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.

• Premise of the study : Climate warming might have resulted in altered initiation and termination dates of stem xylem growth in
  boreal stands. A systematic understanding of intra-annual xylem formation is thus needed for precise simulation of future
  growth in the context of sustainable forest management.
• Methods : A recently developed novel microsampling approach was employed over two growing seasons (2005 – 2006) to investigate
  the intra-annual stem xylem formation of Picea mariana at three sites along a latitudinal gradient (approximately 47.5 –
  50 ° N) in western Quebec, Canada. The critical timings of xylem cell formation were determined and compared among sites/
  years. The relationships between xylem cell formation and meteorological variables were examined.
• Key results : From south to north, the onset of xylem cell production was detected on 20 May (SD ± 3) at Angliers, 24 May (SD ± 3)
  at Chicobi and 24 May (SD ± 4) at Muskuchii in 2005, and on 12 May (SD ± 4) at Angliers, 14 May (SD ± 3) at Chicobi and 20
  May (SD ± 3) at Muskuchii in 2006, respectively. Xylem cell production at each respective site terminated on 11 August (SD ± 4),
  7 August (SD ± 3), and 7 August (SD ± 4) in 2005, and on 8 August (SD ± 4), 4 August (SD ± 4), and 4 August (SD ± 4) in 2006,
  respectively.
• Conclusion : Our study implies that despite the expected occurrence of earlier phenological development due to early spring
  climate warming, boreal trees like P. mariana might not be producing wider rings if cold temperatures occur later in the growing
  season in June to August. These results may challenge the view that boreal trees could be benefi ting from spring warming
  to enhance growth.

Tree-ring formation of eastern white-cedar (Thuja occidentalis L.) at a boreal lake in northwestern Quebec, Canada, was monitored using manual band dendrometers to (i) retrace cambial activity phases, (ii) evaluate the effects of flooding on radial growth, and (iii) analyze the relationships with meteorological factors. The daily circumferential activity of four trees at each of two sites, a riparian and an upland site, was recorded during the growing season of 1996, a year with an extreme spring flood. First cambium cell divisions occurred near June 9, followed by a distinct and sustained upward trend in the stem basal area until mid-July that reflected the earlywood formation. The strongly synchronous circumferential activity at both sites suggests no adverse flooding effect on growth of the riparian trees, which is explained by the rapid retreat of the water just before growth initiation in early June. The following month until mid-August was characterized by strong short-term fluctuations caused by alternating drought and rain periods and a slight downward trend of the basal area for six of the eight banded white-cedars. The dendrometers of two trees, the closest to the lake, showed a slight upward trend probably reflecting latewood formation. Pearson correlation with meteorological data indicated that precipitation was positively related to the daily changes in basal area of all trees except during the period of earlywood formation, which probably resulted from the high soil moisture after spring snow-melting. Mean and minimum air humidity were positively related and maximum temperature negatively related to the daily variations in stem circumference during the whole monitoring period, emphasizing the importance of the internal water status on stem size.

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:

The relative importance of fire and flooding on the population dynamics of eastern white-cedar (Thuja occidentalis L.) and black ash (Fraxinus nigra Marsh.) was evaluated in eight old-growth riparian stands of southwestern boreal Quebec, Canada. Rising water levels and decreasing fire frequency since the end of the Little Ice Age (ca. 1850) were expected to have favoured an inland migration of the riparian forest fringe, with the flood-tolerant black ash colonizing the lower parts of the shore terraces and eastern white-cedar the upper parts. Black ash was found to be restricted to the riparian zone (<200 cm elevation), whereas eastern white-cedar trees did not occur below 100 cm above lake level. Gaps of postfire eastern white-cedar recruitment were noted in stands exposed to riparian disturbances, whereas relatively continuous recruitment occurred at protected sites. Black ash, more tolerant to flooding and ice push, invaded the shore terrace sites left vacant by eastern white-cedar. The riparian forest fringe surrounding Lake Duparquet is currently migrating upland and this trend is expected to continue as water levels continue to increase and fire frequency continues to decrease during the 21st century.© 2008 NRC Canada.

Résumé

L’importance relative de deux des principales perturbations de la forêt boréale, le feu et les crues, sur la dynamique des populations de thuya occidental (Thuja occidentalis L.) et de frêne noir (Fraxinus nigra Marsh.) a été évaluée dans huit peuplements riverains du sud-ouest de la forêt boréale du Québec (Canada). L’hypothèse était que l’augmentation des niveaux d’eau et la diminution de la fréquence de feux depuis la fin du Petit Âge Glaciaire (ca. 1850) auraient favorisé une migration vers l’intérieur des terres de la frange forestière riveraine, le frêne noir, tolérant aux inondations, colonisant les parties basses des terrasses et le thuya occidental les parties hautes. Le frêne noir était restreint à la zone riveraine (<200 cm au-dessus du niveau d’eau), tandis qu’aucun thuya occidental mature ne se trouvait sous 100 cm au-dessus du niveau d’eau. Des épisodes de faible recrutement ont été notés dans les peuplements de thuya occidental exposés aux perturbations riveraines, tandis que le recrutement était relativement continu dans les sites protégés. Le frêne noir, plus tolérant aux crues et aux poussées glacielles, a envahi les terrasses riveraines abandonnées par le thuya occidental. La frange forestière riveraine du lac Duparquet est présentement en migration vers l’intérieur des terres et la tendance devrait continuer suivant l’augmentation des niveaux d’eau et la diminution des fréquences de feu au XXIe siècle.© 2008 NRC Canada.

