The hemi-boreal zone, marking North America’s southern boreal forest boundary, has evolved post-glaciation, hosting diverse ecosystems including mixed forests with savannas, grasslands, and wetlands. While human, climate, and fire interactions shape vegetation dynamics therein, specific influences remain unclear. Here we unveil 12,000 years of hemi-boreal zone dynamics, exploring wildfire, vegetation, climate, and human population size interactions at such long time scales. Postglacial biomass burning exhibited episodes of persistent elevated activity, and a pivotal shift around 7000 years ago saw the boreal forest transition to an oak-pine barren ecosystem for about 2000 years before reverting. This mid-Holocene shift occurred during a period of more frequent burning and a sudden uptick in mean annual temperatures. Population size of Indigenous peoples mirrored wildfire fluctuations, decreasing with more frequent burning. Anticipated increases of fire activity with climate change are expected to echo transformations observed 7000 years ago, reducing boreal forest extent, and impacting land use.

Study region

In northwestern Québec, the Upper Harricana River is representative of the Abitibi Plains’ hydrological dynamics over the last 250 years.

Study focus

Planning for future spring flood risks involves uncertainties. This research presents a multicentury evaluation of changes in spring mean discharge and flood drivers using streamflow reconstruction (1771–2016), observations (1915–2020) and projections (2021–2100).

New hydrological insights for the region

Using a downscaled CMIP5 ensemble of 10 global climate models (GCMs), generalized additive mixed modeling of mean spring discharge projections matched those of an independent mechanistic model and eight GCMs projected variability in spring discharge by 2100 to be similar to the historical variability reconstructed for the last 250 years across the Abitibi Plains. Results indicate that the projected decline in snow cover (–20 to –30% annual snowfall) and rise in winter and spring temperature may be offset by a greater contribution of rainfall to spring high discharge (+100 to +125 mm). However, two GCMs projected an increase in the magnitude and frequency of high mean spring discharge for the Abitibi Plains. By investigating future mean spring discharge for the Upper Harricana River in reference to past reconstructed variability, this study provides insights to inform the future management of regional water resources. The importance of estimating future regional flood risks from the behavior of multi-model ensembles is highlighted.

Few records of spring paleoclimate are available for boreal Canada, as biological proxies recording the beginning of the warm season are uncommon. Given the spring warming observed during the last decades, and its impact on snowmelt and hydrological processes, searching for spring climate proxies is receiving increasing attention. Tree-ring anatomical features and intra-annual widths were used to reconstruct the regional March to May mean air temperature from 1770 to 2016 in eastern boreal Canada. Nested principal component regressions calibrated on 116 years of gridded temperature data were developed from one Fraxinus nigra and 10 Pinus banksiana sites. The reconstruction indicated three distinct phases in spring temperature variability since 1770. Ample phases of multi-decadal warm and cold springs persisted until the end of the Little Ice Age (1850–1870 CE) and were gradually replaced since the 1940s by decadal to interannual variability associated with an increase in the frequency and magnitude of warm springs. Significant correlations with other paleotemperature records, gridded snow cover extent and runoff support that historical high flooding were associated with late, cold springs with heavy snow cover. Most of the high magnitude spring floods reconstructed for the nearby Harricana River also coincided with the lowest reconstructed spring temperature per decade. However, the last 40 years of observed and reconstructed mean spring temperature showed a reduction in the number of extreme cold springs contrasting with the last few decades of extreme flooding in the eastern Canadian boreal region. This result indicates that warmer late spring mean temperatures on average may contribute, among other factors, to advance the spring break-up and to likely shift the contribution of snow to rain in spring flooding processes.

In northeastern boreal Canada, the long-term perspective on spring flooding is hampered by the absence of long gage records. Changes in the tree-ring anatomy of periodically flooded trees have allowed the reconstruction of historical floods in unregulated hydrological systems. In regulated rivers, the study of flood rings could recover past flood history, assuming that the effects of hydrological regulation on their production can be understood. This study analyzes the effect of regulation on the flood-ring occurrence (visual intensity and relative frequency) and on ring widths in Fraxinus nigra trees growing at five sites distributed along the Driftwood River floodplain. Driftwood River was regulated by a dam in 1917 that was replaced at the same location in 1953. Ring width revealed little, to no evidence, of the impact of river regulation, in contrast to the flood rings. Prior to 1917, high relative frequencies of well-defined flood rings were recorded during known flood years, as indicated by significant correlations with reconstructed spring discharge of the nearby Harricana River. After the construction and the replacement of the dam, relative frequencies of flood rings and their intensities gradually decreased. Flood-ring relative frequencies after 1917, and particularly after 1953, were mostly composed of weakly defined (less distinct) flood rings with some corresponding to known flood years and others likely reflecting dam management. The strength of the correlations with the instrumental Harricana River discharge also gradually decrease starting after 1917. Compared with upper floodplain trees, shoreline trees at each site recorded flood rings less frequently following the construction of the first but especially of the second dam, indicating that water level regulation limited flooding in the floodplains. Compared with the downstream site to the dam, the upstream ones recorded significantly more flood rings in the postdam period, reemphasizing the importance of considering the position of the site along with the river continuum and site conditions in relation to flood exposure. The results demonstrated that sampling trees in multiple riparian stands and along with various hydrological contexts at a far distance of the dams could help disentangle the flooding signal from the dam management signal.

