Jud, Nathan A. , Wing, Scott L. .
The Roots of Alnus in North America.
The genus Alnus consists of about 35 species of actinorhyzal trees and shrubs distributed throughout the Northern Hemisphere and into montane tropical regions. Three subgenera are recognized on the basis of overall habit, floral morphology, and ITS sequence data: Alnobetula, Clethropsis and Alnus. We have found characters of the leaves, reproductive structures, and wood that can be used to identify fossils to subgenera, allowing the extensive fossil record of the genus to reveal more about its biogeography. The modern distribution of the subgenera in North America suggests strong control by climate, especially winter temperatures. Species of the cold tolerant Alnobetula are restricted to Canada, Alaska, and the Northern US, where Cold Month Mean (CMM) is <-15 Degrees C. Subgenus Alnus ranges from Canada to the Bolivian Andes, where CMM is >-10 Degrees C. Subgenus Clethropsis is represented by a single species with small populations in Delaware, Georgia, and Oklahoma, and is probably too relictual to express climate tolerances well. Alnus has long been thought to originate in Asia, currently center of diversity for the subgenera. Additionally, putatively Paleocene macrofossils occur in NE Asia. Although Alnus-like pollen is common in the Late Cretaceous, the earliest unequivocal macrofossils come from the early Eocene of the Bighorn Basin, WY. These fossils belong to subgenus Alnus, and for the remainder of the globally warm Paleogene most fossils from North America can be assigned to subgenus Alnus. The appearance of subgenus Alnus in North America at the close of the Paleocene Eocene Thermal Maximum is consistent with the hypothesis that the genus radiated in the Old World before arriving in North America and parallels the pattern seen in several mammalian taxa, suggesting migration across high latitude land bridges during a period of globally warm climate.
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1 - National Meuseum of Natural History, Smithsonian Institution, Department of Paleobiology, PO Box 37012, MRC 121, Washington, DC, 20013-7012, USA
Presentation Type: Oral Paper:Papers for BSA Sections
Location: Superior A/Cliff Lodge - Level C
Date: Monday, July 27th, 2009
Time: 11:15 AM