Cell formation in growth rings of balsam fir (Abies balsamea (L.) Mill) in the boreal forest was studied to describe the timing of ring formation and the development patterns of earlywood and latewood. Wood micro-cores were extracted during the growing season from 1998 to 2000. The micro-cores were stained with cresyl fast violet to facilitate counting the number of cells in the radial enlargement, wall thickening phases, and mature cell phases. The periods required to complete these various phases were then estimated. Variations in the beginning of the growing season (May 7 – June 7), the earlywood–latewood transition (July 2 – July 19), and the end of the growing season ( August 20 – September 20) were observed. Short cell enlargement durations of less than a week for earlywood and 5–10 days for latewood were observed. Time required for cell wall thickening was about 20 days for earlywood and longer than 10–15 days for latewood. A certain flexibility was observed in the ring formation patterns and in the cell development rate, providing an advantage in the boreal forest where optimal growth conditions change from year to year. These findings on the spatial and temporal patterns of ring development may be useful for understanding tree relationships with climate or other environmental parameters.

Significance The cooling effect on the Earth's climate system of sulfate aerosols injected into the stratosphere by large volcanic eruptions remains a topic of debate. While some simulation and field data show that these effects are short-term (less than about 10 years), other evidence suggests that large and successive eruptions can lead to the onset of cooling episodes that can persist over several decades when sustained by consequent sea ice/ocean feedbacks. Here, we present a new network of millennial tree-ring chronologies suitable for temperature reconstructions from northeastern North America where no similar records are available, and we show that during the last millennium, persistent shifts toward lower average temperatures in this region coincide with series of large eruptions.

Tracheid cell number, cell diameter, and cell-wall thickness of black spruce (Picea mariana (Mill.) BSP) tree rings at the tree line (northern Quebec, Canada) were measured using image analysis. The densitometric data from the same samples were used to evaluate the features of image analysis. The anatomical data were correlated with summer temperature variables, including: means of pentad temperature (five consecutive days), growing season (MaySeptember), sum of degree-days, and number of frost-free days. Our results suggest that the main cause of the pale appearance of light rings is thinner latewood cell-wall thickness. Some latewood variables are strongly correlated with corresponding indices for ring cell number and diameter, and ring cell wall thickness. Anatomical ring cell number and the sum of cell diameters (ring widths) were correlated to tree-ring width parameters derived from densitometry. Ring cell number and annual sum of cell diameter also showed very similar trends with both chronologies, suggesting that ring-width length may depend on the number of cells within a ring. Ratio diagram of double cell-wall thickness to cell radial diameter showed similar trend to wood density profile. There is a statistically significant correlation between maximum density and the highest annual ratio between cell wall thickness and lumen diameter. Cell-wall thickness was significantly correlated to maximum density, and both were significantly correlated with summer temperature variables. Our results suggest that wood anatomy may be used as a substitute to densitometry for climate reconstruction as densitometric data require expensive equipments. Also the anatomical method allows the recording of intra-annual information for dendroecological purposes.Résumé : Le nombre de trachéides, le diamètre des cellules et l'épaisseur des parois cellulaires des cernes annuels de l'épinette noire (Picea mariana (Mill.) BSP) située à la limite des arbres (nord du Québec, Canada) ont été mesurés par analyse d'images. Les données densitométriques provenant des mêmes échantillons ont servi à déterminer les caractéristiques pour l'analyse d'images. Les données anatomiques ont été corrélées aux températures estivales (moyenne des températures pour 5 jours consécutifs, saison de croissance (mai à septembre), somme des degrés-jours et nombre de jours sans gel). Nos résultats suggèrent que les cernes pâles sont causés par un faible épaississement de la paroi des cellules de bois final. Certaines variables du bois final sont fortement corrélées avec les indices correspondants du nombre de cellules, de la largeur et de l'épaisseur des parois cellulaires du cerne entier. Le nombre de cellules et la somme du diamètre des cellules (largeur des cernes) sont étroitement liés aux paramètres de la largeur des cernes obtenus par densitométrie. Le nombre de cellules et la largeur des cernes montrent aussi des tendances analogues dans les deux chronologies utilisées, ce qui suggère que l'épaisseur d'un cerne varie en fonction du nombre de cellules qu...

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The forest age mosaic is a fundamental attribute of the North American boreal forest. Given that fires are generally lethal to trees, the time since last fire largely determines the composition and structure of forest stands and landscapes. Although the spatiotemporal dynamics of such mosaics has long been assumed to be random under the overwhelming influence of severe fire weather, no long‐term reconstruction of mosaic dynamics has been performed from direct field evidence. In this study, we use fire length as a proxy for fire extent across the fire‐prone eastern Canadian taiga and systematically reconstruct the spatiotemporal variability of fire extent and fire intervals, as well as the resulting forest age along a 340‐km transect for the 1840–2013 time period. Our results indicate an extremely active fire regime over the last two centuries, with an overall burn rate of 2.1% of the land area yr−1, mainly triggered by seasonal anomalies of high temperature and severe drought. However, the rejuvenation of the age mosaic was strongly patterned in space and time due to the intrinsically lower burn rates in wetland‐dominated areas and, more importantly, to the much‐reduced likelihood of burning of stands up to 50 years postfire. An extremely high burn rate of ~5% yr−1 would have characterized our study region during the last century in the absence of such fuel age effect. Although recent burn rates and fire sizes are within their range of variability of the last 175 years, a particularly severe weather event allowed a 2013 fire to spread across a large fire refuge, thus shifting the abundance of mature and old forest to a historic low. These results provide reference conditions to evaluate the significance and predict the spatiotemporal dynamics and impacts of the currently strengthening fire activity in the North American boreal forest.

Dendroecological analysis of black spruces (Picea mariana (Mill.) BSP.) near the shore of Clearwater Lake in the eastern Canadian subarctic yielded information on water level over the past three centuries. Tree positions, growth froms, layering, tree ring patterns, and ice scars provide direct evidence of a major rise in lake level. From the 17th century to the present, trees on the lake shore were progressively submerged by the increasing water level, which reached a maximum in this century. Higher levels began around the mid-18th century when shoreline spruce trees started to lean because extensive wave erosion caused landward shore displacement. High mortality of leaning spruce and abundant ice scar formation occurred during this century. The rise in water level in inferred to have been caused by snowier winters occurring parallel to climatic warming.

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