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Effects of Past Global Change on Life 9 The Late Cretaceous and Cenozoic History of Vegetation and Climate at Northern and Southern High Latitudes: A Comparison ROSEMARY A. ASKIN University of California, Riverside ROBERT A. SPICER Oxford University ABSTRACT The Late Cretaceous and Cenozoic high latitude land vegetation bequeathed a sensitive paleobotanical and palynological record of regional and global environmental change. Foliar physiognomy provides the most reliable indicators. This record is frequently available for northern localities, augmented by wood and palynomorph data. Southern data are provided mainly by palynomorphs, with some foliar, cuticular, and wood information. The northern high latitude vegetation was mainly deciduous, whereas evergreen taxa locally predominate in the south. Major northern clades were all derived from lower latitudes; in contrast, Antarctica was a center of evolutionary innovation and dispersal. Differences in northern and southern vegetation are a function of continental configurations, interrelated with continentality (winter-summer temperature range), seasonality, moisture/aridity regimes, sea-level cycles, and overprinted by biotic stress or selective mechanisms. Vast land areas encircled the North Pole (to within 85°N), enhancing climatically driven northward and southward migrations, whereas an Antarctic continent continuously occupied the South Polar latitudes, had relatively restricted dispersal corridors, and became increasingly isolated as the other Gondwana fragments spread northward. INTRODUCTION Fossils of plants that lived at high latitudes provide a sensitive and unparalleled record of the complex interplay of global climatic change and polar conditions through time. High latitude plants, presently portrayed by extant polar desert, tundra, and taiga floras, require adaptations for stringent "icehouse" conditions. An icehouse world is, however, infrequently encountered in earth history. More typical "greenhouse" conditions necessitate other strategies for plant survival (Spicer, 1989a; Spicer and Chapman, 1990). In the Mesozoic and Cenozoic greenhouse world, forests thrived near the poles despite the seasonal stress of polar light cycles (the near congruity of rotational and magnetic poles is assumed here).
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Effects of Past Global Change on Life Light and particularly temperature are the principal controlling factors for polar vegetation, whereas precipitation is generally the prime factor at lower latitudes (Ziegler, 1990). Realistic quantitative estimates of these physical conditions can be ascertained from fossil floras. These estimates are crucial data for climatic modeling and for understanding and predicting global climate change. Environmental parameters (mean annual temperature, MAT; mean annual temperature range, MAR; coldest month mean temperature, CMM) that can be determined from fossil floras, and their validity and limitations are reviewed by Spicer and Parrish (1990a). Parameters derived from fossil floras reflect vegetational response to the environment and are obtained by careful analysis and interpretation of leaf physiognomy (size and margin morphology, Bailey and Sinnott, 1915; Wolfe, 1971, 1979, 1985; Wolfe and Upchurch, 1987) and overall floral composition (Wolfe, 1979). Such information is available for northern high latitude floras, especially from upper Cretaceous to Eocene strata from northern Alaska. In southern high latitudes, fossil leaf assemblages are less common, and environmental interpretations are not yet well developed. Ongoing work on leaf floras of southern basins (e.g., Daniel et al., 1990; Parrish et al., 1991) should help correct this imbalance in data types, but in the meantime much of the southern data are derived, by