In recent years, the utility of earlywood vessels anatomical characteristics in identifying and reconstructing hydrological conditions has been fully recognized. In riparian ring-porous species, flood rings have been used to identify discrete flood events, and chronologies developed from cross-sectional lumen areas of earlywood vessels have been used to successfully reconstruct seasonal discharge. In contrast, the utility of the earlywood vessel chronologies in non-riparian habitats has been less compelling. No studies have contrasted within species their earlywood vessel anatomical characteristics, specifically from trees that are inversely exposed to flooding. In this study, earlywood vessel and ring-width chronologies were compared between flooded and non-flooded control Fraxinus nigra trees. The association between chronologies and hydroclimate variables was also assessed. Fraxinus nigra trees from both settings shared similar mean tree-ring width but floodplain trees did produce, on average, thicker earlywood. Vessel chronologies from the floodplain trees generally recorded higher mean sensitivity (standard deviation) and lower autocorrelation than corresponding control chronologies indicating higher year-to-year variations. Principal components analysis (PCA) revealed that control and floodplain chronologies shared little variance indicating habitat-specific signals. At the habitat level, the PCA indicated that vessel characteristics were strongly associated with tree-ring width descriptors in control trees whereas, in floodplain trees, they were decoupled from the width. The most striking difference found between flood exposures related to the chronologies' associations with hydroclimatic variables. Floodplain vessel chronologies were strongly associated with climate variables modulating spring-flood conditions as well as with spring discharge whereas control ones showed weaker and few consistent correlations. Our results illustrated how spring flood conditions modulate earlywood vessel plasticity. In floodplain F. nigra trees, the use of earlywood vessel characteristics could potentially be extended to assess and/or mitigate anthropogenic modifications of hydrological regimes. In absence of major recurring environmental stressors like spring flooding, our results support the idea that the production of continuous earlywood vessel chronologies may be of limited utility in dendroclimatology.

Tree rings from ring-porous species have often been used as flood proxy. Many ring-porous species produce characteristic flood rings in response to stem submersion during vessel formation. Flood rings have earlywood vessels that are more numerous and/or of smaller cross-sectional area than “normal” rings. This study aimed at determining if diffuse-porous balsam poplar and trembling aspen, like ring-porous black ash, produce anatomically distinct annual tree rings in response to flooding. More specifically, we asked (i) if periodic tangential bands of vessels (hereafter PTBV) could be as easily identified/quantified as flood rings and (ii) if PTBV could be associated with spring flooding. Sampling of black ash, balsam poplar and trembling aspen trees took place along a flood exposure gradient in the floodplain of Lake Duparquet in northwestern Québec. Two observers recorded flood rings and PTBV. Consistency between observers was greatest when identifying flood rings. In both diffuse-porous species, PTBV occurred less abundantly than flood rings in black ash. They also occurred less often in balsam poplar than in trembling aspen. Years in which PTBV were initiated early in the growing season were associated with years in which flood rings occurred. Like flood rings, early occurring PTBV were more abundant in springs characterized by high mean river discharge, extensive snow cover, cold temperatures and heavy precipitation. Early-occurring PTBV dominated in flooded sites and late-occurring ones dominated in the control site. However, PTBV of the late-types were also observed in both flood exposures indicating that spring flood may not be the only factor modulating their formation. While flood rings seem to be associated with a change in the transport of growth regulators resulting from stem submergence and excess water, PTBV may be reflective of rhythmic alterations in the transport of growth regulators resulting from either water excess or deficit. Despite promising findings, many questions remain before PTBV in riparian diffuse-porous species can be widely used as a flood proxy. Why do species and individual trees differ in their ability to record them? What is the full range of environmental conditions triggering PTBV’s formation especially in unflooded sites and in the late growing season?

In eastern boreal Canada, long-term perspective in water resources and hydroelectric dam management is currently limited by the lack of long-term hydrological records. The research for new paleohydrological proxies would help fill this hydrological data gap and provide regional hydroclimatic predictive trajectories in the context of climate change. The development of long annually resolved series of earlywood vessel cross-sectional area has recently demonstrated a high potential for reconstructing high and low discharges. This study analyzes a network of 10 sites scattered around Lake Duparquet. The region covers an area of about 20 000 km2 including four river basins characterized by natural and regulated rivers, and unflooded control sites. The objectives were to assess 1) the spatial coherency in flood-rings chronologies among sites and Lake Duparquet, and among hydrological regimes (natural, regulated and unflooded control) and 2) their degree of association with i) annually resolved chronologies of earlywood vessel cross-sectional area and number, ii) a reconstruction of the Harricana River spring discharge and iii) discharge data from eleven hydrometric stations distributed in the study area. It was hypothesized that flood rings would be consistent among natural rivers and absent from regulated rivers. Results showed high spatial coherency among natural rivers with flood rings recording the major floods of the last 250 years. Flood ring and earlywood vessel chronologies were strongly correlated to both reconstructed and instrumental discharge data. On regulated rivers, trees were younger than at the other sites and mainly spring floods that occurred prior to dam creation and the few extreme floods after dam creation were recorded by flood rings. One hypothesis is that older trees (before dam) most likely recorded the natural dynamic of the river, while younger trees (after dam) most likely recorded dam management maneuvers and spring flood exceeding dam capacity. Flood rings and earlywood vessel chronologies provided comparable and complementary hydrological evidence. Flood rings were easily identified visually allowing fast determination of major flood years whereas developing earlywood vessel chronologies, while being more tedious and time consuming, allowed capturing a larger spectrum of hydrological conditions

In eastern boreal Canada, variability in river discharge is poorly understood at the multi-century scale due to short instrumental records. In recent decades, increased magnitude and frequency of spring floods have raised concerns about the potential effects of climate change on flood risk. Unlike tree-ring width, flood rings have a demonstrated dendrochronological utility for reconstructing high discharge in boreal environments. In this study, twelve chronologies of earlywood vessel cross-sectional area (a new hydrological proxy) and ring width were developed from riparian Fraxinus nigra trees periodically flooded in spring. These chronologies were used as predictors of Harricana River spring discharge, which was reconstructed for the period 1771–2016. The reconstruction captured 69% of the variance over a 102-year calibration period. The reconstruction indicates that the magnitude and frequency of spring high discharge has increased since the end of the Little Ice Age (1850–1870 CE) and since 1950. The change from a multi-decadal frequency in the late 19th century to a decadal and then interannual frequency in the late 20th century is associated with an increase in snow cover over much of central-eastern Canada. The association between the reconstructed spring discharge and spring atmospheric circulation indices NINO3.4, AMO, NAO may also have changed in these periods and further work is needed to assess the stability of these associations. The correlation between reconstructed and instrumental spring discharge at the regional scale, as well as the shared features in reconstructed discharge and other paleorecords from subarctic Québec suggest a common hydrological signal across the study area and for the early 20th to 21st centuries. The unprecedently low and high spring discharge in recent decades compared to the historical natural variability of the last 250 years also suggests that the increase in flood frequency and magnitude originates from climate change.

