Ambrosia is an important weed, especially in the Midwest, but also farther east. The "Ambrosia-rise" is indicative of European settlement throughout this region. Ambrosia becomes established on disturbed ground, especially plowed areas. Small-grain farming (as opposed to corn and beans) is especially favorable for Ambrosia, because it reaches maturity and flowers in late summer, tyically after small grains are harvested. In Midwestern sites we often see a decline in Ambrosia post-WWII with the decrease in small-grain acerage and the increase in corn and beans, which are harvested later. Herbicides probably also played a role. Ambrosia is still plenty abundant in the Midwest, however. Ambrosia is abundant in areas that were cleared of forest and then converted to agricultural lands, but not particularly after logging or fires. Today, Ambrosia is not the most important weed in the Great Plains, at least north of the Sandhills. Chenopods, especially the introduced Salsola and Kochia, are the important weeds in the drier Great Plains, and the Salsola-rise is the important indicator of European settlement. However, in the mid-Holocene, Ambrosia was extremely abundant in the Dakotas, and farther east as well. Ambrosia percentages fluctuated greatly in the mid-Holocene, and we have evidence that high Ambrosia is indicative of droughts. Studies by Weaver during the 1930's drought support this interpretation. 3/30/00 Eric Grimm grimm@MUSEUM.STATE.IL.US
The initial reference may be:
Ogden, J.G. III 1966.
Forest history of Ohio. I. Radiocarbon dates and pollen stratigraphy of Silver Lake, Logan County, Ohio. Ohio J Sci 66: 387-
Wright, H.E.Jr. 1971.
Late Quaternary Vegetational History of North America. Pp. 425-464 IN: K.K. Turekian (ed.) The Late Cenozoic Glacial Ages. Yale Univ Press.
this was followed by McAndrews' and Waddington's papers. etc.
Baker, R.G., Schwert, D.P., Bettis, E.A.III, Chumbley, C.A. 1993.
Impact of Euro-American settlement on a riparian landscape in northwest Iowa, Midwestern USA: an integrated approach based on historical evidence, floodplain sediments, fossil pollen, plant macrofossils and insects. Holocene 3: 314-323.
Bradbury, J.P. and Waddington, J.C.B. 1973.
The impact of European settlement on Shagawa Lake, northeastern Minnesota, U.S.A. pp. 289-308 IN: Birks, J.H.B. and West, R.G. (Eds) Quaternary Plant Ecology. John Wiley and Sons, NY.
Brugham, R.B. 1978.
Pollen indicators of land-use change in southern Connecticut. Quaternary Research 9:349-362.
Grimm, E.C. 1983.
Chronology and dynamics of vegetation change in the prairie-woodland region of southern Minnesota, U.S.A. New Phytologist 93: 311-350.
McAndrews, J.H. 1968.
Pollen evidence for the prehistoric development of the “Big Woods” in Minnesota, U.S.A. Review Palaeobotany Palynology 7: 201-211.
McAndrews, J.H. 1988.
Human disturbance of North American forests and grasslands: The fossil pollen record. Pp. 673-697 IN: Huntly, B & Webb, T. III (eds.) Vegetation History. Kluwer Academic.
McAndrews, J.H. and Boyko-Diakonow, M.. 1987.
Pollen analysis of varved sediment at Crawford Lake, Ontario: evidence of Indian and European farming. IN: Fulton, R.J. and Heiginbottom,. J.A. (eds.) Quaternary geology of Canada and Greenland. Geol. Surv. Canada.
Strong, W.L. 1977.
Pre- and postsettlement palynology of southern Alberta. Review Palaeobotany Palynology 23: 383-396.
Van Zant, K. 1979.
Late-glacial and postglacial pollen and plant macrofossils from Lake West Okoboji, Northwestern Iowa. Quaternary Research 12: 358-380.
Waddington, J.C.B. 1969.
A stratigraphic record of pollen influx to a like in the Big Woods of Minnesota. Geol. Soc. Amer. Special Pap. 124: 263-282.
Webb, T. III 1973.
A comparison of modern and presettlement pollen from southern Michigan (U.S.A.). Review Palaeobotany Palynology 16: 137-156.