necessity, from palynomorphs. Useful qualitative information can be obtained from wood anatomy, tree-ring data, and palynomorphs. Palynomorph assemblages contribute a broad-brush view of the regional vegetation or, if locally derived, can provide a more detailed picture. Generalized climatic conditions can be inferred from palynomorph and leaf fossils by analogy with presumed modern counterparts (nearest living relatives), but this method can be risky because it assumes evolutionary stasis. For conservative taxa (conifers, ferns), such conclusions may be reasonably reliable. The northern and southern high latitude (-60 to 90°) vegetational history from the middle Cretaceous through the Cenozoic is presented in this overview, along with its suggested relationship to global change, in particular climatic change. Vegetational changes evolved along different pathways in the northern and southern regions, although basic physiologic constraints of polar conditions are similar. Physiognomic parallels at both poles (e.g., highly dissected ginkgo leaves, the broad-leaved conifers Podozamites and Agathis-type, Sphenopteris-like ferns) illustrate this latter point. PALEOGEOGRAPHIC FRAMEWORK The difference in continental configurations between the northern and southern polar regions is the overriding cause of differences in the evolution of their respective floras. These differences are illustrated (Figures 9.1 and 9.2) in the paleogeographic reconstructions of Smith et al. (1981). During the Late Cretaceous and Cenozoic, vast land areas encircled the North Pole (to within 85°N), facilitating climatically driven northward and southward floral migrations, whereas an Antarctic continent continuously occupied the South Polar position, had relatively restricted dispersal corridors, and became increasingly isolated as other Gondwana fragments spread northward. SUMMARY OF HIGH-LATITUDE VEGETATIONAL CHANGES Significant botanical events, and vegetational types and trends for northern and southern high latitudes are outlined in Figures 9.3 to 9.6, plotted alongside "global change" information. These charts are based on studies and fossil localities cited below and in Figures 9.1 and 9.2. Northern Cretaceous Albian-Cenomanian and Arrival of Angiosperms In the middle Cretaceous, land areas extended northward to 75°N. In these latitudes, prior to the arrival of angiosperms near the end of the Albian, forests were conifer dominated with Podozamites, Arthrotaxopsis, and Elatocladus being the most common foliage (Smiley, 1966, 1967, 1969a,b; Samylina, 1973, 1974; Spicer and Parrish, 1986, 1990a; Spicer, 1987). Needle-leaved conifers were common. Ginkgophytes (Ginkgo, Sphenobaiera or Sphenarion, Ginkgoites) were diverse, but restricted to river margins (Ginkgo-like forms) or back levees (Sphenobaiera), and cycads were relatively common but spatially restricted. Ferns (e.g., Onychiopsis, Sphenopteris like forms) and Equisetites were early colonizers and common as ground cover. Tree productivity and water availability were high, and temperatures were typical of cool temperate regimes (Spicer and Parrish, 1986; Parrish and Spicer, 1988a). All vegetation was deciduous, could enter dormancy, or could over winter as underground organs or seeds (Spicer and Parrish, 1986). Palynological evidence shows that bryophytes, lycopods, and fungi were prevalent (May and Shane, 1985; Spicer et al., 1988), particularly in mire environments that gave rise to extensive coals (Youtcheff et al., 1987; Grant et al., 1988). Angiosperms that produced tricolpate pollen reached the northern high latitudes, including the Canadian Arctic, during the Albian (Jarzen and Norris, 1975; Singh, 1975; Scott and Smiley, 1979). In northern Alaska (Spicer, 1987) and Siberia (Samylina, 1974; Lebedev, 1978), leaf floras indicate that in the Cenomanian, platanoid angiosperms (e.g., "Platanus," Protophyllum, Pseudoprotophyllum,
