Tree growth at the northern limit of the range of boreal forests is primarily limited by temperature-related factors. Thus the position of this range limit, and the growth rates of trees along the northern forest border, may undergo significant change if predictions of enhanced greenhouse warming at northern latitudes are realized. In this paper we evaluate tree ring width and maximum latewood density chronologies of white spruce for three temperature-sensitive tree line sites in northern North America: in the Brooks Range, Alaska, the Franklin Mountains, Northwest Territories, and Churchill, Manitoba. The ring width data, which more strongly integrate low-frequency temperature trends than the density series, show overall enhanced growth and inferred warming during the period of anthropogenic increase in greenhouse gases. The recent growth at these sites equals or exceeds that which has occurred during earlier centuries of more clearly natural climate variability. When the ring width and density variations are estimated using temperature and precipitation data in principal components regession analysis, no substantial residual trends are detected which might require CO2 or other nutrient fertilization as an additional explanation for recent growth changes.
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