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THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 141 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE (tree, vine, shrub, ground cover); degree of xeromorphy; and expense of construction. The latter two are comparative, and hence relative, depending on the spectrum of morphologies present in Carboniferous swamps. Major Intraswamp Habitats Habitats within a peat-forming swamp are all highly modified from extraswamp analogues by the presence of the predominantly organic substrate. The edaphic qualities of this substrate are variable, and these variations define many of the intraswamp habitats. Flooded habitats are recognized mainly on biotic or ecomorphic criteria: low species richness, little ground cover, few free-sporing plants (which need an exposed substrate to complete their life cycle), and dominance by plants with specialized semiaquatic or flood-tolerant morphologies. Such habitats also are relatively low in charcoal. Lycopsids were the most common elements of these environments in the Westphalian. Peat-to-clastic ecotonal habitats cover a wide range of conditions, recognizable mainly on physical criteria, although they encompass plants with corroborative life history strategies. There are several variants. Peat-to-mud transitional environments are associated with underclays, clastic partings, or usually high ash and mineral matter in coal; they are populated in the Westphalian by a small arboreous lycopsid (Paralycopodites), and often by medullosan seed ferns. Fire- prone habitats are associated with elevated levels of fusain and often with increased clastic material; the most common components of these environments are medullosan pteridosperms, sphenopsids, and in some cases, small, scrambling cordaitean gymnosperms. Habitats with long periods of exposure and presumed drying of the peat surface are characterized by heavily rotted and rerooted peats, often with evidence of extensive invertebrate burrowing; such environments are associated with larger cordaites, some sigillarian lycopsids, and medullosan pteridosperms. Cryptic, or irregular disturbance, habitats are recognizable by physical and ecomorphic attributes. They generally have little fusain or mineral matter. Evidence from an unusual buried forest deposit (Wnuk and Pfefferkorn, 1987), drawn from species compositional similarities, suggests irregular floods. In some swamps, storms associated with the influx of marine waters may have been a major disturbance agent, suggested by multiple coal-ball and coal layers containing marine invertebrates. Such habitats are generally dominated by polycarpic, long-lived trees, lycopsids in the Westphalian, and possibly tree ferns in the Stephanian. Species richness is intermediate, growth architectures often are very variable among the subdominants, and a ground cover component is generally important. PATTERNS OF CHANGE IN COAL-SWAMP COMMUNITIES DURING THE PENNSYLVANIAN PERIOD Objectives In this section we focus on the temporal patterns of change in coal swamps. This pattern is examined first at the landscape level-changes that are the easiest to describe and thus to relate to larger questions of environmental influence. The timing and extent of landscape-level changes is compared with patterns in habitat and species composition of successive swamps. We examine the relative timing and extent of change in these elements, and the relationship between species turnover and habitat persistence. We then argue that these relationships suggest a hierarchical structure in which biotic factors influence patterns of species replacement. Changes at the Landscape Level Change in the relative abundance of the major plant groups comprising swamp communities is the major, and simplest, indicator of community change and has been discussed elsewhere (Phillips and Peppers, 1984; Phillips et al., 1985). Because the major plant groups (lycopsids, ferns, pteridosperms, sphenopsids, cordaites) are broadly distinct in habitat preference, changes in their relative abundances also reflect changes in the physical characteristics of swamps. Figure 8.5 summarizes the changes by geographic region, with the general pattern summarized in the right-hand column. Biomass distribution is spread among enough major tree groups, and the patterns are sufficiently distinct, that some important generalities can be resolved at this level. We use mostly western European chronostratigraphic terminology because that of the United States varies widely among geographic regions. 1. Lycopsids, of several ecomorphic forms, dominate most coal swamps for the 9 m.y. (using the Hess and Lippolt, 1986, time scale) of the Westphalian (late early and middle Pennsylvanian). Major extinctions in North America occurred near the Westphalian-Stephanian (middle-upper Pennsylvanian) boundary, eliminating most of the lepidodendrids and removing the lycopsids from a position of ecological dominance in swamps (Phillips et al., 1974). 2. Cordaitean gymnosperms are the major subdominants or dominants during the midportion of this time interval, from the Westphalian B to the early Westphalian D. Two distinct phases are represented. During the Westphalian B and C, cordaitean taxa that produced Mitrospermum-type seeds (ovules) were the most common
THE RESPONSE OF HIERARCHIALLY STRUCTURED ECOSYSTEMS TO LONG-TERM CLIMATIC CHANGE: A CASE 142 STUDY USING TROPICAL PEAT SWAMPS OF PENNSYLVANIAN AGE Figure 8.5 Patterns of change in the abundance of the major plant groups in 28 selected upper Carboniferous coals. Summary at right includes coals from western Europe (labeled with an asterisk). forms. During the late Westphalian C and early Westphalian D the most abundant cordaites produced Cardiocarpus type seeds; these may have had a more shrubby or scrambling habit than earlier coal-swamp forms (Cridland, 1964; Costanza, 1985). 3. Tree ferns replace lycopsids as the ecological dominants of the Stephanian (Late Pennsylvanian). Tree fern abundances begin to rise during the Westphalian C, and especially during the Westphalian D. However, systematic studies suggest that largely different suites of Psaronius species and ecomorphs occupied Westphalian and Stephanian swamps (Lesnikowska, 1989). 4. Pteridosperms (seed ferns) fluctuate in abundance during the Westphalian, but attain uniformly higher and less variable abundances in the Stephanian. Studies of seed morphology (Taylor, 1965) suggest substantial species turnover at the Westphalian-Stephanian boundary. 5. Sphenopsids are present in coal swamps throughout the Late Carboniferous, but generally at low levels. Palynology suggests local times and regions of abundance (Peppers, 1979). The existence of breakpoints, marked by minor to major extinctions of swamp-centered lineages, is one of the most conspicuous aspects of this data summary. These breakpoints also are associated with landscape-level changes in dominance- diversity patterns (e.g., the rise and decline of cordaites, and the dramatic expansion of tree ferns). Palynological analyses (Peppers, 1979; Phillips et al., 1985) reveal the same basic patterns as the coarser megafossil data and point particularly to the Westphalian A-B boundary and the Westphalian-Stephanian boundary as times of major species turnover (Peppers, 1979; Phillips et al., 1974). The patterns at this level led us to suggest a typological classification of coal swamps some time ago (DiMichele and Phillips, 1981). This classification was based on the premise that there were periods of little change in dominance and diversity from one coal swamp to the next in stratigraphic sequenceâperiods of persistenceâpunctuated by much shorter time intervals of abrupt vegetational change. Our original formulation was based on a smaller data base, but the basic pattern has remained as further data have accrued. In order to evaluate this pattern more formally we have used some statistical methods to examine the data for floristic breakpoints (Figure 8.6). Jaccard coefficients and sign tests were used to assess similarity based on species presence or absence; through inspection we examined the data for points of low similarity. Although these are weak tests, used on the weakest manifestation of the data (presence-absence of species), they strongly corroborate the breakpoints at the Westphalian A-B boundary and at the Westphalian/Stephanian boundary. They support less strongly the presence of breakpoints at other boundaries where proportions of dominant tree taxa shift but are associated with few extinctions.