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Effects of Past Global Change on Life FIGURE 9.1 Cretaceous high latitude plant fossil localities on 100 and 80 Ma paleocontinental reconstructions (polar Lambert equal-area projections) of Smith et al. (1981): (a) Northern Albian-Cenomanian localities, 1: Smiley (1966, 1967, 1969a,b), Scott and Smiley (1979); 2: May and Shane (1985), Spicer and Parrish (1986, 1990a,b), Spicer (1987), Parrish and Spicer (1988a,b), Grant et al. (1988), Youtcheffetal. (1987); 3: Jarzen and Norris (1975); 4: Singh (1975); 5: Samylina (1973, 1974), Lebedev (1978). (b) Southern Aptian, Albian-Cenomanian localities, 1: Volkheimer and Salas (1975), Archangelsky (1980), Romero and Archangelsky (1986); 2:Rees and Smellie (1989), Rees (1990), Chapman and Smellie (1992); 3: Dettmann and Thomson (1987), Baldoni and Medina (1989); 4:Truswell (1983), Truswell and Anderson (1985); 5: Truswell (1990); 6: Truswell (1983); 7: Couper (1960), Raine (1984); 8: Douglas and Williams (1982), Dettmann (1986a), Taylor and Hickey (1990), Parrish et al. (1991), Dettmann et al. (1992). (c) Northern Turonian to Maastrichtian localities, 1: Hollick (1930), Spicer(1983); 2: Parrish et al. (1987), Frederiksen et al. (1988), Parrish and Spicer (1988a), Frederiksen (1989), Spicer and Parrish (1990a,b); 3: Hickey et al. (1983); 4: Krassilov (1979). (d) Southern Turonian to Maastrichtian localities, 1: Birkenmajer and Zastawniak (1989); 2: Cranwell (1969), Baldoni and Barreda (1986), Francis (1986), Dettmann and Thomson (1987), Askin (1988a,b, 1989, 1990a,b), Dettmann and Jarzen (1988), Baldoni and Medina (1989), Dettmann (1989), Jarzen and Dettmann (1990), Askin et al. (1991); 3: Truswell (1983), Truswell and Anderson (1985); 4: Truswell (1983); 5: Couper (1960), Mildenhall (1980), Raine (1984, 1988), Daniel et al. (1990); 6: Stover and Partridge (1973), Martin (1977), Dettmann and Jarzen (1988), Dettmann (1989), Dettmann et al. (1992). Pseudoaspidiophyllum, Crednaria) locally dominated riparian habitats, where they successfully replaced Ginkgo and Ginkgo-like plants. By the late Cenomanian, angiosperm diversity had risen to more than 60 leaf forms in Alaska (Spicer and Parrish, 1990a). Pollen diversity in Alaska has yet to be fully evaluated. The vegetation was still conifer dominated, but needle-leaved conifers were less common. Angiosperm leaf sizes were large, and leaf physiognomy suggests a wet regime with MATs of 10°C (Parrish and Spicer, 1988a). Tree rings show little intra-annual variation (few false rings). There is some inter-annual variation, possibly a result of fluctuations in water availability (Parrish and Spicer, 1988b), although overall water stress was lacking, based on the high productivity and large cell size. Latewood was very limited, suggesting rapid onset of dark induced dormancy (Spicer and Parrish, 1990b). There are no periglacial sediments known, or any features indicative of sea ice. Turonian-Coniacian-Santonian The Turonian was characterized by a major global sea-level highstand, reducing the nonmarine sedimentary record. By the Coniacian, needle-leaved conifers and Podozamites had disappeared and taxodiaceous foliage was common. Platanoid angiosperms were still dominant along river and lake margins, and had begun to penetrate forests. Cycads were rare or absent, Ginkgo diversity was much reduced, and Equisetites and ferns formed the main ground cover. Leaf margin analysis of a small number of specimens imply an MAT of 13°C at about 78°N (Parrish and Spicer, 1988a), and conditions were still wet although coals are thinner and less numerous. Angiosperm diversity was high, but possibly less than in the Cenomanian. All taxa were deciduous or capable of winter dormancy. Santonian nonmarine sediments are rare and not yet sampled for plant fossils.