La réaction des peuplements riverains de cèdre blanc (Thuja occidentalis L.) à la construction de deux barrages sur le lac Abitibi — un grand plan d’eau situé dans la région boréale méridionale de l’est du Canada — a été analysée afin de déterminer si la limite lacustre du cèdre blanc est déterminée par le stress physiologique dû à l’inondation ou par les perturbations riveraines. La hausse d’environ 1,2 m du niveau d’eau, engendrée par la construction du premier barrage en 1915, a causé la mort instantanée de tous les arbres qui formaient l’ancienne lisière de la forêt et provoqué des blessures et l’inclinaison des arbres survivants (formant la marge forestière actuelle), par l’action des vagues ou l’activité glacielle. La construction du deuxième barrage, en 1922, n’a pas augmenté significativement le niveau d’eau, mais a plutôt entraîné un retard et une baisse de magnitude des crues printanières, sans toutefois affecter la croissance des cèdres blancs riverains. La croissance radiale n’a pas été affectée par l’inondation, probablement parce que les crues printanières étaient déjà terminées avant le début de la saison de croissance (1915–1921), ou parce qu’elles étaient de trop courte durée pour nuire au métabolisme des arbres (après 1921). Il s’ensuit que (i) la limite lacustre du cèdre blanc est une limite mécanique liée à l’action des vagues et à l’activité glacielle, plutôt qu’une limite physiologique due à l’inondation, et (ii) que les paramètres de croissance reliés aux perturbations, tels les cicatrices glacielles, la formation de bois de compression et la mortalité partielle du cambium, sont de meilleurs indicateurs que la largeur des cernes de croissance, pour reconstituer l’augmentation à long terme du niveau d’eau de lacs naturels.

Abstract:

Effect of interannual climate variations on radial growth was compared among jack pine (Pinus banksiana Lamb.) of diverse geographical origins in a 41-year-old common-garden experiment in Petawawa, Ontario. Provenance experiments established from seeds transferred from different parts of a species range (from the northern United States to northern Canada) to the same environment might be considered as a simulation model of climate change and a shift of climate zones. The following questions are addressed: Did the response of growth to interannual climate variations differ among the provenances transferred within the experimental site? What climatic factors affect interannual growth variations of jack pine provenances? Tree-ring chronologies for 16 populations were developed for the period 1970–2004. The best climate predictors of radial growth were precipitation of June and March of the current year and precipitation of December of the previous year. Although, climatic factors affecting growth were similar between the provenances, absolute radial growth was proportional to the growth potential of the provenances. We conclude that variability due to seeds origins is not a significant source of variation for dendroclimatic studies of jack pine. Increased frequency of summer droughts might result in a growth decrease of jack pine © 2008 NRC Canada.

Résumé

Nous avons étudié les relations entre le climat et la croissance radiale annuelle de provenance de pin gris (Pinus banksiana Lamb.) établis depuis 41 ans dans un jardin commun à Petawawa en Ontario. Des tests de descendance établis dans des conditions similaires à partir de graines provenant de différentes parties de l’aire de distribution de l’espèce (depuis le Nord des Etats-unis jusqu’au nord du Canada) simulent des changements climatiques et des transitions entre différentes zones bioclimatiques. Nous avons tentés de répondre aux questions suivantes : Est-ce que la relation entre la croissance radiale interannuelle et le climat varie en fonction des provenances? Quelles sont les facteurs climatiques qui contrôlent la croissance radiale des différentes provenances. Des chronologies de la croissance radiale couvrant la période 1970–2004 ont été développées pour 16 provenances. Les facteurs climatiques les plus corrélés avec la croissance radiale sont les précipitations pour juin et mars de l’année en cours et les précipitations de décembre pour l’année précédente. Bien que les facteurs climatiques contrôlant la croissance sont les mêmes pour toutes les provenances, la croissance absolue variait en fonction du potentiel de croissance de chaque provenance. Nous concluons que la variabilité due à la provenance des graines ne constitue pas une source importante de variation dans la réponse dendroclimatique chez le pin gris. Une augmentation de la fréquence des sécheresses durant la période estivale pourrait entraîner une diminution de la croissance du pin gris © 2008 NRC Canada.

