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THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 143 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE Figure 8.6 Quantitative assessment of landscape-level patterns of change through the Westphalian and Stephanian. Dotted lines mark breakpoints in coal bed average vegetational composition; breakpoints were determined by inspection and statistical analysis. Jaccard similarity was calculated for adjacent coal. Sign tests are listed where significant differences were found between adjacent coals. A more intuitive, or subtle, way to analyze landscape-level vegetational patterns is to use ordinations of zones or coals balls to examine the structure within individual coals. Patterns within successive coals then can be compared. Ordinations cluster the stands, be those stands individual coal balls or zones of coal balls from profiles, based on their quantitative taxonomic similarity. In two or three dimensions they serve as base maps on which additional information can be superimposed: for example, life history occurrences, diversity, physical attributes associated with assemblages, and ecomorphic characteristics. Ordinations thus serve as a transition between landscape-level and habitat-level patterns in that they illustrate the biotic and physical variability of the landscape in detail. Because of the nature of this review, only selected coals can be illustrated to show the basic patterns of vegetational, landscape-level changes through time (Figure 8.7). The nature of landscape changes detected in ordination again corroborates the pattern detected at the level of average coal seam composition: change in swamp organization at the Westphalian A-B boundary, minor changes between the Westphalian B and early Westphalian D, substantial restructuring during the early Westphalian D, and total overhaul of swamp community organization near the Westphalian-Stephanian (middle-upper Pennsylvanian) boundary. Changes in the Habitat Composition of Landscapes Ordination patterns reveal how the species assemblages of landscapes changed during the Late Carboniferous. During this time there is a close correlation between physical attributes of swamps and species assemblages, particularly in the Westphalian. Because physical attributes of swamps, such as fusain or mineral matter, can be mapped onto ordinations, as can taphonomic, life history, and structural patterns, it is possible to translate ordinations based on taxa into maps of habitat diversity. This permits us to infer patterns of habitat change through time. Westphalian A swamps had complex habitat organization, harboring a variety of taxa with different physical preferences. The Union Seam of England (Figure 8.7) had four basic assemblagesâeach, we believe, characteristic of a distinct subset of swamp environments: assemblages dominated by monocarpic lycopsids (Lepidophloios harcourtii or Lepidodendron hickii), growing in areas with long periods of standing water sufficient to reduce the abundance and diversity of ground cover and free-sporing plants; ecotonal assemblages (Paralycopodites and medullosan seed ferns) enriched in fusain, and ecomorphically distinct (no data exist on their position within the seam); cryptic disturbance habitats dominated by polycarpic lycopsids (Diaphorodendron vasculare and Sigillaria spp.), with a diversity of growth architectures, including abundant ground cover; Lyginopteris assemblages for which the physical attributes are unclear, but which overlap to some extent with Diaphorodendron vasculare-dominated assemblages and may have been part of the broader cryptic disturbance set of environments. Other Namurian and Westphalian A swamps offer variations on these themes (Holmes and Fairon- Demaret, 1984; Bertram, 1989). The transition from the Westphalian A to the Westphalian B was marked by changes in the dominance-diversity composition of swamps. The patterns that appeared persisted through the Westphalian B and C, and into the early
THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 144 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE Figure 8.7 Detrended correspondence analysis ordinations of six Westphalian swamps. Oldest to youngest in stratigraphic order: Union seam (Westphalian A); Rock Springs coal (Westphalian C); Murphysboro equivalent coal (early Westphalian D); Fleming coal (early Westphalian D); Bevier coal (early Westphalian D); combined ordination of Iron Post, Springfield, and Herrin coals (late Westphalian D). VS = based on vertical profiles of coal balls; each point represents one zone from a profile. RS = based on random samples of profiles; each point represents one coal ball.
THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 145 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE Westphalian D. The most notable changes were the extinction of Lyginopteris and the increase in the abundance of cordaitean gymnosperms. The cause of the extinction of Lyginopteris is not clear; the genus disappeared both in coal swamps and in surrounding clastic lowland settings of Euramerica. Post-Westphalian A swamps not only lacked Lyginopteris but differed in other major aspects from swamps of the Westphalian A. In general, there was a decline in the habitat diversity of any one coal swamp, with the habitats present representing a subset of those in the Westphalian A, complemented by the addition of cordaitean-dominated assemblages. Cordaites appeared initially in ecotonal assemblages characterized by heavily rotted peats and abundant fusain, sometimes associated with medullosans, ultimately becoming a locally dominant component of some swamps (Phillips et al., 1985). Later in the interval, in the late Westphalian C and early Westphalian D, opportunistic cordaites and tree ferns began to expand in importance. The tree ferns transcended habitats, appearing in ecotonal assemblages in numbers following one of the minor breakpoints, becoming interstitial opportunists by the early Westphalian D, and eventually occurring in all but persistently flooded assemblages during the Westphalian. Despite the breakup of Westphalian A patterns of landscape organization, the basic habitats of the earlier time are recognizable in Westphalian B to early Westphalian D swamps. The persistent habitats, recognizable by the physical, taphonomic, and structural attributes (as discussed earlier), retained their ecomorphic character, although the component species were largely different. Any one of the several possible habitats may have been either absent from a given coal, or present in such low frequency that it was not sampled, including those characterized by cordaitean dominance. Various combination of habitats thus appear throughout the interval (Figure 8.8). Late Westphalian D coal swamps represent a return to the basic habitat organization characteristic of the Westphalian A: flooded habitats dominated by monocarpic lycopsids, ecotonal habitats dominated by the lycopsid Paralycopodites and medullosans, and cryptic disturbance habitats dominated by polycarpic lycopsids with structurally complex vegetation. Cordaiteans disappeared as an ecologically significant component, associated with the extinctions of several numerically important species. The reassembly of the Westphalian A type of swamp habitat organization is modified by the persistence and further expansion of tree ferns as interstitial opportunists. Identifiably opportunistic species equivalent to the tree ferns were rare in Westphalian A swamps. Psaronius expansion in coal swamps correlates with a similar expansion in lowland wetlands in general (Pfefferkorn and Thomson, 1982). Figure 8.8 Patterns of change in physically determined habitats through time. Habitat characteristics discussed in text. Dotted lines are breakpoints determined by inspection and statistical analysis. MONO = habitats dominated by monocarpic lycopsids; POLY = habitats dominated by polycarpic lycopsids; ECOTONE = habitats dominated by medullosans and/or Paralycopodites; CRD1/CRD2 = variants on ecotonal habitats enriched in or dominated by cordaitean gymnosperms; TREE FERN = designates the occurrence of significant amounts of tree ferns as interstitial elements in other associations. The habitats of the late Westphalian D each have an ecomorphic character that had persisted throughout the 9 m.y. of the Westphalian. This character persisted through landscape-level breakpoint boundaries, despite species-level changes within the habitats. In relatively few instances, the same species can be identified within these habitats over the entire 9-m.y. period. Species replacements within
