

Figure 2. Forest age distribution in 10-yr classes of stands in the mixedwood boreal landscape of northwestern Quebec (see Fig. 1 A). The negative exponential curve shows the theoretical stand age distribution expected under a constant fire regime (data from Bergeron et al. 2001).

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178 Y. Bergeron et al. Fire history was reconstructed in more detail for the area in the vicinity of Lake Duparquet (Bergeron 1991; Dansereau, Bergeron 1993; Fig. 1 B). Seven major fire events occurred around Lake Duparquet since 1720: 1760, 1797, 1823, 1847, 1870, 1916, and 1944. These large fires, together with another located a few kilometers north of Lake Duparquet and dating back to 1964, were used to reconstruct post-fire stand dynamics (Bergeron 2000). Tree composition and ecological characteristics were assessed in 624 plots distributed systematically within the burned areas. One representative stand per fire was selected for detailed dendroecological analysis. All living and dead stems (> 1 cm dbh) within the 20 ´ 20 m2 plots were mapped and cut down. Cross sections collected at the root collar and at every meter were analyzed using standard dendrochronological techniques. Polynomial regression analysis of basal area by time since fire gives insight in the changing importance of the tree species along the chronosequence (Fig. 3 A). Basal area of the willows (Salix spp.) and pin cherry (Prunus pensylvanica L.) are only significant for the

A Figure 3. Post-fire species and stand dynamics in the area of Lake Duparquet, combining 313 plots in eight stands on clay and with fire years ranging from 1760 to 1964 (modified from Bergeron 2000). (A) Polynomial regressions of species basal areas in relationship to time since fire. Regressions of all seven species were significant at P < 0.05. (B) Age structure profiles of five tree species, including living and dead stems of > 1 m height.

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B

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26 year old stand. Trembling aspen (Populus tremuloides Michx.) constitutes the most important species during the early successional stages. The bi-modal presence of paper birch (Betula papyrifera Marsh.) is significant throughout succession, the greatest basal area values occurring in the youngest and oldest stands. Still rare in the 56 year old stand, white spruce (Picea glauca [Moench] Voss) gains continually in importance and culminates about 150 years after fire. Basal area of balsam fir (Abies balsamea [L.] Mill.) increases throughout succession whereas that of eastern white-cedar (Thuja occidentalis L.) is important only in the two oldest stands. With the exception of white-cedar, all species show a peak in recruitment just after fire (Fig. 3 B). The secondary peaks of trembling aspen 70 and 120 years after fire, respectively, may be a response to canopy

gaps created by the gradual dismissal of the post-fire cohort. A low but constant recruitment of white birch and white spruce is observed after the initial peak, but, only few stems succeeded to grow into the canopy. In contrast, balsam fir recruited constantly and abundantly, with important peaks centered 10, 75, 125, and 170 years after fire. Those are roughly synchronous with similar peaks in aspen and white birch recruitment, suggesting a common cause such as stand break-up or spruce budworm outbreaks. Although present in low abundance in the post-fire cohort, white-cedar recruitment increased drastically between 70 and 130 years and declined as time since fire increased. The observed recruitment patterns are congruent with the changes in species dominance observed in Fig. 3 A. Decreases in importance of pin cherry, wil-

Figure 4. Age structures of red pine (Pinus resinosa) and Jack pine (P. banksiana) from a small island in Lake Duparquet affected by surface fires (triangles) in 1800, 1849, 1881, 1901, and 1914 (modified from Bergeron and Brisson 1990). The fire years were identified by dendrochronological dating of fire scars, surviving trees, post-fire recruitment, and abrupt changes in radial increment.

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180 Y. Bergeron et al. low, aspen, and white birch are linked to lower recruitment between fire events. In contrast, the constant recruitment of balsam fir and the late arrival of white-cedar are responsible for their dominance at late stages of succession. The maximum basal area of white spruce observed at mid-successional stages does not coincide with a late peak in recruitment, but can be explained by slower growth under the deciduous canopy and a therefore late establishment in the canopy. The presence of white spruce and white birch in old-growth forests is ensured by constant but low recruitment and extraordinary longevity, particularly of white birch (> 230 years). Stand dynamics can be characterized by three important stages of development: i) the post-fire cohort dominated by broadleaf trees, ii) the dismissal of the first aspen cohort (aspen stand break-up) with the recruitment of subsequent cohorts of aspen-dominated mixedwoods, and iii), spruce budworm outbreaks (see below) in stands dominated by coniferous species. Although Canada's boreal forests are mainly characterized by intense crown fires, surface fires are also locally observed. At Lake Duparquet, surface fire regimes are typical of pine stands located on islands and peninsulas (Bergeron, Brisson 1990). In contrast to crown fires, surface fires rarely kill all pine trees in a stand leaving many survivors, some of them bearing fire scars. A typical age structure of sparse pine populations on xeric sites shows regeneration waves following each fire (Fig. 4). Jack pine (Pinus banksiana Lamb.), with its serotinous cones, appears better adapted to a mixed regime of lethal crown and non-lethal surface fires than red pine (P. resinosa Ait.), which relies only on its thick bark for survival. Since crown fires become predominant to the north, a lack of survivors may explain why red pine attains its northern limit in the Lake Duparquet area (Flannigan, Bergeron 1998).