The boreal forests of Central Asia play a vital role in biodiversity protection and regional economic development. It is important to study potential changes in the growth dynamics of boreal species in a context of global change. In this study, we developed a network of 34 tree-ring chronologies for two tree species, Siberian larch (Larix sibirica Ledeb.) and Siberian pine (Pinus sibirica Du Tour). The network extended across a large latitudinal gradient (45°N to 55°N). Principal component analysis (PCA) was used to detect spatial patterns in tree radial growth during a common period 1943–2004. Results indicated an obvious clustering pattern with chronologies being divided into a northeastern (NR) and a southwestern (SR) region. Bootstrapped correlation analyses of regional climate versus aggregated chronologies showed that tree radial growth in both regions was positively associated with summer temperature (June and July). Tree radial growth in the northeastern region was however positively associated with early spring precipitation and spring Palmer Drought Severity Index (PDSI) whereas, in the southwestern region, it was characterized by negative correlations with early summer precipitation and summer PDSI. The warm pool El Niño-Southern Oscillation (WP ENSO) and North Atlantic Oscillation (NAO) regulated tree radial growth through their influence on regional precipitation and temperature. Results suggest that tree radial growth in the region may decline with future projected climate change. This study provides a more comprehensive understanding to tree growth-climate associations across Central Asia.

The North American boreal forest has been developing since the end of the last glaciation approximately 10,000 yr ago. With climate warming and human occupation, it is anticipated that fire danger, ignition, and activity will be increasing, compromising forests’ benefits for generations to come. In this study, we show, however, that a century of rapid climate changes and human densification has had the opposite effect in the boreal eastern interior of the North American continent, reducing biomass burning to values below two millennia of historical levels. A multi-millennial fire history was reconstructed for eight forested landscapes from the Lake of the Woods Ecoregion (LWE) located at the boreal–prairie ecotone. Fire history was reconstructed using a combination of archival (period 1920–2010), tree-ring (stand initiations and fire scars: period 1690–2010), and lake sediment charcoal (2500 BP to present) records. The archival record revealed recent large fires (>200 ha) in 1948, 1980, and 1988. An additional 19 fires were identified by the fire-scar record. Fire events in 1805, 1840, 1863, and the 1890s were identified in numerous locations around multiple lakes suggesting that they were of large extents. In accordance with the tree-ring record, the charcoal accumulation rate (CHAR) peak record generally identified the major fires but tended to lag from the tree-ring records by several decades. Within LWE, the long-term charcoal record revealed that CHAR was higher for each lake in the earlier portion of the record including the warm Medieval Climate Anomaly (AD
900 to AD 1000), followed by a progressive decrease toward the cool Little Ice Age period. This decline was abruptly interrupted in the mid- to late 19th century with large synchronized fires, also reported over western and central North America, and resumed approximately four decades later. Fire disturbance level is today below the historical range, despite the accentuated climate warming. Aging of the forest landscape may create biodiversity loss notably in fire-adapted species while at the same time setting the tone for major fires in upcoming decades if no action is taken for managing fuels.

Plants respond to environmental stimuli through changes in growth and development. Characteristics of wood cells such as the cross-sectional area of vessel elements (hereafter referred to as vessels) may store information about environmental factors present at the time of vessel differentiation. The analysis of vessel characteristics therefore offers a different time resolution than annual ring width because vessels in tree rings differentiate within days to a few weeks. Little research has been conducted on the sensitivity of earlywood vessels in ring-porous species in response to flooding. The general objectives of this study were to determine the plasticity of earlywood vessel to high flows and spring flooding in floodplain black ash (Fraxinus nigra Marsh.) trees and to assess the utility of developing continuous earlywood vessel chronologies in dendrohydrological reconstruction. In contrast, most dendrohydrological studies until now have mainly used vessel anomalies (flood rings) as discrete variables to identify exceptional flood events. The study area is located in the boreal region of northwestern Québec. Vessel and ring-width chronologies were generated from F. nigra trees growing on the floodplain of Lake Duparquet. Spring discharge had among all hydro-climatic variables the strongest impact on vessel formation and this signal was coherent spatially and in the frequency domain. The mean vessel area chronology was significantly and negatively correlated to discharge and both the linearity and the strength of this association were unique. In floodplain F. nigra trees, spring flooding promoted the formation of more abundant but smaller earlywood vessels. Earlywood vessels chronologies were also significantly associated with other hydrological indicators like Lake Duparquet’s ice break-up date and both ice-scar frequency and height chronologies. These significant relationships stress the utility of developing continuous vessels chronologies for hydrological reconstructions prior to instrumental data. Continuous earlywood vessel chronologies may also be useful in determining the impact of altered hydrological regime in floodplain habitat regulated by spring floods. Future research should involve quantifying the impact of high flows and flooding on other cell constituents and also determining the plasticity and utility of continuous anatomical series in floodplain diffuse-porous species.

Immediate phenotypic variation and the lagged effect of evolutionary adaptation to climate change appear to be two key processes in tree responses to climate warming. This study examines these components in two types of growth models for predicting the 2010–2099 diameter growth change of four major boreal species Betula papyrifera, Pinus banksiana, Picea mariana, and Populus tremuloides along a broad latitudinal gradient in eastern Canada under future climate projections. Climate-growth response models for 34 stands over nine latitudes were calibrated and cross-validated. An adaptive response model (A-model), in which the climate-growth relationship varies over time, and a fixed response model (F-model), in which the relationship is constant over time, were constructed to predict future growth. For the former, we examined how future growth of stands in northern latitudes could be forecasted using growth-climate equations derived from stands currently growing in southern latitudes assuming that current climate in southern locations provide an analogue for future conditions in the north. For the latter, we tested if future growth of stands would be maximally predicted using the growth-climate equation obtained from the given local stand assuming a lagged response to climate due to genetic constraints. Both models predicted a large growth increase in northern stands due to more benign temperatures, whereas there was a minimal growth change in southern stands due to potentially warm-temperature induced drought-stress. The A-model demonstrates a changing environment whereas the F-model highlights a constant growth response to future warming. As time elapses we can predict a gradual transition between a response to climate associated with the current conditions (F-model) to a more adapted response to future climate (A-model). Our modeling approach provides a template to predict tree growth response to climate warming at mid-high latitudes of the Northern Hemisphere.

