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Harvard Forest REU Symposium Abstract 2011
- Title: History of New England Vegetation: Pollen Morphology of Eastern Hemlock in New England Lake-Sediment Records
- Author: Lindsay T Day (Emerson College)
- Abstract:
The mid-Holocene decline of eastern hemlock (Tsuga canadensis) is one of the most
studied yet least understood events in the vegetation history of Eastern North America. The
widespread collapse of hemlock 5500 years ago originally was interpreted as disease-caused
mortality followed by the evolution of resistance and the recovery of hemlock around 3000 years
ago. Subsequent research on the event suggests that major droughts initiated and sustained the
decline, but questions remain about evolutionary changes during the mid-Holocene. We explored
the question of evolutionary change by analyzing the morphology of fossil hemlock pollen. We
measured the diameters of hemlock pollen grains from lake cores in two sites in New England,
Little Pond in Royalston, MA and Knob Hill Pond in Marshfield,VT. We found that pollen
grains from within the hemlock decline were significantly smaller than those from before and
after. The trends in pollen size parallel the dramatic decline and recovery of pollen percentages
and water levels in sites across the Northeast. These data are not consistent with unidirectional
evolutionary change but instead suggest that the changes in pollen morphology are linked to
climate. The fluctuation in pollen size could illustrate population dynamics in which the
subpopulation of hemlock trees with smaller grains was favored during the interval of dry
conditions. Alternatively, the pattern may represent the effect of drought on individual hemlock
trees. Under stressful conditions, trees may devote fewer resources to reproduction, including the
growth of pollen grains. Taken with other evidence from the region, our data lend further support
to climate as the driver of the mid-Holocene decline. - Research Category: Historical and Retrospective Studies
