Tree growth varies closely with high–frequency climate variability. Since the 1930s detrending climate data prior to comparing them with tree growth data has been shown to better capture tree growth sensitivity to climate. However, in a context of increasingly pronounced trends in climate, this practice remains surprisingly rare in dendroecology. In a review of Dendrochronologia over the 2018–2021 period, we found that less than 20 % of dendroecological studies detrended climate data prior to climate-growth analyses. With an illustrative study, we want to remind the dendroecology community that such a procedure is still, if not more than ever, rational and relevant. We investigated the effects of detrending climate data on climate–growth relationships across North America over the 1951–2000 period. We used a network of 2536 tree individual ring-width series from the Canadian and Western US forest inventories. We compared correlations between tree growth and seasonal climate data (Tmin, Tmax, Prec) both raw and detrended. Detrending approaches included a linear regression, 30-yr and 100-yr cubic smoothing splines. Our results indicate that on average the detrending of climate data increased climate–growth correlations. In addition, we observed that strong trends in climate data translated to higher variability in inferred correlations based on raw vs. detrended climate data. We provide further evidence that our results hold true for the entire spectrum of dendroecological studies using either mean site chronologies and correlations coefficients, or individual tree time series within a mixed-effects model framework where regression coefficients are used more commonly. We show that even without a change in correlation, regression coefficients can change a lot and we tend to underestimate the true climate impact on growth in case of climate variables containing trends. This study demonstrates that treating climate and tree-ring time series “like-for-like” is a necessary procedure to reduce false negatives and positives in dendroecological studies. Concluding, we recommend using the same detrending for climate and tree growth data when tree-ring time series are detrended with splines or similar frequency-based filters.

Spatially explicit reconstructions of fire activity in European boreal forest are rare, which limits our understanding of factors driving vegetation dynamics in this part of the boreal domain. We have developed a spatially explicit dendrochronological reconstruction of a fire regime in a 25 × 50 km2 area within boreal biome located within the Kalevalsky National Park (Kalevalsky NP), over the 1400–2010 CE period. We dated 184 fire years using 212 fire-scarred living and dead Scots pine (Pinus sylvestris L.) trees collected on 38 sites.

The studied period revealed a pronounced century-long variability in forest fire cycles (FC). The early period (1400–1620 CE) had low fire activity (FC = 178 years), which increased during the 1630–1920 period (FC = 46 years) and then decreased over the 1930–2000 period (FC = 283 years). Dendrochronological results did not provide a conclusive answer on the origins of FC dynamics, although several lines of evidence suggest that climate drove the increase in fire activity in the early 1600s, while human-related factors were largely responsible for its decline in the early 1900s. The current FC in the Kalevalsky NP is close to the estimates reported for the pre-industrial colonisation period in Scandinavia, which suggests that the forests of the area currently maintain their close-to-natural fire regime. Fire has been the pivotal factor of forest dynamics in this biome and forest management should acknowledge that fact in developing conservation strategies in Karelia and other areas of European boreal forest. Introduction of prescribed burns of varying severity could be an important element of such strategies.