Insect outbreaks After fire, tree defoliation by insect outbreaks might be considered the second most important disturbance type in the southern boreal forest. Common defoliating insects are larch sawfly (Pristiphora erichsonii), forest tent (Malacasoma disstria) and western tent

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caterpillar (M. californicum), and spruce budworm (Choristoneura fumiferana). Spruce budworm is, by far, the most important enemy of balsam fir, but can also cause serious damage to white spruce, black spruce (Picea mariana [Mill.] B.S.P.), and red spruce (P. rubens Sarg.) (Frank 1990). Similarly, larch sawfly can have devastating effects on eastern larch (Larix laricina (Du Roi) K. Koch) (Muldrew 1955; Turnock 1972; Marie-Victorin et al. 1995), and outbreaks of both forest and western tent caterpillars were responsible for the defoliation of trembling aspen in large parts of hardwood forests (Perala 1990). Since the area of Lake Duparquet is periodically affected by outbreaks of these insects, several studies were performed to reconstruct their history and dynamics.

Spruce budworm Growth pattern analysis of the host species balsam fir and white spruce compared to the non-host species eastern white-cedar allowed the establishment of a chronology of spruce budworm outbreaks in the vicinity of Lake Duparquet covering the last 200 years (Morin et al. 1993; Bergeron 2000). The identification of outbreak periods was based on ring-width measurement series as well as visual determination of periods of both reduced and released radial growth of 86 white spruce, 20 balsam fir, and 37 white-cedar from several late-successional riparian stands. Growth reductions of the host species associated with insect defoliation revealed two outbreak periods in the 20th century: 1919 to 1950, and 1970 to 1987 (Fig. 5). However, the white spruce standard chronology shows several waves of devastation within the outbreak periods such as 1932±40, 1945±50, 1970±78, and 1981±85 (Fig. 5 B). The budworm outbreaks had a major impact on the population dynamics of balsam fir as shown by massive invasions between 1915 and 1940 as well as after 1970 (Fig. 5 A). Fir mortality caused by the last spruce budworm outbreak in the area of Lake Duparquet (1970 to 1987) was shown to be independent of site characteristics but the diameter of the trees, the basal area of fir, and the size of the conifer stands were factors positively related to mortality (Bergeron et al. 1995; Bergeron, Leduc 1998; Cappuccino et al. 1998).

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with dry periods that were reflected in the growth pattern of the non-host species white-cedar as periods of reduced increment.

Figure 5. Recruitment of balsam fir and tree-ring patterns of white spruce in relation to two 20th-century outbreaks of spruce budworm in the vicinity of Lake Duparquet from 1919 to 1950, and 1970 to 1987 (shaded areas), respectively (data from Bergeron 2000 (A) and Morin et al. 1993 (B and C)). (A) Balsam fir frequency distribution in 5-year age classes for living (solid bars) and dead (open bars) stems from a stand established after 1760 (n = 218). (B) Ring-width chronology of white spruce (n = 86 trees) standardized by orthogonal polynomial regressions. Sample depth is shown for every 25 years. (C) Frequency of white spruce trees with growth reduction and release, respectively. A tree ring was considered reduced when it was the first of a series of at least five consecutive relatively narrow rings, and released when it was the first tree ring after a period of suppression.

