FOREST DISEQUILIBRIUM CAUSED BY RAPID LITTLE ICE-AGE COOLING

GLOBAL climatic change may alter species' ranges as well as restructuring ecosystems1-3. Models simulating forest growth predict that the area covered by different forest types may be affected2, which may in turn further affect climate3. In the mixed forests of southern Ontario, pollen analyses have demonstrated that after AD 1400, Fagus (beech), the formerly dominant warmth-loving species, was replaced first by oak (Quercus) and subsequently by pine (Pinus strobus). Although these changes had been attributed to aboriginal forest clearance4-6, they have also been seen in areas unaffected by aboriginal farming, and are now thought to reflect Little Ice Age cooling7. Although modelling suggests that some forests may take several centuries to reach equilibrium after a climatic change8,9, a real forest showing this behaviour has not previously been identified. Here we model the Little Ice Age by a 2-degrees-C decrease in mean annual temperature from AD 1200 to 1850, and show that the changes predicted by a forest simulator derived from FORET10 match those seen in southern Ontario. These forests thus appear to have remained in disequilibrium with the prevailing climate for more than 650 years.