The mid-20th century changes in North Atlantic Ocean dynamics, e.g. slow-down of the Atlantic meridional overturning thermohaline circulation (AMOC), have been considered as early signs of tipping points in the Earth climate system. We hypothesized that these changes have significantly altered boreal forest growth dynamics in northeastern North America (NA) and northern Europe (NE), two areas geographically adjacent to the North Atlantic Ocean. To test our hypothesis, we investigated tree growth responses to seasonal large-scale oceanic and atmospheric indices (the AMOC, North Atlantic Oscillation (NAO), and Arctic Oscillation (AO)) and climate (temperature and precipitation) from 1950 onwards, both at the regional and local levels. We developed a network of 6876 black spruce (NA) and 14437 Norway spruce (NE) tree-ring width series, extracted from forest inventory databases. Analyses revealed post-1980 shifts from insignificant to significant tree growth responses to summer oceanic and atmospheric dynamics both in NA (negative responses to NAO and AO indices) and NE (positive response to NAO and AMOC indices). The strength and sign of these responses varied, however, through space with stronger responses in western and central boreal Quebec and in central and northern boreal Sweden, and across scales with stronger responses at the regional level than at the local level. Emerging post-1980 associations with North Atlantic Ocean dynamics synchronized with stronger tree growth responses to local seasonal climate, particularly to winter temperatures. Our results suggest that ongoing and future anomalies in oceanic and atmospheric dynamics may impact forest growth and carbon sequestration to a greater extent than previously thought. Cross-scale differences in responses to North Atlantic Ocean dynamics highlight complex interplays in the effects of local climate and ocean-atmosphere dynamics on tree growth processes and advocate for the use of different spatial scales in climate-growth research to better understand factors controlling tree growth.

In the early 2000s, an extensive epidemic of the fungus Gremmeniella abietina (Lagerb.) Morelet occurred in Sweden and caused severe damages to coniferous species. This study aimed to evaluate the impacts of this outbreak on the stem growth of the surviving Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) trees, using the country-wide data on pine and spruce from the Swedish National Forest Inventory and data from stem analyses of pine growth in four infected plots (ntrees = 12) in the Bergslagen area of middle Sweden. Analyses of volume dynamics in the four stands indicated a decade-long negative effect of pathogen infections on volume growth of pine trees. Over 2000–2012, the difference between projected (assuming no infection) and observed volume growth was 10–62%, depending on infection-related crown transparency (varying between less than 20% to above 80%). Height growth of pine in affected stands was reduced by 64–85%, although the reduction did not correlate with levels of crown transparency. The average reduction in basal area increment (BAI) in affected areas country-wide, accumulated over 2000–2006, was ?21% for pine and ?4% for spruce. The use of regular ring-width chronologies, as compared to volume increment chronologies, resulted in underestimation of volume losses by 25–30%.

The growth of high-latitude temperature-limited boreal forest ecosystems is projected to become more constrained by soil water availability with continued warming. The purpose of this study was to document ongoing shifts in tree growth sensitivity to the evolving local climate in unmanaged black spruce (Picea mariana (Miller) B.S.P.) forests of eastern boreal North America (49°N–52°N, 58°W–82°W) using a comparative study of field and modeled data. We investigated growth relationships to climate (gridded monthly data) from observed (50 site tree-ring width chronologies) and simulated growth data (stand-level forest growth model) over 1908–2013. No clear strengthening of moisture control over tree growth in recent decades was detected. Despite climate warming, photosynthesis (main driver of the forest growth model) and xylem production (main driver of radial growth) have remained temperature-limited. Analyses revealed, however, a weakening of the influence of growing season temperature on growth during the mid- to late twentieth century in the observed data, particularly in high-latitude (> 51.5°N) mountainous sites. This shift was absent from simulated data, which resulted in clear model-data desynchronization. Thorough investigations revealed that desynchronization was mostly linked to the quality of climate data, with precipitation data being of particular concern. The scarce network of weather stations over eastern boreal North America (> 51.5°N) affects the accuracy of estimated local climate variability and critically limits our ability to detect climate change effects on high-latitude ecosystems, especially at high altitudinal sites. Climate estimates from remote sensing could help address some of these issues in the future.

The length of the fire cycle is a critical factor affecting the vegetation cover in boreal and temperate regions. However, its responses to climate change remain poorly understood. We reanalyzed data from earlier studies of forest age structures at the landscape level, in order to map the evolution of regional fire cycles across Eastern North American boreal and temperate forests, following the termination of the Little Ice Age (LIA). We demonstrated a well-defined spatial pattern of post-LIA changes in the length of fire cycles toward lower fire activity during the 1800s and 1900s. The western section of Eastern North America (west of 77°W) experienced a decline in fire activity as early as the first half of the 1800s. By contrast, the eastern section showed these declines as late as the early 1900s. During a regionally fire-prone period of the 1910s–1920s, forests in the western section of Eastern boreal North America burned more than forests in the eastern section. The climate appeared to dominate over vegetation composition and human impacts in shaping the geographical pattern of the post-LIA change in fire activity. Changes in the atmospheric circulation patterns following the termination of the LIA, specifically changes in Arctic Oscillation and the strengthening of the Continental Polar Trough, were likely drivers of the regional fire dynamics.