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree-ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site-specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current-year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.

Abstract: A dendrochronological reconstruction of insect outbreaks was conducted along a latitudinal gradient from 46°N to 54°N in the boreal forest of western Quebec, Canada. Tree-ring chronologies of the host species, trembling aspen (Populus tremuloides Michx.), were constructed to identify periods of severe defoliation and comparisons were made with tree-ring chronologies of nonhost species. In addition, the frequency of white and narrow rings was used to further confirm the occurrence of insect outbreaks at these latitudes. Some major outbreaks occurred in relatively close synchrony at the regional scale, but the initiation year, intensity, and extent of the outbreaks varied spatially. For example, the 1950s outbreaks were observed from 1951 to 1952 at 46°N, from 1953 to 1954 at 47°N, and from 1954 to 1956 at 48°N. Other major outbreaks like the 1964 and 1980 outbreaks were fairly well synchronized at northern latitudes. The observed outbreaks in trembling aspen stands at 54°N also provided clear evidence that severe insect defoliation occurs much further north than the currently reported range limit, that is, between 49°N and 51°N, of the most important trembling aspen defoliator, the forest tent caterpillar (Malacosoma disstria Hubner). Our study demonstrated that careful identification of white rings in host species can provide valid information allowing the expansion of the forestry insect inventory database both at temporal and spatial scales.

Résumé : Une reconstitution dendrochronologique des épidémies d’insecte a été réalisée le long d’un gradient allant de 46°N à 54°N dans la forêt boréale de l’ouest du Québec, au Canada. Les chronologies de l’espèce hôte, le peuplier faux-tremble, ont été construites de manière à identifier les périodes de défoliation sévère et des comparaisons ont été effectuées avec les chronologies d’espèces non hôtes. De plus, la fréquence des cernes pâles et étroits a été utilisée pour valider l’occurrence des épidémies d’insecte à ces latitudes. Quelques épidémies majeures sont survenues avec une synchronicité relativement étroite à l’échelle régionale mais l’année du début, l’intensité et l’étendue des épidémies variaient dans l’espace. Par exemple, les épidémies des années 1950 ont été observées de 1951 à 1952 à 46°N, de 1953 à 1954 à 47°N et de 1954 à 1956 à 48°N. D’autres épidémies importantes comme celles de 1964 et 1980 étaient assez bien synchronisées aux latitudes nordiques. Les épidémies observées dans les peuplements de peuplier faux-tremble à la latitude 54°N fournissent des preuves manifestes que des défoliations sévères causées par les insectes surviennent beaucoup plus au nord que la limite de l’aire de répartition couramment rapportée, soit entre 49°N et 51°N, dans le cas du plus important défoliateur du peuplier, la livrée des forêts. Notre étude montre que l’identification minutieuse des cernes pâles chez l’espèce hôte peut fournir une information valide qui permet d’élargir la base de données de l’inventaire des insectes forestiers tant à l’échelle temporelle que spatiale.

[Traduit par la Rédaction]

Abstract:

Xylem production was studied by repeatedly taking microcore samples from the stems of six tree species growing on the “réserve écologique des Vieux-Arbres”, on Lake Duparquet, Québec, throughout the 1999 growing season. Species examined were paper birch (Betula papyrifera Marsh.), white spruce (Picea glauca (Moench) Voss), black spruce (Picea mariana (Mill.) BSP), jack pine (Pinus banksiana Lamb.), red pine (Pinus resinosa Ait.), and eastern white cedar (Thuja occidentalis L.). Onset of xylem cell production was observed in all species by 22 May 1999, and ended as early as mid-July and early August for white spruce and eastern white cedar, respectively. Xylem cell production in the remaining species ended between late August and mid-September. In general, the onset of latewood production ranged from the start of July to the first week of August. Typical sigmoidal curves were characteristic of ring width, number of cells, and number of earlywood cells over the growing season. Completion of the annual growth increment was quickest for white spruce and eastern white cedar, while it continued longest in both pine species. Numerous similarities in xylem production and tree ring formation over the course of the growing season were observed among the six species, suggesting that weather, along with photoperiod, plays a critical role in xylem production.

Résumé

Les auteurs ont étudié la production de xylème, en prélevant de façon répétitive des microcarottes à partir de la tige de six espèces venant dans la « réserve écologique des Vieux-Arbres », sur le lac Duparquet, au Québec, tout au long de la saison de croissance en 1999. Ils ont examiné le bouleau à papier (Betula papyrifera Marsh.), l’épinette blanche (Picea glauca (Moench) Voss), l’épinette noire (Picea mariana Mill.) BSP), le pin gris (Pinus banksiana Lamb.), le pin rouge (Pinus resinosa Ait.), et le thuya blanc de l’Est (Thuja occidentalis L.). Chez toutes les espèces observées, le début de la production des cellules du xylème est survenue vers le 22 mai, en 1999, et s’est terminé aussi tôt que vers la mi-juillet début d’août, chez l’épinette blanche et le thuya, respectivement. Chez les autres espèces,, la production des cellules du xylème se termine entre la mi-août et la mi-septembre. En général, le début de la formation du bois final va du début de juillet à la première semaine d’août. Typiquement, on observe des courbes sigmoïdes pour la largeur des anneaux, les nombres de cellules, et les nombres de cellules du bois hâtif, au cours de la saison de croissance. L’achèvement de l’accroissement annuel est plus rapide chez l’épinette blanche et le thuya de l’Est, alors qu’elle se poursuit plus longuement chez les deux espèces de pins. On observe de nombreuses similarités dans la production du xylème et la formation des anneaux de croissance, au cours de la saison de croissance, parmi les six espèces, ce qui suggère que la lumière, avec la photopériode, joue un rôle critique dans la production du xylème.   © 2007 NRC Canada

