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THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 148 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE the craton (Stevens and Stone, 1988). Such botanical evidence has led paleobotanists (e.g., White, 1933) to suggest climatic factors as the driving force behind floristic change. Over the past ten years there has been a reawakening of interest in late Paleozoic climate prompted by studies of paleogeography (Scotese et al., 1979; Ziegler et al., 1981), and the resulting models of climatic dynamics (Parrish, 1982; Parrish et al., 1989). Most of the focus has been on climatic fluctuations during the time of Pennsylvanian coal formation. The most explicit of these have focused on the availability of moisture in the lowland tropics. Inferences have been based on the stratigraphic patterns of several indicators: coal resource abundance (Phillips and Peppers, 1984), coal sulfur and ash (Cecil et al., 1985), coal underclay mineralogy (Dulong and Cecil, 1989), chemical characteristics of rocks associated with coal-bearing strata (Cecil et al., 1985; Cecil, 1990), and the abundances of environmentally sensitive fossil plants in coal (Winston and Stanton, 1989). Additional evidence for climatic changes during the Pennsylvanian and Permian comes from smaller-scale studies of rocks with depositional histories indicating alternation of wet and dry conditions on a regional level. Examples are underclay mineralogies indicative of long periods of subaerial exposure within coal-bearing sequences (Prather, 1985; Spears and Sezgin, 1985), complex paleosols indicating marine regression and increasingly drier climate (Prather, 1985; Goebel et al., 1989), alternating wetter and drier intervals in ancient dune deposits (Driese, 1985), and mixed sequences including both fluvial and eolian deposits (Johnson, 1987, 1989a,b). Deposits that indicate alternation of wet and dry conditions are generally attributed to changes in base level and associated changes in regional rainfall patterns. Eustasy, linked to South Polar glaciation, has been cited in most instances as the proximal cause of climatic variations (e.g., Wanless and Shepard, 1936; Heckel, 1986, 1989; Rust et al., 1987; Rust and Gibling, 1990; and virtually all of the above citations in this paragraph), and there is considerable evidence of Gondwanan glaciation in the late Paleozoic (Veevers and Powell, 1987). Regional and global climate may have been affected by a variety of additional factors related to regional tectonics. The effects of plate collisions on crustal deformation (Klein and Willard, 1989), changes in circulation patterns associated with the uplift of mountain ranges (Parrish, 1982), and the movement of continents and climatic belts relative to each other (Ziegler, 1990) all complicate climatic patterns. The principal difficulty with most climatic scenarios is correlation. It is extremely difficult, some would say impossible, to correlate identifiable glacial deposits with identifiable changes in base level half the world away. We sympathize with such concerns and recognize that local, autocyclic depositional factors will overprint and often confound climatic interpretation; how does one differentiate dry climate from locally well-drained conditions based on the limited exposures usually available? Nonetheless, it is clear that one cannot turn to a ''default" climate. It is equally clear that climate has a major, if not the major, impact on many aspects of sedimentation and the distribution of biotas. Ziegler (1990) demonstrates this well in his summary of an enormous amount of physical and paleontological data on Permian climatic and biogeographic patterns. He notes that sharp biogeographic boundaries, often attributed by paleontologists to physiographic barriers, can be the result of subtle climatic variability, a phenomenon well marked in the modern world and in the postglacial (Holocene) migrational patterns of plants (e.g., Delcourt and Delcourt, 1987). Relationships of Climatic Patterns to Vegetational Patterns The landscape-level breakpoint boundaries detected in Late Carboniferous coal swamps correspond closely to times of inferred climatic change during the Westphalian and Stephanian (DiMichele et al., 1986). In eastern North America, comparison of three Pennsylvanian climatic scenarios with points of vegetational reorganization is illustrated in Figure 8.10. The correspondence is remarkably close, and the data bases are independent, suggesting a correlation-causation relationship. The differences in the inferred climatic patterns are not as important as the points at which departures from a norm are detected. It is these departures, or major inflections in the climate curves, that appear to dictate the times of vegetational change. The vegetational changes at the landscape and species levels appear to scale approximately to the magnitude of inferred climatic variability. The largest inferred climatic changes were near the Westphalian A-B boundary and the Westphalian- Stephanian boundary, also the times of the greatest vegetational changes. However, habitat-level changes offer a different perspective. Habitats persist through several climatic excursions during the Westphalian, retaining ecomorphic attributes and basic generic composition. It is only during the major climatic changes at the end of the Westphalian that habitat structure crumbles. This suggests that there are aspects of the structure of at least some ecosystems that do not follow climatic patterns in a linear manner. Rather, thresholds may exist, and once exceeded, a breakdown in organization or a reduction in the number of hierarchical levels within the system may occur rapidly.