Depuis la fin du 20e siècle, d'importantes baisses de croissance et de nombreuses baisses de corrélation entre croissance et températures estivales ont été observées dans plusieurs régions boréales. L'origine de tels changements, bien que variable d'une région à une autre, a souvent été expliquée par des étés de plus en plus chauds et secs qui forcent les arbres à fermer leurs stomates afin d'éviter de trop importantes pertes d'eau par transpiration. La disponibilité en eau et la respiration semblent donc avoir un contrôle grandissant sur les processus de croissance face au réchauffement climatique. A l'échelle d'un peuplement forestier, l'attribution de ces changements de croissance aux seuls effets des changements climatiques reste cependant difficile car de nombreuses et complexes interactions existent entre le climat, les processus physiologiques de croissance et les facteurs de site (topographie, type de sol, etc.). En revanche, l'apparition de synchronisations de croissance de peuplements géographiquement éloignés présentant des caractéristiques pédologiques et bioclimatiques différentes, traduit de manière plus certaine un forçage climatique de large échelle sur la croissance, tels que ceux induits par les changements climatiques.

Cette thèse a pour objectifs de tester si (1) le réchauffement climatique homogénéise la croissance des arbres et leurs réponses au climat sur de larges échelles et si (2) les variations observées dans les corrélations entre la croissance et les températures estivales émanent d'un contrôle de plus en plus important des facteurs hydrauliques sur la croissance. Contrairement aux études précédentes, elle s'appuie sur une méthode novatrice : l'apparition de téléconnexions dans les patrons de croissance comme indicateur des impacts du réchauffement climatique sur les dynamiques de croissance. Elle compare la croissance des forêts boréales du Québec (50°N-52°N, 58°W-82°W) et de la Suède (59°N-68°N, 12°E-24°E), deux régions séparées par l'Océan Atlantique, et au climat fortement contrôlé par les dynamiques océaniques et climatiques nord-atlantiques. L'ensemble des hypothèses est testé à différentes échelles spatiales et temporelles en comparant des données de croissance provenant à la fois de larges bases de données d'inventaires (103-104 arbres échantillonnés) et de modélisation forestière.

Dans un premier chapitre, cette thèse étudie la fréquence et l'origine climatique d'anomalies de croissance interannuelles et multi-décennales à travers le Québec boréal de 1901 à 2001. L'origine climatique de chaque type d'anomalie est étudiée par analyses d'époques superposées en incluant des facteurs climatiques locaux (températures et précipitation mensuelles) et globaux (Oscillations Nord Atlantique/ONA et Oscillation Arctique/OA). Dans un deuxième chapitre, cette thèse analyse les impacts des dynamiques nord-atlantiques post-1950 sur la croissance des forêts boréales du Québec et de la Suède en étudiant les téléconnexions de croissance entre ces deux régions, et leurs réponses à certains indices climatiques nord-atlantiques (ex. ONA et OA) et aux températures saisonnières. Dans un troisième et dernier chapitre, cette thèse explore si le renforcement prévu des contraintes hydrauliques sur la croissance des forêts boréales du Québec peut être détecté par une instabilité des relations climat-croissance (observée et simulée) entre 1908 et 2013. Les données de croissance observée proviennent de récents inventaires forestiers et les données de croissance simulée sont obtenues à l'aide d'un modèle bioclimatique de croissance des peuplements basé sur des processus physiologiques. La corrélation au cours du temps entre les données observées et simulées, ainsi que leurs réponses respectives au climat mensuel (températures minimum et maximum, précipitations totales), sont étudiées par corrélations mobiles.