The CO2 fertilization hypothesis stipulates that rising atmospheric CO2 has a positive effect on tree growth due to increasing availability of carbon. The objective of this paper is to compare the recent literature related to both field CO2-enriched experiments with trees and empirical dendrochronological studies detecting CO2 fertilization effects in tree-rings. This will allow evaluation of tree growth responses to atmospheric CO2 enrichment by combining evidence from both ecophysiology and tree-ring research. Based on considerable experimental evidence of direct CO2 fertilization effect (increased photosynthesis, water use efficiency, and above- and belowground biomass), and predications from the interactions of enriched CO2 with temperature, nitrogen and drought, we propose that warm, moderately droughtstressed ecosystems with an ample nitrogen supply might be the most CO2 responsive ecosystems. Empirical tree-ring studies took the following three viewpoints on detecting CO2 fertilization effect in tree-rings: 1) finding evidence of CO2 fertilization effect in tree-rings, 2) attributing growth enhancement to favorable climate rather than atmospheric CO2 enrichment, and 3) considering that tree growth enhancement might be caused by synergistic effects of several factors such as favorable climate change, CO2 fertilization, and anthropogenic atmospheric deposition (e.g., nitrogen). At temperature-limiting sites such as high elevations, nonfindings of CO2 fertilization evidence could be ascribed to the following possibilities: 1) cold temperatures, a short season of cambial division, and nitrogen deficiency that preclude a direct CO2 response, 2) old trees past half of their maximum life expectancy and consequently only a small increase in biomass increment due to CO2 fertilization effect might be diminished, 3) the elimination of age/size-related trends by statistical detrending of tree-ring series that might remove some long-term CO2-related trends in tree-rings, and 4) carbon partitioning and growth within a plant that is species-specific. Our review supports the atmospheric CO2 fertilization effect hypothesis, at least in trees growing in semi-arid or arid conditions because the drought-stressed trees could benefit from increased water use efficiency to enhance growth.

Five independent multicentury reconstructions of the July Canadian Drought Code and one reconstruction of the mean July–August temperature were developed using a network of 120 well-replicated tree-ring chronologies covering the area of the eastern Boreal Plains to the eastern Boreal Shield of Canada. The reconstructions were performed using 54 time-varying reconstruction submodels that explained up to 50% of the regional drought variance during the period of 1919–84. Spatial correlation fields on the six reconstructions revealed that the meridional component of the climate system from central to eastern Canada increased since the mid–nineteenth century. The most obvious change was observed in the decadal scale of variability. Using 500-hPa geopotential height and wind composites, this zonal to meridional transition was interpreted as a response to an amplification of long waves flowing over the eastern North Pacific into boreal Canada, from approximately 1851 to 1940. Composites with NOAA Extended Reconstructed SSTs indicated a coupling between the meridional component and tropical and North Pacific SST for a period covering at least the past 150 yr, supporting previous findings of a summertime global ocean–atmosphere– land surface coupling. This change in the global atmospheric circulation could be a key element toward understanding the observed temporal changes in the Canadian boreal forest fire regimes over the past 150 yr.

Area burned variability in the province of Ontario, Canada, was inferred from 25 treering width chronologies covering AD 1781-1982 and distributed largely across the Boreal Shield. The area burned estimates account for 39.5% of the variance in the actual area burned recorded from 1917 to 1981 and were verified using a split sample calibrationverification scheme. The reconstruction showed that a positive trend in area burned from ca. 1970-1981 was preceded by three decades during which area burned was amongst the lowest during the past 200 years. The area burned exhibited a trend toward increasing variance during the past century, recently reaching magnitudes similar to those seen prior to 1850. Signal analyses further identified the presence of two prominent periodic components in area burned that related to decade-to-decade variations. This will help to place the recent increase in area burned in a context relative to the long-term history of the province.

Forest fires in Canada are directly influenced by the state of the climate system. The strong connection between climate and fire, along with the dynamic nature of the climate system, causes the extent, severity and frequency of fires to change over time. For instance, many reconstructions of the history of forest fires across boreal Canada report a general decrease in fire activity since ~1850 which could, in part, result from changes in climate. Here we describe progress in characterizing the variability in fire-conducive droughts in the central and eastern Canadian boreal forests during the past three centuries. An extensive network of drought-sensitive tree-ring records from Manitoba, Ontario and Quebec was used to develop five multi-century reconstructions of the mean July Canadian Drought Code and one reconstruction of mean July and August temperatures. Correlation analyses with regional fire statistics (common period 1959-1998) showed that drought estimates have some skill to approximate fire activity and, hence, the estimates are relevant for the study of climate change impacts on Canadian forests. Spatial correlation analysis over the period 1768-1998 revealed that variability between the west and east has increased after the mid-19th century, specifically the decade-to-decade variability and the frequency of extreme events. Based on the synoptic characteristics of recent droughts, we interpret this change in variability as a response to an increasing frequency of upper level ridging and troughing over western and eastern Canada, respectively. The increasing horizontal movement of humid air masses over eastern Canada since ~1850 could have contributed to the creation of moister conditions that are less suitable for fire.

Résumé

Les feux de forêt sont fortement influencés par l'état du système climatique. Le lien étroit qui existe entre le climat et les feux ainsi que la nature dynamique de l'état du système climatique conduisent à des variations temporelles dans l'étendu, la sévérité et la fréquence des feux. Par exemple, plusieurs reconstitutions historiques des feux de forêt à travers le Canada boréal rapportent une diminution de l'activité des feux depuis ~1850 qui pourrait en partie être due à des changements du climat. Dans la présente étude, nous décrivons les progrès réalisés dans la caractérisation de la variabilité des sécheresses propices aux feux de forêt au cours des trois derniers siècles du centre à l'est de la forêt boréale canadienne. Un réseau étendu de séries d'accroissement radial d'espèces arborescentes sensibles à la sécheresse en provenance du Manitoba, du Québec, et de l'Ontario, a été utilisé pour développer cinq reconstitutions multi-centenaires de l'indice de sécheresse canadien (CDC) de juillet et une reconstitution des températures moyennes de juillet et août. Des analyses de corrélation effectuées sur des données régionales de l'activité des feux (période commune 1959-1998) ont démontré que les estimés de sécheresses étaient suffisamment fiables pour inférer la variabilité temporelle de l'activité des feux. Ces estimés sont donc pertinents pour l'étude de l'impact des changements climatiques sur la forêt canadienne. Des analyses de corrélation spatiale sur la période 1768-1998 ont démontrés que la variabilité entre l'ouest et l'est s'est accentuée depuis le milieu du 19e siècle, en particulier pour la variabilité inter-décennale et la fréquence d'événements extrêmes. À partir des caractéristiques synoptiques des sécheresses récentes, nous interprétons ce changement dans la variabilité comme une réponse à une augmentation de la fréquence des crêtes et creux barométriques au-dessus de l'ouest et de l'est du Canada, respectivement. L'accroissement du mouvement horizontal d'air humide sur l'est du Canada depuis ~1850 pourrait avoir contribué à la création de conditions plus humides qui sont moins propices aux feux.