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Effects of Past Global Change on Life Campanian-Maastrichtian The North Slope of Alaska was at 85°N, and overall diversity had dropped in the megafloral record. Woody angiosperm diversity was much reduced, and conifers were represented by only two foliage and five wood taxa (Spicer and Parrish, 1990a). Equisetites and ferns continued as the main ground cover, and Ginkgo and cycads had disappeared. Fires were common. The palynomorph record is extensive and indicates angiosperm predominance in the Maastrichtian, with a high turnover and spatial heterogeneity of taxa (Frederiksen et al., 1988; Frederiksen, 1989). Plants that produced the pollen are believed to be mostly herbaceous and probably annuals. A ''weedy" strategy would have been favored by short growing seasons and fire disturbance. Some woody Betulaceae/Ulmaceae may have been present (Parrish et al., 1987; Frederiksen, 1989); however tree taxa were dominated by the limited variety of conifers. Tree rings reflect lower productivity and more inter-and intra-annual variation. Latewood to earlywood ratios are higher, suggesting thermal limitations on spring or summer growth rather than sudden dark-induced dormancy (Spicer and Parrish, 1990b). By the Maastrichtian, vegetation was more open, with smaller trees. The environment was not as wet, with periodic drying. Vegetational physiognomy suggests MAT of 2.5 to 5°C and long, cold, dark winters with CMM probably no lower than -11°C (Parrish et al., 1987). Large wood tracheid cross sections, and lack of periglacial sediments, imply no severe drought or freezing (Spicer and Parrish, 1990b). On the Alaskan Peninsula, conditions were much warmer than on the North Slope. Leaf forms were more advanced, migration from the south continued, and Nilssonia (cycad) and diminuitive Ginkgo dawsonii survived into the Maastrichtian (Hollick, 1930; Spicer, 1983). Floras in both the northern (Spicer, 1989b) and the southern (Askin, 1988b, 1990a; Raine, 1988) high latitudes apparently did not suffer any major ecological trauma at the Cretaceous-Tertiary boundary. Southern Cretaceous Albian-Cenomanian and Early Angiosperms By the middle Cretaceous, angiosperms were well established in the southern high latitudes. At least 8 an FIGURE 9.2 Cenozoic high latitude plant fossil localities on 40 and 10 Ma paleocontinental reconstructions (polar Lambert equalarea projections) of Smith et al. (1981): (a) Northern Paleocene-Eocene localities, 1: Wolfe (1980); 2: Wolfe (1966), Wolfe and Poore (1982); 3:Hickey et al. (1983), Parrish et al. (1987), Frederiksen et al. (1988), Spicer and Parrish (1990a); 4: Francis and McMillan (1987); 5: Schweitzer (1974). (b) Southern Paleocene-Eocene localities, 1: Stuchlik (1981), Lyra (1986), Torres and LeMoigne (1988), Birkenmajer and Zastawniak (1989), Torres and Meon (1990); 2: Dusen (1908), Fleming and Askin (1982), Zamaloa et al. (1987), Askin (1988a,b, 1990b), Case (1988), Askin et al. (1991); 3. Mohr, 1990; 4:Kennett and Barker (1990), Mohr (1990); 5: Truswell (1983). (c) Northern Oligocene to Pliocene localities, 1: Sher et al. (1979); 2: Hills et al. (1974); 3: Hills and Matthews (1974), Kuc et al. (1983); 4: Funder et al. (1985). (d) Southern Oligocene to Pliocene localities, 1: Palma-Heldt (1987), Birkenmajer and Zastawniak (1989); 2:Mohr (1990); 3: Askin and Markgraf (1986), Carlquist (1987), Webb etal. (1987), Harwood (1988); 4:Kemp (1975), Kemp and Barrett (1975), Truswell (1983, 1990), Hill (1989), Mildenhall (1989).
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Effects of Past Global Change on Life FIGURE 9.3 Vegetational trends and global change for the Cretaceous northern high latitudes. Sources of data are in the text. Sea-level curve for Figures 9.3 to 9.6 from Haq et al. (1987); time scale for Figures 9.3 and 9.4 from Harland et al. (1990).
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Effects of Past Global Change on Life FIGURE 9.4 Vegetational trends and global change for the Cretaceous southern high latitudes. Isotope data for Figures 9.4 and 9.6 from Barrera et al. (1987), Stott and Kennett (1990), and Stott et al. (1990).
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Effects of Past Global Change on Life FIGURE 9.5 Vegetational trends and global change for the Cenozoic northern high latitudes. Time scale for Figures 9.5 and 9.6 from Berggren et al. (1985) and Harland et al. (1990).
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Effects of Past Global Change on Life FIGURE 9.6 Vegetational trends and global change for the Cenozoic southern high latitudes.
