Impact of an icehouse climate interval on tropical vegetation and plant evolution

Abstract

Complex plant ecosystems first experienced the effects of major glaciation during the Late Paleozoic Ice Age. The general response of Carboniferous tropical vegetation to these climatic fluctuations, especially the transitions from greenhouse to icehouse conditions (ice age sensu lato) and a return to warm times, now can be characterized based on large paleobotanical data sets originally collected to solve stratigraphic and paleoecologic questions. These data come primarily from North America and central Europe, which at the time were part of a single continental mass situated in the tropics. At the onset of icehouse conditions, innovation (speciation leading to novel forms and ecologies) occurred in environments subjected to perhumid (everwet) climates, while floras in better drained, drier, and more seasonal conditions remained dominated by holdovers/survivors from older biomes. This pattern is termed the 'Havlena effect'. During the height of the ice age, glacial-interglacial cycles produced large sea-level fluctuations and concomitant climatic changes, such that significant areas of continents in the tropics were alternately covered by shallow seas or densely vegetated terrestrial coastal plains. In spite of the repeated destruction of wet lowland habitats during each marine transgression and their further fragmentation that accompanied a climate change from humid to sub-humid, seasonally dry conditions, most ofthe species and the basic configuration ofthe plant communities in the wetland biome remained stable. This resilience demonstrates that glacial-interglacial cycles by themselves are not responsible for either extirpation or extinction of these biomes. At the transition from icehouse-to-greenhouse conditions, dry-biome forms, which had been evolving outside of the taphonomic preservational window, became dominant across basinal landscapes while wet landscapes retained their 'conservative' species composition. This pattern is termed the 'Elias effect'. Thus, environmental threshold-crossing marked both the beginning and end of this cold interval, with the loci of response in different environmental settings. In contrast, the minor systematic changes that occurred during glacial-interglacial cycles did not influence the composition or structure of tropical lowland vegetation substantially.