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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. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of surface climate and atmospheric circulation on radial growth of eight boreal tree species growing in the Duck Mountain Provincial Forest, Manitoba, Canada. Tree-ring residual chronologies were built, transformed into principal components (PCs), and analysed through correlation and response functions to reveal their associations to climate (temperature, precipitation, and drought data for the period 1912–1999, as well as local geopotential height data for the period 1948–1999). Geopotential height correlation and composite charts for the Northern Hemisphere were also constructed. Correlation and response function coefficients indicated that radial growth of all species was negatively affected by temperature-induced drought stresses from the summers previous and current to ring formation. The summer drought stress alone explained nearly 28% of the variance in PC1. Warm spring temperature was also a positive factor for Pinus banksiana Lamb. and Picea glauca (Moench) Voss, but a negative one for all hardwoods. Analyses performed on geopotential height highlighted the importance of the Northern Hemispheric atmospheric circulation in the species' response to climate. The variability within the 500-hPa level over southern Manitoba explained 39% and 58% of the variability in PC1 and PC2, respectively. The relationships were highly significant with the middle and high troposphere during spring and late summer (determinant factor for growing season length) and with the troposphere and stratosphere during summer. The sensitivity of tree growth to atmospheric circulation exceeded the synoptic scale, with a response associated with yearly variations in the amplitude of the mid-tropospheric longwaves. [ABSTRACT FROM AUTHOR]