A numerical analysis of Holocene forest and prairie vegetation in central Minnesota

Fossil-pollen samples from Billy's Lake in central Minnesota are compared with 105 presettlement pollen samples from Minnesota and adjacent states by ordinating both sets of data with detrended correspondence analysis. The pollen record from Billy's Lake reveals that the vegetation changed from pine forest (10 000-8020 BP), to prairie (8020-3400 BP), to deciduous forest (3400-1000 BP), and finally back to pine forest (1000 BP-present). The numerical comparison indicates that most of the fossil samples have analogs in the presettlement pollen assemblages from Minnesota. Fossil samples from the early Holocene pine forest/prairie transition have no analogs because a belt of deciduous forest presently occurs between pine forests and prairie. The early prairie also has no analogs in Minnesota because of abundant Artemisia, which today is characteristic of prairie farther west. This Artemisia-rich prairie may indicate that the climatic gradient across the region was steeper in the early Holocene than at present. The rate of polynological change is assessed by smoothing the Billy's Lake pollen curve through the ordination. Change is continual throughout the last 10 000 yr, but is most rapid in the early and late Holocene and least rapid 7000-6000 BP, when prairie occurred in the region. Inasmuch as pollen-assemblage change reveals vegetational change, these results show that, in central Minnesota, vegetational constancy has been low for at least 9000 of the past 10 000 yr.