The Summer Drought Related Hemlock ( Tsuga canadensis ) Decline in Eastern North America 5 , 700 to 5 , 100 Years Ago

High resolution paleoecological analyses from Shepherd Lake, Ontario, Canada, show that 10 to 100 year lake level fluctuations due to climatic change were responsible for alterations in the aquatic biodiversity 5,700 to 5,100 years ago. Thermophilic aquatics such as the Bushy pondweed Najas flexilis, charophyte algae and aquatic invertebrates indicate water level fluctuations of several meters, which were likely linked to a warm, dry summer climate. Pollen analysis and radiocarbon dating at Wilcox Lake shows a hemlock decline at 5800 years ago that lasted 1,000 years. Multiple regression analysis indicates that the decline coincides with a drop in mean annual precipitation from 830 to 700 mm. From fossil pollen analysis the impact of these droughts on the upland forest is poorly visible, except for the hemlock decline during the first half of the 6th millennium ago. The two-phased reduction of hemlock from 30% tree cover to less than 5% within centuries is found in large parts of northeastern America. Our results imply drought-weakened hemlock trees and stands, and that drought may have triggered local insect calamities, such as hemlock looper attack. However, this also implies that climatic change and not a pathogen-pest attack was responsible for the synchronous decline of hemlock all over its range in eastern North America. Reconstructing and understanding the hemlock decline is, therefore, of interest to the public and to ecosystem managers when anticipating the effect of pathogen-pest attacks combined with climatic change.