Trees with ring-porous wood are limited to the higher latitudes and form an important component of the vegetation of North America. Analysis of the occurrence of trees with this wood type in North America east of the Rockies shows that they make up an increasing percentage of the upland tree flora as precipitation decreases. The northern limit of ring-porous trees is probably related to winter temperature extremes and a mechanism of avoiding frost damage based on supercooling of cellular water. Percent of ring-porous species has the potential for use in the reconstruction of paleoclimates. As an example, the calculated value for the Toronto Don Beds flora, which dates from the last (Sangamon) interglacial period, is only slightly greater than that of the present-day flora and thus suggests that the interglacial climate in this region was much the same or only somewhat different from today's. The differences were probably related to lower winter temperatures or lower frequency of winter temperature extremes. The climatic requirements of zonal vegetation and individual species indicate that summer temperatures were not higher than at present. The extralocal species must be evaluated carefully since nonclimatic considerations may be important in explaining their presence. © 1989.
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