Inter-annual and -decadal scale variability in drought over the Abitibi Plains ecoregion (eastern Canada) was investigated using a 380-year dendroclimatic reconstruction of the Canadian Drought Code (CDC; July monthly average) i.e., a daily numerical rating of the average moisture content of deep organic layers. Spectral analyses conducted on the reconstructed CDC indicated a shift in spectral power after 1850 leading toward a reduction in interdecadal variability and an increase in interannual variability. Investigation on the causes for this shift suggested a decrease in North Pacific forcing after the mid-nineteenth century. Cross-continuous wavelet transformation analyses indicated coherency in the 8-16 and 17-32-year per cycle oscillation bands between the CDC reconstruction and the Pacific Decadal Oscillation (PDO) prior to 1850. Following 1850, the coherency shifted toward the North Atlantic Oscillation (NAO). Principal component analysis conducted over varying time windows reaffirmed that the Pacific forcing was restricted to the period about 1750-1850. Prior to and after this period, the CDC was correlated with the NAO. The shift around 1850 could reflect a northward displacement of the polar jet stream induced by a warming of the sea surface temperature along the North Pacific coast. A northward displacement of the jet stream, which inhibits the outflow of cold and dry Arctic air, could have allowed the incursion of air masses from the Atlantic subtropical regions.

Trends and periodicities in summer drought severity are investigated on a network of Canadian Drought Code (CDC) monthly average indices extending from central Quebec to western Manitoba and covering the instrumental period 1913–1998. The relationship and coherency between CDC indices and ocean–atmosphere circulation patterns are also examined. Trend analyses indicate that drought severity is unchanged in eastern and central Canada. Composite analyses indicate that for most of the corridor, severe drought seasons occur with a combination of positive 500-hPa geopotential height anomalies centered over the Gulf of Alaska and over the Baffin Bay. Additional severe drought seasons develop across the corridor in the presence of positive height anomalies located over or upstream of the affected regions. According to spectral analyses, the North Atlantic and the North Pacific circulation patterns modulate the drought variability at the decadal scale. Our results lead us to conclude that climate warming and the increases in the amount and frequency of precipitation in eastern Canada during the last century had no significant impact on summer drought severity. It is unlikely that linear climate change contributed to the change in the boreal forest dynamics observed over the past 150 years.

Les tendances et périodicités dans la sévérité des sécheresses estivales sont analysées sur un réseau d'indices de sécheresse (CDC) moyens mensuels couvrant le corridor Québec–Manitoba et la période déterminante 1913–1998. Les relations et les cohérences entre les indices CDC et les principaux patrons de circulation atmosphérique sont également examinées. Les résultats obtenus n'indiquent aucun changement linéaire des conditions de sécheresse estivale, ni dans l'Est ni au centre du Canada. Des analyses composites indiquent qu'à travers le corridor, des saisons de sécheresse sévère ont lieu en combinaison avec des anomalies positives de la hauteur géopotentielle à 500 hPa centrées au-dessus du Golfe de l'Alaska et au-dessus de la Baie de Baffin. Des saisons de sécheresse additionnelles ont lieu à travers le corridor avec le développement d'anomalies positives au-dessus des régions affectées. D'après nos analyses spectrales, les patrons de circulations de l'Atlantique Nord et du Pacifique Nord agissent sur la variabilité des sécheresses dans l'échelle décennale. Nos résultats nous amènent à conclure que le réchauffement climatique et les augmentations de la quantité et de la fréquence des précipitations dans l'est du Canada au cours du dernier siècle n'ont eu aucun effet important sur la sévérité des sécheresses estivales. Il est donc peu probable que le changement climatique linéaire ait contribué au changement dans la dynamique de la forêt boréale enregistré au cours des 150 dernières années.©2004 NRC Canada

Climate change, fires, and insects outbreaks can affect eastern larch (Larix laricina (Du Roi) K. Koch) stand dynamics. To determine which of these factors had the greatest influence on stand dynamics, we sampled four wetlands dominated by larch on the margin of Lake Duparquet in the northern Clay Belt of Quebec. The ages of seedlings, saplings, and trees were determined in twelve 400-m² plots. Increment cores were taken at breast height to determine past disturbance episodes related, among others, to larch sawfly (Pristiphora erichsonii (Hartig)) activity. Stem analysis was conducted using larch and black spruce (Picea mariana (Mill.) BSP) for identification of post-disturbance releases in height growth. Analyses at the stand and cluster levels showed that larch age structures were characterized by many unsynchronized establishment periods. In addition, a seedlings bank not much older than 10 years characterized many plots. Two larch sawfly outbreak episodes (1895-1912 and 1955-1962) identified by tree-ring and stem analysis were associated with larch establishment. A smaller outbreak in the late 1970s could also have contributed to the initiation of establishment in one stand. Our results suggest that the length (severity) of an outbreak may be a critical factor in explaining the different patterns of establishment observed in these stands. During a severe outbreak, larch establishment may occur mainly from surviving stems (increased seed production), whereas during a mild outbreak, larch establishment may occur from increased survival of pre-established seedlings and saplings. Further studies on the distribution of gaps within larch stands may provide better information on the pattern of mortality (gap dynamics) during an outbreak and may help to better understand larch establishment in these stands.©2002 NRC Canada

A dendrochronological study was performed at sis sites dominated by eastern larch, Larix laricina in Ouebec's southwestern boreal forest. The objectives were to reconstruct periods of larch sawfly (Pristiphora erichsonii) outbreak in the region and to determine which physical factors (precipitation, temperature, water level or drought) explained the greatest variation in radial growth. From the presence of light latewood rings followed by periods of growth suppression. we identified larch sawfly outbreaks for the years 1895-1912, 1937-1942, and 1955-1962. We suspect that additional outbreaks occurred in the early 1920s, late 1970s and early 1980s as well, but at the same time as spruce budworm outbreaks (Choristoneura fumiferana). Response function analysis demonstrated negative relationships between larch radial growth and May and August precipitation and May and September current year water level, and demonstrated positive relationships with May current year drought index and September previous year drought index. These results suggest that flooding in the early growing season and excessive water levels at the end of the growing season may negatively affect larch radial growth. Our results also indicate an increase in the year-to-year variation in radial growth in larch sites subjected to flooding. This may reflect the increase in the Lake Duparquet water level at spring break up.

