Thursday, August 6, 2009

PS 67-116: Establishment of a disjunct population of hemlock (Tsuga canadensis) by long-distance seed dispersal

Randy Calcote, University of Minnesota, Sara C. Hotchkiss, University of Wisconsin, and Elizabeth A. Lynch, Luther College.

Background/Question/Methods Migration rates of tree species would be maximized by long-distance seed dispersal and establishment of disjunct populations well ahead of the continuous range of the species. Long-distance seed dispersal is difficult to document, however, because of the infrequent and unpredictable nature of the event. The modern range limit of hemlock has many disjunct populations up to 150 km from the species continuous range, suggesting establishment by long-distance dispersal. We use fossil pollen data from sediments of two AMS radiocarbon dated cores and several less well dated sites as indicators of the abundance of eastern hemlock (Tsuga canadensis) over the last 2000 years at sites that were at least 20km outside the continuous range limit of hemlock in Public Land Survey (PLS) data. All the study sites are small, deep lakes (~10ha, 10m deep) on sandy soil in northwestern Wisconsin.

Results/Conclusions

Pine Lake has 1-2% hemlock pollen throughout most of the last thousand years and a peak of 3-6% hemlock pollen from 850-650 years ago. One to two percent hemlock pollen is present throughout the last 1000 years in Sawdust Lake, 10 km southwest of Pine Lake. Other cores from nearby sites outside the continuous range of hemlock seldom exceed 1% hemlock pollen and none exceed 2%. Analogs to PLS-age pollen samples of 30 other sites on sandy soils in the region shift from jack-red pine dominated vegetation to red pine-birch vegetation in Pine Lake at about 850 cal yr BP. The peak of hemlock pollen at Pine Lake is well above the regional background level of 0-2% represented in Sawdust and other lakes. We interpret this peak of hemlock pollen at Pine Lake as evidence for the establishment of a disjunct population of hemlock by long-distance seed dispersal. After ~600 cal. yr BP hemlock pollen percentages at Pine Lake decrease to about the level found at Sawdust Lake. This site provides rare documentation of the establishment of a disjunct population 10s of km from the range limit of hemlock at the time.


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