LANDSCAPE RESPONSES TO ENVIRONMENTAL CHANGE: HERE TODAY, GONE YESTERDAY, GONE TOMORROW?

Landscape responses to environmental change: here today, gone yesterday, gone tomorrow?

John W. Williams and Stephen T. Jackson

Modern analog analysis, the comparison of Quaternary fossil assemblages with modern assemblages, has conclusively shown that many fossil assemblages lack close modern analogs, suggesting widespread reorganizations of biotic communities in response to environmental change. Pollen assemblages lacking modern analogs are well documented for the late-glacial period in eastern North America and other regions. These pollen assemblages comprise high abundances of selected temperate deciduous taxa in association with boreal and herbaceous taxa, as well as the low abundances for arboreal taxa that today are widespread (e.g. Pinus, Alnus, Betula). It is not yet known definitively whether these ‘no-analog’ pollen assemblages indicate the existence of anomalous mixtures of species at the stand, landscape, or regional scale. Macrofossil evidence for the presence of temperate deciduous tree taxa in the late-glacial upper Midwest suggests that the fossil pollen assemblages are unlikely to be artifacts of long-distance pollen transport. Simulated climates for the late-glacial period also lack modern analogs owing to increased seasonality of insolation, lowered atmospheric [CO2], and persistent ice sheets. A doubled-CO2 simulation from the same model series suggests that future climates may also have no modern analog. Most pollen assemblages from the last glacial maximum have modern analogs, but macrofossil and other evidence suggest that the vegetation may have lacked modern analogs, owing to unique climate realizations and perhaps direct physiological effects of lowered CO2. Better understanding of the nature of past no-analog vegetation, and the underlying causes, will address important issues in ecology and evolutionary biology and help anticipate biotic responses to the no-analog greenhouse world of the near-future.