We analysed the radial growth response of seven boreal tree species growing on an island of Lake Duparquet, northwestern Québec. The species investigated were Betula papyrifera, Abies balsamea, Thuja occidentalis, Picea glauca, Picea mariana, Pinus banksiana and Pinus resinosa. Seven species chronologies were developed as well as seventy individual tree chronologies. Coniferous species were positively correlated to warm April and to cool-wet July. This indicates that early spring and positive water balance during the growth season favour radial growth. In contrast, the radial growth of B. papyrifera was mainly correlated to June precipitation. The response of individual trees to climate was variable but the differences between B. papyrifera and the coniferous species were maintained. No microsite factors or tree characteristics were associated with this variability. Except for B. papyrifera, it is speculated that climate change could have a similar qualitative physiological consequence on the growth of coniferous species found on homogeneous insular landscapes.

La réponse climatique de sept espèces arborescentes de la forêt boréale croissant sur une île du Lac Duparquet, nord-ouest du Québec, fut analysée. Les sept espèces étudiées furent le Betula papyrifera, le Abies balsamea, le Thuja occidentalis, le Picea glauca, le Picea mariana, le Pinus banksiana et le Pinus resinosa. Une chronologie fut développée pour chacune des espèces de même que pour chacun de 70 arbres inclus dans l’étude. Toutes les espèces, sauf le B. papyrifera, ont démontré une commune influence du climat. L’accroissement radial du B. papyrifera fut significativement relié aux précipitations de juin. La réponse climatique des six espèces de conifères fut principalement associée à l’effet positif des températures chaudes d’avril et aux conditions fraîches et humides de juillet. Bien que plus variable, l’analyse des 70 chronologies a démontré que les différences entre le B. papyrifera et les conifères étaient maintenues. Aucun facteur du microsite ou caractérisant les arbres (age ou diamètre) ne fut associé à la réponse climatique des arbres. Encore une fois, les arbres des six espèces de conifères ont démontré une réponse climatique très semblable. Sauf pour le B. papyrifera, il est spéculé que l’effet des changements climatiques aura un impact physiologique qualitativement similaire pour les espèces de conifère croissant dans des milieux insulaires homogènes.

In the `Des Vieux Arbres' ecological reserve situated within northwestern Québec, 40 band dendrometers were installed on 7 of the major boreal tree species. The late Spring–early Summer daily radial activity registered in 1997 was related to daily weather variables. For each tree species, the daily mean i) cumulative radial increment and ii) radial activity indexed series obtained by first-difference standardization were analyzed. The results indicate the existence of strong similarities among the 7 species. All showed strong synchronous fluctuations in radius during late winter and early spring. This period ended with a short but sharp increase in radial increments that marked the passage of water into the stem. This initial swelling, less obvious in Pinus species was followed by a prolonged period of little change in radial activity. Meteorological data indicated that air temperature was positively related to stem swelling during the late winter–early spring period. Both air and soil temperatures became negatively related to radial expansion once the passage of water has occurred in the stem. Starting in early June, all species registered a sustained increase in radial increments possibly associated with active cell division. After this, radial expansion was negatively related to air temperature and positively to rainfall.

With the objective of understanding how vegetation was structured in four Larix laricina (Du Roi) K. Koch dominated wetlands in north-western Quebec, 186 point-centred quarters were sampled in four stands. For each point, both biotic and abiotic variables were collected and species cover was recorded. Divisive hierarchical classification analysis (Twinspan) identified nine vegetation clusters: i) Larix laricina & Spiraea alba, ii) Larix laricina & Kalmia angustifolia, iii) Larix laricina, Picea mariana & Alnus rugosa, iv) Larix laricina & Betula pumila, v) Thuja occidentalis & Trientalis borealis, vi) Abies balsamea & Betula papyrifera, vii) Fraxinus nigra & Onoclea sensibilis, viii) Alnus rugosa, and ix) Eleocharis smallii. Results of the canonical correspondence analyses indicated that the distribution of these clusters was mainly related to (i) distance from shore, (ii) shade (canopy cover), (iii) substrate nitrate concentration (in relation to the abundance of Kalmia angustifolia and Alnus rugosa), (iv) substrate pH (in relation to the abundance of Sphagnum spp.), and (v) substrate conductivity. Several characteristics of the water table also affected species distribution, including pH, depth, and carbon concentration. Further studies should address the effect of the presence of Kalmia angustifolia and Alnus rugosa on larch growth.

To decipher spatial and temporal tree-growth responses to climate change we used tree-ring data from Picea mariana (Mill.) BSP and Pinus banksiana Lamb. along a latitudinal transect in western Quebec. The transect encompassed the distinct transition between mixed and coniferous forests at approximately 49 degrees N. Correlation analyses and principal component analyses were used to identify common spatiotemporal growth patterns, and site- and species-specific patterns since 1825. A moist summer in the year t - 1 and an early start of the current growing season favored growth of both species. A prolongation of the growing season into fall was the most distinguishing factor between the species. A long and gradual climatic gradient shifted to a short gradient with a clear segregation between the southern and northern parts of the transect. This shift, around 1875, was abrupt and characterized by a turbulent climatic period. The observed pattern was likely related to a large-scale shift in the mean position of the Arctic Front that occurred at the end of the 1800s. No discrete climatic setting explained the present switch from mixedwoods to conifers at 49 degrees N. Awareness of such nonequilibrial relations between climate and species distribution is essential when assessing vegetation responses to future climate change. ©1999 NRC Canada

