As temperatures in southwestern North America increase 3 to 5°C over the next 60 to 90 years, rapidly colonizing species should increase, while slow colonizing species will at first decrease, eventually becoming re-established in their new range. Using long-term plots, this successional process has been estimated to require from 100 to 300+ years in small areas, under a stable climate, with a nearby seed source. How much longer will it require on a continental scale, under a changing climate, without a nearby seed source? This question is tested using the response of fossil plant assemblages from the Grand Canyon, Arizona to the most recent rapid warming of similar magnitude.
At the start of the Holocene, 11,700 years ago, temperatures increased about 4 oC over less than a century. Grand Canyon plant species responded at different rates to this warming climate. Early-successional species rapidly increased, while late-successional species decreased. This shift persisted throughout the following 2700 years. Two similar but less pronounced shifts followed rapid warming events around 14,700 and 16,800 years ago. Late-successional species only predominated following 4000 years or more of relatively stable temperatures during the full glacial Wisconsinan and late Holocene. These results suggest the potential magnitude, duration, and nature of future ecological changes. When these concepts are extended to include the most rapid early-successional colonizers, the herbaceous species, they imply that the recent increases in invasive exotics may be only the most noticeable part of a new resurgence of herbaceous early-successional vegetation. These results also caution against models of natural vegetation and carbon balance projecting future conditions based upon the assumption that vegetation approaches equilibrium within only a century.