PARENT SESSION
Symposium 15: Severe Insect Outbreaks in North American Forests: Recent Trends, Long-Term Recurrence, and the Role of Climate
Organized by: D Gavin and M Ayers
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Chatham Ballroom C.

Climate change, insect outbreaks and the mid-Holocene hemlock decline.

Calcote, Randy¹, Shuman, Bryan¹, ¹ University of Minnesota, Minneapolis, MN

ABSTRACT- Fossil pollen percentages of eastern hemlock (Tsuga canadensis) decreased dramatically about 5400 cal yrs BP and did not recover significantly for 1000-2000 years. The traditional interpretation of the decline is that hemlock populations were decimated by insect or pathogen outbreaks. Climatic change was not considered to be a cause. Several independent lines of paleoclimatic data now suggest that climatic changes in eastern North America were associated with the hemlock decline. Lake level studies using transects of sediment cores across lake basins indicate low lake levels and dry conditions in New England, Quebec, Ontario, and Michigan from about 5300 to 3200 cal yr BP. Modern analog climate reconstructions, based on fossil pollen assemblages near hemlock's mid-Holocene western range limit, suggest decreased winter temperatures at the time of the decline, probably in combination with periods of drought. Sites just outside the range of hemlock, which should not be affected by insect or pathogens attacking hemlocks, also indicate lower winter temperatures during the period of the decline. Stable isotope studies at Crooked Pond, Massachusetts, and at Crawford Lake, Ontario, also demonstrate that the interval of the decline differed from previous and later intervals. The combination of temperature and moisture conditions is likely to have contributed to insect outbreaks and the hemlock decline either by stressing the tree populations and/or by directly favoring insect/pathogen populations. The mid-Holocene hemlock decline is the classic paleoecological example of a probable insect outbreak and it's effect on forests, and improvements in our understanding of the paleo-record suggest that the interactions of multiple factors caused the decline. The inevitability of future climate changes makes it increasingly important to understand the combined effects of interactions.

Key words: mid-Holocene, paleoecology, hemlock decline, climate change