Population dynamics of black ash (Fraxinus nigra) were investigated in five stands with different exposures to spring flooding at the southwestern limit of Quebec's boreal forest. Fifty-four 100-m² plots were sampled along 25 transects laid out perpendicular to Lake Duparquet, In each plot, all black ash >5 cm in diameter at breast height (dbh) were aged using dendrochronological methods. An age-height regression was used to calculate the age of those individuals <5 cm in dbh. Stump sprouts had a significantly faster height and diameter growth rate than individuals established from sexual origin. Two stands in this study showed deficits in sexual recruitment. One stand, located on a deep organic layer, also showed recent colonization by F. nigra that may have started during the 1930s. This stand had the highest stem density, and sprouting was restricted to the younger age classes. Ordination analysis of dispersion indices characterizing the age distribution of F. nigra indicated a definite pattern toward higher sexual recruitment in plots and stands that were less exposed to flooding. In contrast, poor sexual recruitment was observed on more exposed sites. However, this situation was compensated for by abundant vegetative recruitment, Analysis of the periods of growth release and suppression of 153 trees revealed that F. nigra population dynamics are not regulated by major disturbance events. A general upward trend in radial growth and F. nigra expansion toward higher elevation sites observed since the end of the 19th century may be due to changes in Lake Duparquet's hydrological regime. Increases in the severity of flooding events may result in the decrease in seedling abundance observed in the more exposed stands. F. nigra populations were found to be resistant to long-term natural water fluctuations due to their ability to regenerate both by seeds and by sprouts.

Le domaine de la pessière noire à mousse est le plus vaste domaine forestier de la province. Sa superficie totale représente 28% du territoire québécois. Le Québec a la responsabilité d’en assurer la pérennité en y pratiquant un aménagement forestier durable, ce qui assurera le maintien de la viabilité des écosystèmes du domaine et de leur biodiversité. Une bonne partie de ce territoire étant constituée de forêts primaires, il est encore possible d’étudier la dynamique naturelle et la croissance des peuplements pour mieux fonder notre stratégie d’aménagement écosystémique. Les nouvelles connaissances indiquent que les perturbations naturelles conduisent à l’ouverture de plusieurs peuplements fermés d’épinette noire. D’ailleurs, pour ce domaine, certains auteurs parlent de la forêt ouverte comme étant un état alternatif stable à la forêt fermée. À ce changement se superpose une perturbation d’origine anthropique, la coupe, qui s’ajoute aux perturbations naturelles. L’aménagement écosystémique devra donc tenir compte non seulement des impacts des perturbations anthropiques, mais aussi des impacts reliés aux perturbations naturelles, car celles-ci peuvent modifier de façon permanente le paysage forestier. En effet, si les mécanismes conduisant à l’ouverture des peuplements sont maintenant bien documentés, il en va autrement pour les mécanismes de fermeture. L’action des perturbations naturelles dans le paysage de la pessière noire fermée ne permettrait donc pas d’atteindre l’objectif de pérennité. Devant un tel portrait, les méthodes répondant aux exigences de l’approche écosystémique restent à définir.

In northwestern Quebec, black ash (Fraxinus nigra Marsh.) is found mostly in floodplains bordering lakes and rivers. In this habitat, population dynamics are largely controlled by spring water level, and sexual regeneration is more abundant in sites less exposed to flooding. The hypothesis tested in this study is that sexual regeneration of black ash in sites slightly exposed to flooding is influenced by fluctuations of spring flood water level. The age structure of two sapling populations ( lt 5 cm DBH) was analyzed using models assuming a constant (exponential function) and decreasing mortality (power function) with age. Both models were useful in describing the age structure of the saplings. The constant mortality model was selected and analysis of the residuals showed that the populations responded in different ways to climate during the 1914-1989 period. While regeneration is correlated with summer and annual precipitation in population A, it is correlated with winter precipitation in population B. Abundant sexual regeneration in both black ash stands suggests that moisture conditions are sufficient to assure constant sexual regeneration, which is, however, diminished during dry years.

Stands of Fraxinus nigra (black ash) near Lake Duparquet (Abitibi region, NW Quebec) are affected each year by spring or early summer flooding. The relationship between environmental factors and radial growth was studied in 50 100-m-2 quadrats laid out in five Fraxinus-dominated stands. Response function coefficients showed that the weather in the year before growth, i.e. in the time of bud formation, had a strong influence on radial increment. Temperatures in April of the year before (year i - 1) and May (i - 1) were positively correlated with growth. While precipitation in April (i - 1), May (i - 1) and June (i - 1) were negatively correlated. Climatic factors positively associated with growth during the year of bud expansion were April (i) temperature and June (i) precipitation. Redundancy analysis showed that radial growth in Fraxinus nigra responded to variation in elevation and geomorphological landform. These differences in stand location and elevation were also related to different responses to climate. The sites most exposed to flooding responded much more to the negative impact of spring precipitation, while the more elevated sites were more responsive to summer precipitation. This study showed that black ash could be used for long term reconstruction of climatic and hydrological fluctuations.

Spring and early summer flood is one of the main factors affecting the dynamics of black ash (Fraxinus nigra Marsh.) stands growing on lake floodplains in the boreal forest. The effect of flooding on the vegetation was studied in five stands on the shore of Lake Duparquet in Abitibi, northwestern Quebec (Canada). Divisive hierarchical classification analysis (Twinspan) revealed the existence of four different vegetation types: (i) black ash - speckled alder (Alnus rugosa) - bog willow (Salix pedicellaris), (ii) black ash - balsam poplar (Populus balsamifera) - ostrich fern (Matteuccia struthiopteris), (iii) black ash - pussy willow (Salix discolor) - sensitive fern (Onoclea sensibilis), and (iv) black ash - speckled alder - sensitive fern. Detrended correspondance analysis (Decorana) shows that elevation and drainage are the main factors affecting plant distribution and dynamics. Geomorphology also plays a major role in the way each stand responds to flooding. For each of the vegetation types studied, black ash shows a particular regeneration pattern and sexual regeneration tends to become less frequent with increasing exposure to flooding. Finally, the strong sexual regeneration and the youth of the populations found in the most elevated sites could be related to a possible alteration of the flood regime.