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Effects of Past Global Change on Life giosperm pollen taxa were present near the tip of the Antarctic Peninsula (about 66°S) by the Albian-Cenomanian (Dettmann and Thomson, 1987; Baldoni and Medina, 1989). There, and slightly further north in southern South America (Volkheimer and Salas, 1975; Archangelsky, 1980; Romero and Archangelsky, 1986), palynomorphs and leaves represent monocotyledonous and several dicotyledonous families, including "higher" (nonmagnoliid) forms that produced triaperturate pollen. Vegetation was predominantly conifer forest with podocarps (Podocarpus and Microcachrys-type) and araucarians, pteridosperms, and diverse fern understory and ground cover. For a small riparian collection (seven leaf taxa) of mainly microphyllous and deciduous angiosperms from Livingston Island, South Shetland Islands (59 to 65°S), a MAT of 13 to 20°C was suggested (Rees and Smellie, 1989; Rees, 1990), although values are tentative at best for samples with fewer than 20 species (Wolfe, 1971). Associated foliage includes various conifers and cycads. Several types of fern foliage and Equisetites may have formed ground cover, as in northern high latitudes. Well-defined growth rings in conifer wood attests to strong seasonality, but the lack of rings in angiosperm woods (Chapman and Smellie, 1992) is difficult to explain. Angiosperms had penetrated close to 80°S by the Albian-Cenomanian in New Zealand (in Motuan and Ngaterian Stages; Raine, 1984). They have an older record in southeastern Australia (Aptian Koonwarra beds, ~60°S) based on pollen (Dettmann, 1986a) and foliage (with flowers) of a small, rhizomatous, perennial angiosperm (Taylor and Hickey, 1990). Douglas and Williams (1982) interpreted the Albian-Cenomanian climate in southern Australia as wet warm temperate, with moderate seasonality, probably seasonally dry, and no widespread winter freezing. Cooler temperatures are indicated by Parrish et al. (1991) based on reevaluation of the Albian vegetation, though not as cold as suggested by oxygen isotope results (Gregory et al., 1989; MAT less than 0°C). Parrish et al. (1991) noted that the Albian floras include both deciduous, thin-cuticled leaves, commonly occurring as mats (e.g., Phyllopteroides), plus more abundant microphyllous conifers and small-leaved bennettitaleans with thick cuticles and specialized stomata for reducing water loss. The latter could have retained their leaves throughout the winter if temperatures were low enough to slow metabolic processes significantly. In their rift valley continental setting, these floras apparently experienced greater MAR than those in coastal sites, and thus with colder winters could contain evergreen taxa. In contrast, coastal Cenomanian floras from South Island, New Zealand (Daniel et al., 1990) are physiognomically more comparable with coeval floras of Alaska. Many major groups, including angiosperms, conifers, ferns, and cycadophytes, are broad leaved, have mostly thin cuticles, and appear deciduous. On the coast of East Antarctica, in Prydz Bay, Albian palynomorph assemblages in Ocean Drilling Program (ODP) site 119 drillhole samples (Truswell, 1990) record podocarpaceous conifer vegetation with ferns (especially schizaeaceous types) and various hepatics (liverworts). Fungal material is common. Recycled palynomorphs in Recent muds of the Weddell Sea (Truswell, 1983; Truswell and Anderson, 1985) indicate this vegetational type probably extended around much of the East Antarctic coastal margin. Albian conifer wood from James Ross Island (~66°S) has uniform rings, absence of false rings, and large earlywood cell size, which indicate little water stress and no evidence for frost (Francis, 1986). Narrow latewood indicates sudden dark-induced dormancy, similar to coeval northern high latitude wood. All the southern high latitude fossil occurrences fall on the marginal areas of Gondwanaland fragments. Palynomorph assemblages indicate the predominance of temperate, podocarp-araucarian-fern forest vegetation, with lycopod and fern moorland vegetation in some areas (Douglas