IMPACT OF ICE HOUSE CLIMATE ON TROPICAL VEGETATION AND PLANT EVOLUTION

The response of tropical vegetation to climatic changes associated with the Late Carboniferous glaciation, especially the transitions between hot ("greenhouse") and cold (ice age sensu lato; "icehouse") intervals, can now be characterized based on very large paleobotanical data sets of Late Carboniferous age. These data sets were collected in eastern North America and central Europe.

At the onset of the ice age large-scale extinctions and originations occurred in at least two major steps during an interval of several million years. Innovation first occurred in ever-wet climates and environments, while floras in drier environments were still dominated by holdovers/survivors, a pattern termed the “Havlena effect.”

During the height of the ice age glacial-interglacial cycles produced large sea-level fluctuations and concomitant climatic changes, so that significant areas of continents in the tropics were alternately covered by shallow seas and densely vegetated terrestrial environments, including peat-forming mires. In spite of the repeated destruction of wet lowland habitats during each transgression of the sea, all common species and the plant communities in the wetland biome returned again and again. This resilience demonstrates that glacial-interglacial cycles by themselves do not produce changes in the flora.

Major extinctions within the tropical wetland biome were concentrated at the transition from cold climate to warm climate. In the aftermath of this transition, during ecological restructuring, opportunistic plant species rose to dominance in many environments within that biome. The new dominant forms were drawn, however, from pre-extinction wetland evolutionary lineages. Thus, this was an evolutionarily conservative turnover, with much species change but minimal higher-order innovation. Evolutionary innovation was occurring on a major scale in dry environments during this time of transition, while wet environments retained their "conservative" species make up, a pattern termed the “Elias effect”.

Thus, environmental threshold-crossing marked both the beginning and end of this cold interval, and produced extinctions and origination, although a reversal is seen in the environments that support innovation at the beginning and the end.