The results of several dendroecological studies from Quebec and Ontario were combined to assess the spatio-temporal evolution of spruce budworm outbreaks during the 20th century (Morin 1998). This comparison revealed a relatively synchronous increase of the insect populations to epidemic levels in the different areas rather than a directed development from south to north as hypothesized. Morin et al. (1993) assume that the budworm outbreaks are climatically induced because their begin coincided

Larch sawfly A dendroecological study analyzed the role of larch sawfly outbreaks on growth and dynamics of riparian larch stands in alluvial fens of Lake Duparquet (Girardin 2001; Girardin et al. 2001, 2002). Within twelve 400 m2 plots, all larch stems were sampled at ground level (seedlings and saplings were cut, whereas the trees were cored) and aged by tree-ring counting and crossdating. The resulting age structure was then compared to reconstructed sawfly outbreak episodes. Those were identified by tree-ring characteristics caused by severe larch defoliation (narrow tree rings, light rings, and missing rings), and by a host/non-host analysis. The latter consists of the year-by-year subtraction of a non-host chronology (residual chronology of black spruce from the same sites as larch) from each host series (i. e., the individual residual series of larch) to reduce the common influence of climatic variation. Thus, a growth response to the adverse effects of defoliation is present if larch growth declined without a corresponding ring-width decrease of black spruce. For the last 125 years, six periods emerge during which radial increment of a considerable part of the larch trees was reduced relative to the growth of the non-host species (Fig. 6 E). Four of these six peaks, around 1880, 1920, 1940, and 1980, respectively, are considered weak sawfly outbreaks only because there are almost no coinciding signals in the ringwidth chronology of larch (Fig. 6 B) or in the frequency distributions of light and missing rings (Fig. 6 C and 6 D), and, at the landscape level, there seems to be no effect on the population dynamics of the host species (Fig. 6 A). The two other periods, 1895 to 1912 and 1955 to 1962, however, were identified as severe sawfly outbreaks because they are characterized by distinct growth reductions over several years and numerous light and missing rings (Fig. 6 B to D). The prime ecological effect of these two sawfly outbreaks was tree mortality, which resulted in two stand recruitment phases in the 1890s and the late 1930s, respectively (Fig. 6 A). Their tim-

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182 Y. Bergeron et al.

Figure 6. Eastern larch recruitment and growth patterns in relation to larch sawfly outbreaks in riparian stands at Lake Duparquet (data from Girardin 2001 and Girardin et al. 2001, 2002). (A) Frequency of larch stems in five-year age classes. A total of 747 stems of all sizes were cored and cut within twelve 400 m2 plots. (B) Standard chronology of larch computed as biweight mean of 232 standardized tree-ring series by the program ARSTAN, and covering the period from 1857 to 1999. Sample depth is shown for every 25 years. (C) and (D) Vertical bar charts showing relative frequencies of light rings and missing rings, respectively. Both parameters were associated with severe defoliation by the larch sawfly. (E) Percentage of larch series affected by sawfly defoliation as identified by a host/non-host analysis. The individual residual ringwidth series of larch were compared year-by-year with the chronology of the non-host species black spruce from wetland sites close to Lake Duparquet. Years for which the radial increment of larch passed a default threshold of ±1.28 standard deviations relative to black spruce increment for at least four consecutive years were retained. The dashed line indicates 75 % of pair-wise comparisons that satisfied this condition. Two severe larch sawfly outbreaks (shaded areas) were identified for 1895 to 1912 and 1955 to 1962, respectively, since they emerge for all of the analyzed treering parameters.

ing with respect to the corresponding sawfly outbreaks, however, points to major differences of the two events. During the first and longer outbreak from 1895 to 1912, the sawfly larvae killed adult trees, seedlings, and saplings. Therefore, the larch stands recruited principally from the seeds of surviving trees and those in the seed bank, resulting in a peak of regeneration following immediately the outbreak. The second outbreak between 1955 and 1962, however, was shorter and probably less severe, killing only adult larches. Thus, the pre-established seedlings and saplings that had germinated in the late 1930s and the 1940s benefited from the destruction of the canopy to grow up to dominant trees. At the same time, they provided deep shade, reducing

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survivorship of new seedlings for several decades after the outbreak. Hence, length and severity of a larch sawfly outbreak seem to be critical factors in determining whether consecutive recruitment will origin from seeds or from increased survival of the pre-established seedlings and saplings.