and Williams, 1982; Dettmann, 1986a,b; Dettmann and Thomson, 1987; Truswell, 1990; Dettman et al., 1992). The nature of the vegetation in the more inland, continental craton of East Antarctica remains unknown. Turonian-Coniacian-Santonian Palynological data spanning the Turonian-Coniacian-Santonian interval is available from James Ross Island (Baldoni and Medina, 1989) and from the Australasian high latitude sector. The podocarp-araucarian-fern forest association continued through this interval in southern high latitudes. In the Turonian-Coniacian, the abundance and diversity of angiosperms remained low (Dettmann, 1989), while cryptogams were still an important part of the vegetation. This was, however, an important transitional time when the established southern podocarpaceous conifer forest vegetation was diversifying, and new angiosperm families that henceforth typified southern vegetation started to appear. The southern high latitude region was a locus of evolutionary innovation from the Turonian to the end of the Cretaceous. Among the conifers, Lagarostrobus originated in the Turonian (Dettmann, 1989). This important forest tree dominated southern high latitude forests from the Santonian through the Paleocene, and produced pollen (Phyllocladidites mawsonii ) identical to that of the Huon pine (L. franklinii), now restricted to wet, cool temperate, maritime western margins of Tasmania. Dacrydium-type conifers originated in the Coniacian (Dettmann, 1989). Based on the angiosperm pollen record, Ilex (Aquifoliacaeae) originated in southern high latitudes. Its earliest fossil
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Effects of Past Global Change on Life record was in the Turonian of the Otway Basin, southeastern Australia (Martin, 1977; Dettmann, 1989). Proteaceae, now a major southern family, also evolved at about this time (Dettmann, 1989). Campanian-Maastrichtian Evolutionary innovation in southern floras reached its zenith during the Campanian. Evidence is provided by a rich palynomorph record from James Ross, Seymour, and adjacent islands near the tip of the Antarctic Peninsula (e.g., Dettmann and Thomson, 1987; Dettmann and Jarzen, 1988; Askin, 1989, 1990b), from New Zealand (e.g., Couper, 1960; Mildenhall, 1980; Raine, 1984), and from the Otway and Gippsland Basins of southeastern Australia (e.g., Stover and Partridge, 1973; Dettmann and Jarzen, 1988; Dettman et al., 1992). The Nothofagaceae (we accept the family status, discussion and references cited in Dettmann et al., 1990) originated in southern high latitudes in the early Campanian, with ancestral pollen types widespread from Australasia to the Antarctic Peninsula. N. brassii, fusca, and menziesii types differentiated soon after (Dettmann et al., 1990), with the southern South America-Antarctic Peninsula area the center of Nothofagus diversification. In the Proteaceae, several types of both Proteoideae and Grevilleoideae occur in Campanian rocks, with earlier arrival of ancestral Proteaceae from northern Gondwana (Dettmann, 1989). The Antarctic Peninsula and New Zealand-southeastern Australia areas were both early Campanian diversification sites for various proteaceaeous groups (Beauprea type, Macadamia type, Gevuina-Hicksbeachia type, Knightia type, Xylomelum type; Dettmann and Jarzen, 1988; Pocknall and Crosbie, 1988; Dettmann, 1989). Gunnera (Gunneraceae) immigrated southward from northern Gondwana via the Antarctic Peninsula in the Campanian (Jarzen and Dettmann, 1990). Members of Myrtaceae may have entered via this route, while Winteraceae appeared in the New Zealand-southeastern Australian area (Dettmann, 1989). Loranthaceae appeared near the end of the Campanian in the Antarctic Peninsula area (Askin, 1989). The conifer Dacrycarpus also appeared in the high southern latitudes during the Campanian (Dettmann, 1989). Diversification continued through the Maastrichtian, with many endemic angiosperm pollen taxa of unknown botanical affinities appearing in the fossil record. This may represent parallel (to northern) development of a herbaceous, "weedy" strategy during the general cooling