Abstract:

Growth–climate relationships were analyzed for 15 stands of trembling aspen (Populus tremuloides Michx.) along a productivity gradient in northeastern British Columbia. Productivity was evaluated with a site index model whose values varied between 6.65 and 26.93 m height at age 50 years. To assess the role of climate on radial growth, we built a tree-ring chronology for each site and then performed a dendroclimatic analysis by means of bootstrapped correlation functions using monthly and annual climatic variables. We found that trembling aspen sensitivity to climate varied across stands with different productivities in the study area. At the more productive sites, trees were more sensitive to climate, and ring widths were positively correlated, particularly with summer month precipitation prior to the year of growth. The different responses to climate along the productivity gradient are likely related to the differences in substrate properties; substrates at the more productive sites are richer in nutrients and have a better water holding capacity than those of the less productive sites. Our study suggests that the effects of future climate change on tree growth may not only increase towards the limit of a species distribution, but also towards richer and wetter stands.© 2008 NRC Canada.

Résumé

Quinze populations de peuplier faux tremble (Populus tremuloides Michx.) du nord-est de la Colombie Britannique représentant un gradient de productivité ont été analysés en regard des relations entre la croissance radiale et le climat. La productivité a été évaluée par l’indice de qualité de site qui variait entre 6,65 et 26,93 m de hauteur à 50 ans. Afin d’évaluer le rôle du climat sur la croissance radiale, une chronologie maîtresse fut produite pour chacun des sites. Les chronologies ont par la suite été comparées aux données climatiques mensuelles et annuelles à l’aide d’une analyse dendroclimatique basée sur des fonctions de corrélation utilisant une procédure bootstrap. La sensitivité du peuplier faux tremble aux conditions climatiques variait avec le gradient de productivité. Ainsi, les sites les plus productifs montraient une plus grande sensitivité au climat particulièrement avec les précipitations durant les mois d’été de l’année précédente. Les différentes réponses observées selon le gradient de productivité pourrait s’expliquer par les caractéristiques des sols qui dans les sites les plus productifs sont plus riches en nutriments et possède une plus grande capacité à retenir l’eau. Notre étude suggère que l’effet du changement climatique sur la croissance des arbres pourrait se faire sentir non seulement à la limite de distribution des espèces mais aussi dans les sites riches et plus humides.© 2008 NRC Canada.

The effects of seed transfer and climate change on the width and basal area of tree rings were studied in 21 provenances of jack pine (Pinus banksiana Lamb.) grown in a common-garden plantation in Petawawa, Ontario, Canada. Seed-source origin significantly influenced both mean tree-ring width and mean annual basal area increment over a 25-year growth period (1975–1999). Temperature and precipitation transfer functions were developed to predict width and basal area of tree rings of the jack pine populations. The best predictors of growth were the transfer distances mean annual maximum daily temperature and annual precipitation between the plantation site and the seed origins. Radial growth of the jack pine populations was mainly related to temperature at seed origin and, to a lesser degree, to precipitation at seed origin. Extension of the transfer functions to three sets of independent data revealed significant correlations between estimated and predicted mean radial growth characteristics. Seed sources of jack pine originating from warmer and drier climates than that of the plantation site in Petawawa had slightly higher mean ring widths and basal areas than the local populations. The application of different climate change scenarios derived from general circulation models the developed transfer functions indicated that radial growth of jack pine may decline only if significant climate changes occur, which might happen before the mid 21st century. Both a higher radial growth of southern seed sources and a potential negative effect of a significant temperature increase and precipitation decrease in future suggest restricting the northward transfer of southern seed sources to less than 18 latitude. However, provenance specific differences in survivorship, frost- and disease-resistance, and cone serotiny should also be taken into consideration.© 2007 Elsevier B.V. All rights reserved.

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.

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.

The objective of this study was to evaluate the most important environmental factors determining the distribution of tree species within the riparian zone of Lake Duparquet, located in the southern boreal region of Quebec, Canada. Occurrence and relative basal area of 10 species were recorded within an altitudinal range of 200 cm above mean water level along 95 transects. Stepwise logistic regression and canonical correspondence analyses were performed on the overall data set as well as separately for the five geomorphological shore types distinguished (depositional flats, floodplains, beaches, terraces, and rock outcrops). The elevation gradient, representing seasonal floodings, is the main factor determining the distribution of the species. The differences between the geomorphological shore types with respect to composition and arrangement of the arborescent vegetation along the elevation gradient are at least partially explained by surficial substratum, topography, aspect, and fire. Exposure to wave activity seems to be of minor importance only. However, since they are the driving force of erosion and sedimentation, the waves are to a great part responsible for the morphological differentiation of the shoreline. The distribution of the tree species along a characteristic physiographic cross-section is illustrated for each geomorphological shore type.