Forest tent caterpillar The forest tent caterpillar is native to North America with a wide distribution that corresponds roughly to that of trembling aspen, its principal host. Historical outbreak records show that forest tent caterpillar populations exhibit cycles of 3 to 6 years with explosive growth to epidemic levels followed by abrupt decline. To assess the effects of documented forest

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Figure 7. Forest tent caterpillar outbreaks in the 20th century reflected in the growth pattern of the two host species trembling aspen and white birch from a stand at Lake Duparquet established after the fire of 1916 (modified data from Bergeron and Charron 1994). (A) Frequency distribution in 5-year age classes of dead and living aspen (solid bars) and birch (open bars) trees with > 5 cm dbh. (B) Standardized chronologies of aspen from the canopy cohort and of birch from the subcanopy cohort (n = 12 trees of each species), respectively. Before averaging, each ringwidth series was divided by its mean in order to give the same weight to all trees without eliminating long- or short-term signals. (C) and (D) Frequency of aspen and birch, respectively, with growth reduction or release of at least 50 % relative to mean ring width of the previous 10 years. Shaded areas indicate documented (1952± 56, 1943±44) and suggested (1931±35) forest tent caterpillar outbreaks, respectively.

tent caterpillar outbreaks on growth of trembling aspen and paper birch, the secondary host, and to reconstruct past outbreak events during the 20th century, chronologies for these two species were produced from a stand located close to Lake Duparquet that originated after a forest fire in 1916 (Bergeron, Charron 1994). In addition, all stems with > 1 cm diameter at breast height found within the stand were sampled at the root collar and aged using dendrochronological methods to reconstruct population dynamics. The age structure profile shows that recruitment of both pioneer species analyzed started immediately after the fire in 1916 (Fig. 7 A). The caterpillar outbreaks documented for

the study area of Lake Duparquet occurred from 1943 to 1944, and 1952 to 1956, respectively. Both aspen from the canopy cohort and birch from the subcanopy cohort reflect these events in their growth patterns by concurrent periods of reduced radial increment (Fig. 7 B to D). The abrupt growth decrease of white birch in the early 1930s was very likely caused by another caterpillar outbreak, although growth of trembling aspen was not affected. This interpretation could not be confirmed by direct observation since the records of defoliation only started in 1938. Comparing the two host species, it seems that white birch was more affected by the insect outbreaks than was trembling aspen.

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184 Y. Bergeron et al. The latter species showed distinct but only short growth responses on the two documented outbreaks. Radial growth of white birch, however, never attained the same level after each defoliation period than before, and did not recover since the 1950s out-

break with the exception of some years around 1980. The age structure profile reveals no recruitment periods after the stand initiation that could be related to the caterpillar outbreaks (Fig. 7 A). Thus, we suggest that, in the contrary to spruce budworm and larch sawfly (Fig. 5 A and 6 A), the defoliation of trembling aspen and white birch by the forest tent caterpillar rarely killed the affected trees. The small gaps created by the some dead individuals did not allow the two pioneer species to recruit, only shade-tolerant balsam fir (not shown). In addition, growth of the understory trees, mainly balsam fir and white spruce, also benefited from the higher illumination after defoliation of the canopy deciduous trees (not shown). The opposite situation can be observed for the 1970s and early 1980s when many balsam fir in the stand were killed and growth of white spruce was poor during the same spruce budworm outbreak as shown in Fig. 5. The reduced competition for light resulted in a considerable growth recovery of subcanopy white birch, whereas the canopy aspen did not benefit because their crowns were situated above those of the dying firs (Fig. 7).

Figure 8. Evidences of climate change since the end of Little Ice Age (~ 1850) from trees surrounding Lake Duparquet. (A) Number of ice scars from 1661 to 1990 in 10-yr classes (n = 615), and cumulative age distribution in percent of scar-bearing white-cedars (n = 81) from the lakeshore (modified from Tardif and Bergeron 1997b). (B) Long-term evolution of maximum ice-scar height above lake water level recorded by the 81 white-cedars analyzed from 1660 to 1990 (n = 121) (modified from Tardif and Bergeron 1997b). (C) Log10-transformed static and cumulative age distributions of black ash in 10-yr age classes and differentiated by origin (sexual or vegetative). Post-break bars show, respectively, < 10 cm height and first-year seedlings (modified from Tardif and Bergeron 1999). (D) Growth index curves of black ash (n = 306 radii) from shoreline stands (modified from Tardif and Bergeron 1993), and of eastern white-cedar (n = 38 trees) from xeric sites on lake islands (modified from Archambault and Bergeron 1992). (E) Area burned on 43 islands in Lake Duparquet between 1688 and 1988 (modified from Bergeron and Archambault 1993). Stand initiation maps based on the establishment of cohorts of fire-sensitive tree species were used to estimate the area burned by each fire. Notice that the values represent only minimum estimates, particularly for old fires where a part of the area might have reburned subsequently. Nevertheless, the burned area diminished considerably since the first half of the 19th century.