trend. The latest Maastrichtian on Seymour Island (~66°S) included a short warm interval characterized by members of Bombacaceae, Olacaceae (Anacolosa type), and Sapindaceae (Cupanieae tribe) (Askin, 1989). A rich Campanian leaf assemblage from King George Island (Zamek locality), South Shetland Islands, suggests a broad-leaved forest community, including evergreen types (with thick, coriaceous leaves) and deciduous Nothofagus growing in a subhumid mesothermal climate (Birkenmajer and Zastawniak, 1989). Abundant ferns, especially gleicheniaceous types (Cao, 1989), grew on adjacent moist lowland areas at the end of the Cretaceous. Conifer and angiosperm wood from the James Ross Basin have wide, uniform rings and low latewood-to-earlywood ratios, indicating high productivity, sudden dark-induced dormancy, and no water stress (Francis, 1986). Rainfall of 1000 to 2000 mm/yr was suggested. From the late Maastrichtian (and Early Paleocene) on Seymour Island, dispersed plant cuticle from evergreen foliage of araucarians, Cupressaceae, and podocarp conifers, plus numerous angiosperm taxa, including Myrtaceae and Lauraceae, indicates a wet climate, MAT approximately 8 to 15°C, MAR probably <16°C, and CCM >1°C (G. R. Upchurch, University of Colorado, personal communication, 1990). Podocarpaceous conifer (especially Lagarostrobus)-Nothofagus-Proteaceae forest grew throughout much of the southern high latitudes. Presumed paleoclimates were humid, warm-cool temperate (Cranwell, 1969; Mildenhall, 1980; Francis, 1986; Dettmann and Thomson, 1987; Dettmann and Jarzen, 1988; Askin, 1989, 1990a; Truswell, 1991). Northern Cenozoic Paleocene After a slow start, woody angiosperms became the vegetation dominants, with about 20 leaf taxa in the fossil record. During the Paleocene, angiosperm wood (with vessels and other angiospermous features) occurred on the North Slope of Alaska. Betulaceous-Ulmaceous forms became established, and also migrating from lower latitudes were members of Palmae, Fagaceae, and Juglandaceae (Wolfe, 1966, 1980; Wolfe and Wahrhaftig, 1970; Spicer et al., 1987). "Metasequoia" was the dominant conifer. Leaf assemblages include rare Cupressaceous foliage and the first record of a rosaceous leaf on the Alaskan North Slope (J. A. Wolfe, U.S. Geological Survey, personal communication, 1989). The MAT was 7°C, and the climate periodically dry, although coals were extensive, albeit thin (less than 1-m thick). The palynomorph record has yet to be examined in detail. Eocene The fossil record is good in Canada and southwest Alaska, though poor in northern Alaska. On Ellesmere Island, at 66°N, the vegetation included high-productivity
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Effects of Past Global Change on Life taxodiaceous forests. Floras across this region, and in Svalbard and Greenland, suggest a possible thermal maximum for the past 100 m.y. (Heer, 1868; Schweitzer, 1974; Wolfe and Poore, 1982; Francis and McMillan, 1987). During warm intervals of the Middle and Late Eocene, the vegetation of southern Alaska included multistratal rain forest dominated by Palmae, Lauraceae, Menispermaceae, Fagaceae, Theaceae, and other mesothermal families. Further north, the Alaska Eocene vegetation was dominantly broad-leaved deciduous (e.g., Meshik floras). Important elements were Taxodiaceae, "Cercidiphyllum," "Cocculus," Fagus, Quercus, and Juglandaceae (Wolfe, 1966, 1980, 1985; Spicer etal., 1987). These taxa were all derived from lower-latitude ancestors. Oligocene-Miocene-Pliocene Overall diversity decreased into the Early Oligocene, although Pinaceae and Salicaceae increased in diversity and ecological importance. The vegetation included cool temperate, angiosperm-dominated, broad-leaved deciduous forest. Thick coals (at Healey, Alaska) interspersed with very thick conglomerates suggest very high and evenly distributed rainfall (R.A.S., personal observation). Cooling produced a general pattern of southward migrations of particular lineages of closely related species. Salix, which originated in lower latitudes, is diverse in Early Miocene and even more diverse in the later