Devonian to Carboniferous expansion of land flora

The earliest vascular land plants are known from Late Silurian strata. Colonization of the terrestrial environment, at least in moist lowlands, obviously happened rather quickly, and within only 45 Ma, all major land-plant lineages and organizational grades (except for flowering plants) developed (Niklas 1997, 2004; Willis and McElwain 2002). All the morphological adaptations including a protective outer covering (waxy cuticula) against desiccation, stomata to allow gas diffusion, specialized tissues for the transport of liquids, and rigid cell walls (Chaloner 2003) developed in the Devonian, although accompanied only by a modest diversification at the species level (Niklas 1997; Willis and McElwain 2002). By the end of the Devonian, the terrestrial environment saw the first globally distributed forests with large trees (Chaloner 2003). This resulted in a huge increase in biomass that culminated in the Late Carboniferous. Much of this organic material was not decomposed and recycled but instead buried in moist anoxic soils (acidic swamps; Chaloner 2003).

This expansion of the land flora had a profound impact on earth's environmental conditions. Most important was the removal of large quantities of CO2 from the atmosphere through photosynthetic carbon fixation. The most widely accepted models for Phanerozoic CO2 show a sharp decrease from about 15 times the present level (15 PAL) at the beginning of the Devonian to 10 PAL at the Devonian-Carboniferous boundary and a further decrease to less than 2 PAL in the Late Carboniferous (Berner 1998; Royer et al. 2000). With the massive drop in available CO2, a high stomatal density became crucial, and the laminate leaf rapidly became widespread (Beerling et al. 2001) although at the cost of higher water loss through transpiration (Chaloner 2003).

During the Devonian, the increase in the sizes of the trees was accompanied by increasing depth and complexity of the roots (Algeo and Scheckler 1998). This in turn accelerated silicate weathering and led to a further drawdown of carbon as bicarbonate into rivers and ultimately into the seas (Kump et al. 2004). The most obvious consequence of this huge decrease in atmospheric CO2 was the onset of the Gondwana glaciation. In the Late Devonian, the southern continents were assembled near the South Pole, and the first polar ice caps developed in the latest Devonian. However, the main phase of the Late-Paleozoic glaciation started in the Early Carboniferous. With its duration of more than 80 Ma (Crowley and North 1991; Frakes et al. 1992), well into the Permian, this glaciation was by far the longest and also the latitudinally most extensive of the Phanerozoic ice ages. Yet it was not associated with any major mass extinction pulse.

The very high productivity of the plants during the Devonian and Carboniferous also led to a significant increase in atmospheric oxygen, with 30-38% O2 in the Late Carboniferous (Berner 1999; Berner et al. 2003; Bergman et al. 2004). This seems to have had yet another impact on life. With increasing oxygen partial pressure, the diffusive flux is increased considerably, allowing the evolution of gigantic sizes (Graham et al. 1995; Dudley 1998), most notably among terrestrial arthropods and amphibians (Briggs 1985; Graham et al. 1995). For the giant flying insects, the higher density of the atmosphere might also have played a role (Dudley 1998, 2000).

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