VIEWPOINT - PLANT COMMUNITY THRESHOLDS, MULTIPLE STEADY-STATES, AND MULTIPLE SUCCESSIONAL PATHWAYS - LEGACY OF THE QUATERNARY

The climate cycles of the 2 million years of the Quaternary were a major force in the evolution of plant response to change. Quaternary climate has been primarily glacial with interglacials such as the current Holocene a minor component. Plant species responded individually to climate changes and, consequently, species composition has continually changed. The legacy of Quaternary climate change is that plant communities are far less stable than they appear to be from our perspective. They are unique at each location, difficult to define, and communities that are relics from a previous environment can be sensitive to small or transient environmental changes. Plant communities are variable both in space and time. Many ecological principles and concepts, and ecosystem pardigms derived from them, require revision to incorporate this variation. The concepts of habitat type and condition and trend, for example, do not reflect dynamic vegetation response to changes in climate. Our knowledge is presently insufficient to adequately describe interactions between ecosystems and changing climate, but the patterns of vegetation response to environmental changes of the past may provide important information on vegetation response to present and future climate change. The concepts of thresholds, multiple steady states, and multiple successional pathways are helpful in understanding the dynamic interrelationships between vegetation and environmental changes.