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 maj or force in the evolution of plant response to change. Quaternary clim ate has been primarily glacial with interglacials such as the current Holocene a minor component. Plant species responded individually to cl imate changes and, consequently, species composition has continually c hanged. The legacy of Quaternary climate change is that plant communit ies are far less stable than they appear to be from our perspective. T hey 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 bot h in space and time. Many ecological principles and concepts, and ecos ystem 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 clim ate. Our knowledge is presently insufficient to adequately describe in teractions between ecosystems and changing climate, but the patterns o f vegetation response to environmental changes of the past may provide important information on vegetation response to present and future cl imate change. The concepts of thresholds, multiple steady states, and multiple successional pathways are helpful in understanding the dynami c interrelationships between vegetation and environmental changes.