. "8. The Response of Hierarchically Structured Ecosystems to Long-Term Climate Change: A Case Study Using Tropical Peat Swamps of Pennsylvanian Age." Effects of Past Global Change on Life. Washington, DC: The National Academies Press, 1995.
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Effects of Past Global Change on Life
relatively trivial specifics of change, but the general mechanistic basis underlying change. We need to learn how to recognize responses of ancient ecosystems that are unique products of times and circumstances, as opposed to inherent qualities that reflect principles of organization that transcend evolutionary diversification of the biota. When comparing modern plant communities to any from the Late Carboniferous we must recognize that the levels of Carboniferous species diversity were at least three orders of magnitude lower than those at present, that there are difficulties in recognizing those environments that are most stable and geologically persistent, particularly in extant systems, and that differences in temporal scale can confound comparisons of ecological patterns.
LATE CARBONIFEROUS WETLANDS
The Carboniferous was the first and most extensive coal age. The pantropical wetlands of the Late Devonian and Early Carboniferous from which the coal-forming forests evolved were the Earth's primeval forests (Cleal, 1987). Most of the major lineages of plants that dominated these environments originated during the Late Devonian and earliest Carboniferous. In taxonomic terms, all the generally recognized major class-level groups of vascular plants (save the flowering plants) and many of the major ordinal-level groups originated during this narrow window of time. In the establishment of all basic life history and architectural patterns in vascular plants, the Late Devonian and earliest Carboniferous parallels the Cambrian radiation of marine invertebrates.
The vascular plant radiation involved a strong association of classand ordinal-level clades with specific ecological conditions. In effect, the basic ecologies and habitats of the landscape were strongly partitioned along taxonomic lines. This pattern of ecological partitioning along taxonomic and life history lines was to distinguish the Carboniferous from all subsequent times. The landscape partitioning of the Carboniferous began to break down over an extended time interval, from the end of the Late Carboniferous through the Early Permian. During this time there were major extinctions in the wetlands (Phillips et al., 1974, 1985; Pfefferkorn and Thomson, 1982), accompanied by expansion of seed plants into vacated resource space (Knoll, 1984). The result, by the Late Permian, was dominance of the landscape by one major life history, the seed habit, which ushered in the pattern still prevalent today.
The lowlands were partitioned, in the broadest sense, between seed plants and lycopsids as the dominant tree groups. Seed plants occupied mainly the better-drained environments; lycopsids grew in swampy, semiflooded habitats. These lineages had very different life histories and morphologies, and the ecosystems they dominated had widely differing dynamics (Phillips and DiMichele, 1992). Today the seed plants, particularly flowering plants, dominate nearly all terrestrial ecosystems; despite the diversity of dynamics encompassed by extant plants and ecosystems, they do not include some of the structure and dynamical properties that were part of the Carboniferous ecological spectrum, the lycopsid-dominated swamps, for example.
Peat-forming coal swamps had organic substrates, suggesting chelation of mineral nutrients, typically low pH, and periodic flooding. Such environments had high levels of abiotic stress, a result of the combination of low nutrient levels, anaerobic conditions within water-saturated rooting zones, water-table fluctuations, and regular disturbance. The high abiotic stress (sensu Grime, 1979; DiMichele et al., 1987) appears to have selected strongly against most species, leading to a specialized flora. The tolerant species tended to be parts of peat swamp-centered evolutionary lineages, such as many of the lycopsid genera (DiMichele and Phillips, 1985), or specialized offshoots that became somewhat isolated from main line of the clade, such as cordaitean gymnosperms and some of the marattialean ferns (Rothwell and Warner, 1984; Costanza, 1985; Trivett and Rothwell, 1985; Lesnikowska, 1989).
Late Carboniferous peat swamps were not a single monotonous habitat, as depicted in most museum dioramas. Rather they encompassed a range of subhabitats controlled largely by flooding regime and influenced by disturbances such as incursions of mineral matter, brackish water, and wildfires. Some swamps or parts of swamps may have been domed (ombrotrophic), as in modern areas of Sumatra and Borneo (Cecil et al., 1985; Esterle and Ferm, 1986; Kvale and Archer, 1990; Staub and Esterle, 1992; Eble and Grady, 1993). Others were clearly planar (rheotrophic) swamps, subject to flooding and introduction of clastics in floodwaters (Eggert, 1982; Grady and Eble, 1990; Calder, 1993). Both kinds of swamp, which may have coexisted within a single peat body, were sufficiently variable in physical conditions to support a diversity of habitats and the species specific to them.
Knowledge of the plant composition and paleoecology of peat swamps is provided by coal balls and miospore studies. Coal balls (Figure 8.1) are concretions of the original peat fabric petrified (permineralized) before the peat was coalified. They occur within coal seams, generally as calcium carbonate and occasionally as siliceous concretions. The plants in coal balls are anatomically