P. M. Anderson, P. J. Bartlein, L. B. Brubaker, K. Gajewski, J. C. Ritchie, Vegetation-Pollen-Climate Relationships for the Arcto-Boreal Region of North America and Greenland, Journal of Biogeography, Vol. 18, No. 5 (Sep., 1991), pp. 565-582
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Anderson et al. (1991) studied this question.