Two key observations emerging from pollen analysis in Europe and North America in the early 1960s were that some past pollen assemblages did not appear to match any known modern assemblages, and that plant taxa that are closely associated in communities today seemed to arrive at individual sites at different times from each other after glacial retreat. These observations led many paleoecologists to suggest that "plants react individualistically to their environment (which includes other plants) and not in collective units such as communities and floristic elements" (Cushing, 1965).
Palynologists being cautious acknowledged that assemblages lacking apparent modern analogs might arise from an incompletely representative set of modern assemblages, mixtures of redeposited pollen of different ages, artifacts of percentage calculations as assemblages changed, or distorting effects of pollen productivity and dispersal. They completed studies to confront these issues and rendered inferences from pollen data more robust.
While these methodological issues were addressed, two contrasting frameworks developed to explain the no-analog assemblages and staggered arrival times. The first invoked delayed migration, proposing that characteristic taxa of, for example, the (modern) deciduous forest migrated northward from glacial-age centers at different rates. Differential migration rates, including migration lags, might be expected because of the contrasting size, mass, and dispersal-mechanisms of seeds of different deciduous taxa.
Results/Conclusions
Migration lags were cited as evidence that vegetation was out of equilibrium with climate for much of the Holocene, and even today. Past no-analog plant communities were viewed as transient assemblages of species populations that had not had sufficient time to sort themselves out biogeographically and ecologically after postglacial warming.
An alternative framework postulated that plant taxa differed among themselves in their relationships with climate, a multivariate entity, and that changes in past climates involved changing combinations of climate variables including seasonality. Thus, different multivariate climate realizations would lead to different vegetation realizations, including past communities unlike today’s (under past climates unlike any today), and species would migrate at different rates according to the rates and patterns at which their climatic ‘envelopes’ shifted geographically in the Holocene.
These contrasting explanations, together with the methodological uncertainties of a developing field, led to fruitful tensions and healthy controversies, yielding a science of paleoecology that was more sophisticated, ecologically and climatologically, and more robust, empirically and conceptually. Most importantly, they revealed issues central to global change ecology, including community turnover, novel and disappearing climates, novel communities, and lagged ecological responses.