La synthèse transversale des trois chapitres indique que l'homogénéisation des dynamiques de croissance des forêts boréales de la région de l'Atlantique Nord au cours du 20e siècle n'a été que partielle. Elle s'est manifestée par deux phénomènes seulement : (1) une augmentation de la fréquence des années à faible croissance entre le début et la fin du siècle au Québec, et (2) une baisse de corrélation entre croissance et températures estivales dans de nombreux peuplements de cette même région au milieu du siècle. Si le premier phénomène semble associé au réchauffement climatique, le deuxième semble émaner de causes diverses telles qu'une respiration plus importante ou une diminution du couvert neigeux hivernal. Face à un réchauffement climatique généralisé (dans le temps et l'espace), l'hétérogénéité des dynamiques suggère une importance grandissante (i) du climat de l'année précédente et de dormance sur la respiration de maintenance, (ii) des facteurs locaux, et particulièrement pédologiques et topographiques, dans la modulation de la croissance et (iii) de la variabilité spatiotemporelle des précipitations. Cette étude montre que la complexité des liens existants entre variabilité climatique et croissance est encore trop importante pour pouvoir prédire avec précision l'impact des changements climatiques futurs sur les écosystèmes forestiers.

Nous avons étudié l’origine climatique des anomalies de croissance des forêts boréales en analysant 895 séries de croissance d’épinette noire (Picea mariana (Mill.) B.S.P.) provenant de 46 sites xériques repartis le long de trois transects latitudinaux dans l’Est Canadien. Nous avons identifié les anomalies de croissance interannuelles (comparaison à l’année précédente) et multi-décennales (comparaison à toutes les années) pour chaque site et transect de 1901 à 2001. Les anomalies de croissance apparaissent principalement à l’échelle du site mais rarement à de plus larges échelles géographiques. Les anomalies positives (interannuelles et multi-décennales) sont fortement associées à des températures basses et des précipitations fortes pendant la saison de croissance de l’année précédente. L’origine climatique des anomalies négatives (interannuelles et multi-décennales) est plus complexe et généralement associée à des anomalies climatiques de l’année en cours. Entre le début et la fin du XXe siècle, seules les anomalies multi-décennales négatives sont devenues plus fréquentes. Nos résultats révèlent l’importance du climat de la saison de croissance précédente dans l’apparition d’anomalies de croissance et suggèrent un lien positif entre le réchauffement climatique et l’augmentation de la fréquence des anomalies multi-décennales négatives. L’augmentation prévue des températures dans les prochaines décennies pourrait davantage accroitre la fréquence des anomalies.

An increasing demand for bioenergy has stimulated interest in harvesting of tree stumps after felling operations. To reduce the amount of soil disturbance, it is recommended to retain stumps in wet areas. We wanted to evaluate if that strategy is beneficial also for saproxylic (wood-living) insects, which will have less habitat to breed in at stump harvest, and therefore also need mitigation. We tested if stumps in wet positions on a clear-cut harbour fewer species and a different assemblage of saproxylic insects than do stumps in dry positions. Insects were reared out from wood pieces taken from 100 stumps (50 Norway spruce, Picea abies and 50 birch, Betula spp.) sampled in pairs: one stump from wet and one from dry soil in each pair. In the lab 2201 individuals representing 49 beetle and 6 moth species were encountered. Fewer species were found in spruce stumps on wet soil than on dry soil, both when measured per stump or as an accumulated value over all stumps within the categories. No difference was detected between the number of species found in wet and dry birch stumps. However, three beetle species that live mainly in birch were more common in dry stumps than in wet. No species showed an association with wet stumps. We conclude that stumps in wet positions form an inferior habitat to stumps in dry positions, and that this should be considered when making recommendations concerning the harvesting of stumps for bioenergy.