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


References from Eric Grimm
5/18/00

Weaver, J.E. 1954. North American Prairie. Johnsen Publishing Company,
Lincoln, Nebraska.

Weaver, J.E., and T.J. Fitzpatrick. 1934. The prairie. Ecological
Monographs 4:109-295. [This has been reprinted as a book. The work was done
before the drought so provided the very fortunate baseline data.]

Weaver, J.E., L.A. Stoddart, and W. Noll. 1935. Response of the prairie to
the great drought of 1934. Ecology 16:612-629.

Weaver, J.E., and F.W. Albertson. 1936. Effects of the great drought on the
prairies of Iowa, Nebraska, and Kansas. Ecology 17:567-639.

Weaver, J.E., and F.W. Albertson. 1943. Resurvey of grasses, forbs, and
underground plant parts at the end of the great drought. Ecological
Monographs 13:63-117.

Albertson, F.W., and J.E. Weaver. 1944. Nature and degree of recovery of
grassland from the great drought of 1933 to 1940. Ecological Monographs
14:391-479.

Weaver, J.E., and I.M. Mueller. 1942. Role of seedlings in revovery of
midwestern ranges from drought. Ecology 23:275-294.

Albertson, F.W. 1965. Vegetation changes during a 30-yr period in grassland
communities near Hays, Kansas. Ecology 46:714-720.






To add to Eric Grimm's reply, Ambrosia is abundant in the middle to
late-middle Holocene of much of the central Midwest.  Three species are in
the area today and all were probably represented in the fossil record to
some degree.  A. trifida, giant ragweed, grows in midwestern prairies,
especially along waterways, usually where there is some disturbance. Its
fruits are distinctive, and it is definitely widespread in the fossil
record.  A. artemisiifolia, common ragweed, also grows in disturbed areas,
but it is common even on small disturbances like gopher mounds in native
prairies, at least in Iowa. These first two species are native annual weeds.
A. psilostachya is a perennial ragweed that is found to some degree in
native undisturbed prairies.  It is difficult (impossible?) to distinguish
these latter two as fossils, but one or both are also widespread.

3/30/00
Dick Baker
dick-baker@uiowa.edu