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Climate change Global warming since the end of the Little Ice Age (~ 1850) has been observed all around the northern hemisphere and amounts to approximately 0.6 °C for the last century (Houghton et al. 2001). Precipitation, however, varies much more spatially than does temperature and future changes are difficult to predict. Therefore, regional analyses of past and future changes in precipitation patterns are needed. Several paleoclimatical studies using dendroecological methods were conducted in the area of Lake Duparquet, a lake that has not only preserved its natural water level regime, but also large parts of the original riparian vegetation. The high water levels of Lake Duparquet were reconstructed for the last 150 years by the dating of abrasion scars at the stems of shoreline eastern white-cedars caused by drifting ice during spring thaws (Tardif, Bergeron 1997b). The number of these ice scars has considerably increased since the second half of the 19th century, and particularly since the 1930s (Fig. 8 A). Spring floods not only became more common but also attained higher levels. This is shown by the maximum height above water level of the ice scars that increased about 1 m since the end of Little Ice Age (Fig. 8 B). Climate analyses have shown that major ice-flood events are related to low temperature in autumn, and high winter and spring precipitation (Tardif, Bergeron 1997b). This rise of spring water levels since the late 19th century influenced both recruitment and growth of black ash (Fraxinus nigra Marsh.) bordering Lake Duparquet. In sites severely exposed to flooding, vegetative regeneration (stump sprouting) increased in importance relative to sexual reproduction to allow black ash to persist, whereas regeneration by seeds predominates in rarely inundated sites (Tardif, Bergeron 1992, 1999; Tardif et al. 1994; Fig. 8 C). Contrary to what might be expected, higher spring water levels at Lake Duparquet actually increased growth of black ash rather than decreasing it (Tardif, Bergeron 1993; Fig. 8 D). Dendrochronological studies have shown that high water periods cause inundation stress to trees only if they occur during the growing season whereas early flooding may even

have a beneficial effect on tree growth since the increased soil humidity helps overcome an otherwise dry summer (Broadfoot, Williston 1973; Kozlowski et al. 1991; Tardif, Bergeron 1993, 1997a). The reconstruction of forest fire history in the Lake Duparquet area using fire scars and stand initiation maps revealed a constant decrease of the number of fire years, the portion of the lake perimeter affected by fires, as well as the area burned on the islands in the lake since the end of Little Ice Age (Bergeron 1991; Bergeron, Archambault 1993; Fig. 8 E). This mitigated forest fire regime was suggested to be of climatic origin since a dendroclimatic study on eastern white-cedar from xeric sites on the islands of Lake Duparquet revealed that the upward trend of their radial increment for the same time span (Fig. 8 D) was related to a decrease of summer drought periods (Archambault, Bergeron 1992). The paleoclimatical studies presented in this chapter revealed an increase of precipitation in the area of Lake Duparquet since global warming set in about 150 years ago. This observation is in accordance with similar studies from subarctic Quebec (Payette et al. 1985; BeÂgin, Payette 1988; Payette, Delwaide 1991; Lepage, BeÂgin 1996; BeÂgin 2000). Bergeron and Archambault (1993) hypothesized that these wetter conditions could have been caused by the northward shift of the polar front that allowed warm and relatively humid air masses from the south to advance more frequently to higher latitudes than during the Little Ice Age. Regardless the exact reason, global warming since about 1850 had noticeable consequences on tree growth, and, maybe even more important, affected the vegetation indirectly by changing the natural disturbance regimes in the area of Lake Duparquet (Bergeron 1998). Although future climate change can not be predicted by the analysis of (not yet formed) tree rings, we would like to briefly present some studies about the possible consequences of global warming on the disturbance dynamics in the region of Lake Duparquet. Simulations of the effects of global warming (2 ´ CO2-scenario) on the climate of eastern Canada, including the Lake Duparquet region, predict less frequent summer drought periods and, as consequence, a reduced importance of the forest fire re-

