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Morphological differentiation of Betula (birch) pollen in northwest North America and its palaeoecological application

Department of S. Goodwin Ave, Plant Biology, 265 Morrill Hall, University of Illinois, 505 Urbana, IL 61801, USA; bclegg{at}life.uiuc.edu;fshu@life.uiuc.edu

Willy Tinner

Department of S. Goodwin Ave, Plant Biology, 265 Morrill Hall, University of Illinois, 505 Urbana, IL 61801, USA; Institut für Pflanzenwissenschaften, Universität Bern, Altenbergrain 21, CH-3013 Bern, Switzerland

Daniel G. Gavin

Department of S. Goodwin Ave, Plant Biology, 265 Morrill Hall, University of Illinois, 505 Urbana, IL 61801, USA

Feng Sheng Hu

Department of S. Goodwin Ave, Plant Biology, 265 Morrill Hall, University of Illinois, 505 Urbana, IL 61801, USA; bclegg{at}life.uiuc.edu;fshu@life.uiuc.edu

Lake sediments from arcto-boreal regions commonly contain abundant Betula pollen. However, palaeoenvironmental interpretations of Betula pollen are often ambiguous because of the lack of reliable morphological features to distinguish among ecologically distinct Betula species in western North America. We measured the grain diameters and pore depths of pollen from three tree-birch species (B. papyrifera, B. kenaica and B. neoalaskana) and two shrub-birch species (B. glandulosa and B. nana), and calculated the ratio of grain diameter to pore depth (D/P ratio). No statistical difference exists in all three parameters between the shrub-birch species or between two of the tree-birch species (B. kenaica and B. papyrifera), and B. neoalaskana is intermediate between the shrub-birch and the other two tree-birch species. However, mean pore depth is significantly larger for the tree species than for the shrub species. In contrast, mean grain diameter cannot distinguish tree and shrub species. Mean D/P ratio separates tree and shrub species less clearly than pore depth, but this ratio can be used for verification. The threshold for distinguishing pollen of tree versus shrub birch lies at 2.55 µm and 8.30 for pore depth and D/P ratio, respectively. We'applied these thresholds to the analysis of Betula pollen in an Alaskan lake-sediment core spanning the past 800 years. Results show that shrub birch increased markedly at the expense of tree birch during the‘Little Ice Age’; this patten is not discernible in the profile of total birch pollen.

Key Words: Betula • birch • pollen • western North America • palaeoecology • ‘Little Ice Age’ • Grizzly Lake • Alaska

The Holocene, Vol. 15, No. 2, 229-237 (2005)
DOI: 10.1191/0959683605hl788rp


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