The paleoecological record has revealed that species respond individualistically to climate change; and as climate changes, species assort into different communities in terms of qualitative and quantitative composition. In North America, the first recognized, most studied, and best known example of past “no-analog” novel vegetation, based on pollen assemblages, is from the late-glacial period in the Great Lakes region. Nevertheless, pollen data from the western tall-grass prairie region indicate mid-Holocene assemblages quite unlike any in the recent past. These data show highly varying percentages of the four main grassland pollen types Poaceae, Artemisia, Ambrosia-type, and Amaranthaceae. However, these novel assemblages have received little attention, probably because most analog studies removed Ambrosia-type because it increased greatly after European settlement, removed Poaceae because of occasional over-representation by aquatic grasses, and lumped Artemisiaand Amaranthaceae into “prairie forbs.” Thus, precluding the recognition of novel grassland assemblages. Moreover, absence of pollen data from drier mixed-grass prairies farther west precluded knowing whether mid-Holocene assemblages from the tall-grass prairie had late Holocene mixed-grass analogs. However, newly acquired high-resolution records from the northern Great Plains (NGP) now permit an evaluation of novel grassland ecosystems.
Results/Conclusions
A new high-resolution pollen record from Kettle Lake, in the mixed-grass prairie of northwestern North Dakota, shows high-frequency, high amplitude variations in grassland pollen types, similar to that seen in the tall-grass prairies of the eastern Dakotas. In particular, during the mid-Holocene (~9200-4400 cal yr BP), Ambrosia-type repeatedly spikes to percentages higher than those occurring anywhere in the millennium prior to European settlement. Today, Ambrosia is most abundant in the Midwest where summer and mean annual precipitation are higher than the NGP. Thus, paradoxically, during the mid-Holocene, high abundances of Ambrosia occurred under a climate regime much drier on average than today. Charcoal and mineralogy data from Kettle Lake indicate that the high amplitude variability in pollen is in response to high-amplitude, high-frequency drought. Drought is a disturbance factor, under which Ambrosia can thrive during wetter parts of the drought cycle. Thus, important variables in the fundamental niche of Ambrosia are mean summer and annual precipitation and drought frequency. Ambrosia thrives under a relatively moist climate with artificial disturbance or under a climate with highly variable moisture with alternating droughts and wet summers. Novel mid-Holocene grasslands occurred under a climate regime with a higher amplitude drought cycle than today.