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186 Y. Bergeron et al. gime (Bergeron, Flannigan 1995; Flannigan et al. 1998, 2001). Despite increasing temperatures (Houghton et al. 2001), the decrease of fire frequency as observed since the end of Little Ice Age (see Fig. 2 and 8 E) could thus continue. Longer fire cycles in turn should be beneficial to late successional species resulting in increased importance of balsam fir, eastern white-cedar, and black spruce at the expense of those species dominating during the early successional stages (Bergeron, Gagnon 1987; Bergeron, Brisson 1990; Bergeron, Dansereau 1993; Bergeron, Flannigan 1995; Gauthier et al. 1996; Bergeron 1998; Flannigan, Bergeron 1998). These changes in species composition and importance should influence insect epidemics. The increased abundance of balsam fir under a longer fire cycle could result in a considerably higher mortality due to devastating spruce budworm outbreaks (Bergeron, Leduc 1998). Riparian environments will also be affected by future climate change. The water levels might increase due to abundant precipitation, resulting in a more severe disturbance regime composed of flooding, ice push, wave activity, and erosion. Hydric species such as black ash may benefit and extend their range, whereas species less tolerant to flooding might rather recede. The fringe of large and old eastern white-cedars within the riparian zone around Lake Duparquet, for example,

seems to be in a non-equilibrium state today and might disintegrate completely if the water level of this lake continues to rise (Denneler et al. 1999). The studies presented here show that climate change had and will continue to have considerable effects on composition and dynamics of the boreal mixedwood forests, particularly through the alteration of the natural disturbance regimes. More research is needed to evaluate the consequences of global warming especially with respect to changes in biodiversity and C sequestration.

Application to forest management The concept of ecosystem management in which regional natural disturbance regimes serve as a template for forest management has received considerable attention in many regions (Hunter 1990). At the landscape level, maintaining ecosystem integrity implies maintaining structural and compositional patterns, within the limits of historical variability of the regional mosaic, produced under the natural disturbance regime (Mladenoff et al. 1993). At the stand level, ecosystem management promotes the use of sylvicultural systems which are inspired by or imitate natural stand dynamics, and in which maintaining structural and biotic attributes or legacies of natural stands is of primary importance (Franklin 1993).

Figure 9. Natural dynamics of the mixedwood boreal forest and associated sylvicultural treatments (modified from Bergeron and Harvey 1997). The abscissa represents time since last major disturbance (clearcut or fire).

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Using dendrochronology to reconstruct disturbance and forest dynamics around Lake Duparquet 187

Studies on disturbance and stand dynamics were used in the development of an ecosystem management approach for the Lake Duparquet Research and Teaching Forest (Bergeron, Harvey 1997; Bergeron et al. 1999; Harvey et al. 2002). A sylvicultural system inspired by natural dynamics, in which fire is emulated by clear-cutting and natural canopy succession is imitated by partial cutting, was developed (Fig. 9). Even-aged stands dominated by deciduous species but with an understory of conifers are partially cut in order to reproduce succession towards mixed stands composed of deciduous and conifers. These stands can be partially cut again to produce pure conifer stands. A proportion of deciduous, mixed and coniferous stands are clear-cut in order to recreate fire disturbances. Insect outbreaks and gap dynamics (Kneeshaw, Bergeron 1998) that occur in late successional conifer stands could also be emulated using partial and selection cutting. At the landscape level, the proportion of stands belonging to deciduous, mixed and conifer compositions are determined in order to represent the proportion that would be observed under a natural disturbance regime. Using a fire cycle of 140 years, corresponding to mean forest age, and a range of stand break-up varying from 80 to 110 years, we estimate that roughly 45±55 % of the forest should consist of post-fire forest types, 23± 26 % of mixed-wood forest types, and 20±30 % of late successional coniferous forest types (Harvey et al. 2002). The emphasis on maintaining forest type diversity contrasts significantly with current even-aged management techniques in the Canadian boreal forest, and has important implications for stand-level interventions, notably in necessitating a greater diversification of sylvicultural practices including more uneven-aged harvesting methods.

Conclusion In conclusion, dendrochronology was an essential tool in the reconstruction of Lake Duparquet disturbance and stand dynamics. Dendroecological studies were also useful to give a long-term perspective to the possible effects of climatic changes. Moreover, in addition to allowing better knowledge of this natural ecosystem, dendroecological studies have con-

tributed significantly to the development of a new approach to forest management for this part of the Canadian boreal forest.

Acknowledgements We first wish to thank the members of the GREFI and URDFAT who have contributed to the research presented in this paper. We are grateful to Patrick Lefort for the creation of Fig. 1, to Elizabeth Campbell for the correction of the English, and to an anonymous reviewer for his critical comments. Research projects forming the basis of this article were made possible by grants from the Natural Sciences and Engineering Research Council of Canada (NSERC), the Quebec Ministry of Education (FCAR program), the Quebec Ministry of Natural Resources, the Canadian Forest Service, the University of Quebec, as well as two forest companies, Norbord Industries and Tembec Forest Products.

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