Miocene of the Kenai Group, contrasting with its lower diversity in coeval midlatitude assemblages (Wolfe, 1985). Early Middle Miocene floras on Banks Island (70°-80°N) include Pinus, Picea, Tsuga, and Juglans (Hills et al., 1974). Grasses first became a major component of the pollen flora at about the Miocene-Pliocene transition, and taiga and tundra began to develop on a wide scale (Wolfe, 1985). Taiga appears to develop first in Siberia at about this time (Baranova et al., 1968; Sher et al., 1979) and provides evidence for periglacial conditions. Southern Cenozoic The present ice cover on Antarctica greatly hampers the retrieval of information on Cenozoic (and older) vegetation and climates. Much of our knowledge is derived from careful interpretation of recycled palynomorphs in recent seafloor muds around the coast of Antarctica, with sediment provenance inferred from tracing ice stream paths (Truswell, 1983; Truswell and Drewry, 1984). Only a generalized view of vegetation and climate could be derived from these data, in part because there was no refined age control for many of the (endemic) palynomorphs. This situation should improve as more Antarctic and Subantarctic sediments are drilled and as precise stratigraphic ranges of distinctive palynomorph species are established from these drillholes, and from the rare outcrop localities such as Seymour Island. Paleocene On the Antarctic Peninsula, the southern high latitude lower Paleocene vegetation apparently had decreased diversity (Askin, 1988a). Cuticular evidence suggests that conditions may have been slightly cooler than in the Maastrichtian; however, the composition of dispersed cuticle assemblages and the presence of frost-sensitive epiphyllous fungi preclude severe frost, at least in coastal areas (G. R. Upchurch, University of Colorado, personal communication, 1990). Tree-ring data from conifer and angiosperm wood also suggest cooler conditions through the upper Maastrichtian into the Paleocene (Francis, 1986, 1991). Podocarpaceous conifer-Nothofagus-Proteaceae forest floras continued through the Paleocene. Lagarostrobus remained an important component, and its predominance in Paleocene coals in Australia (Stover and Partridge, 1973; Martin, 1984; Truswell, 1990) and Seymour Island (Fleming and Askin, 1982) highlight its preference for wet habitats. In marginally high latitudes, Cunoniaceae-dominated vine forests with conifers covered the central Australian landscape (Sluiter, 1990), and further south, Nothofagus was becoming more prominent. By the Paleocene, the New Zealand-Campbell Plateau block had separated and was drifting into lower latitudes. Leaf floras from King George Island with Nothofagus, Myrtaceae, various other angiosperms, and ferns, represent a temperate broad-leaved forest with MAT of 10 to 12°C and rainfall of 1000 to 4000 mm/yr (Birkenmajer and Zastawniak, 1989). Seymour Island Paleocene leaf floras are dominated by fern foliage, with microphyllous Nothofagus, other angiosperms (some notophyllous), and conifer foliage, and probably reflect a cool temperate climate (Dusen, 1908; Case, 1988). Evidence for warm, humid conditions in East Antarctica (adjacent to Maud Rise) in the earliest Paleocene is provided by ODP 113 material, with further evidence from subsequent Paleocene lateritic and possible aeolian sediments for warm, semiarid, continental climate in inland East Antarctica (Kennett and Barker, 1990). Eocene The Eocene is marked by diverse floras and a general shift from conifer-dominated forest to Nothofagus-dominated vegetation in the coastal high latitudes. Warmth-loving (frost-sensitive) taxa are present through much of the Eocene, and a warm, moist climate is indicated. Information is available from leaf floras (Dusen, 1908; Case, 1988) and palynomorphs (Zamaloa et al., 1987; Askin et
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Effects of Past Global Change on Life al., 1991) from the Seymour Island area; recycled palynomorphs from around the Antarctic coast (Truswell, 1983); leaves (e.g., Birkenmajer and Zastawniak, 1989; Li and Shen, 1989), wood (Torres and LeMoigne, 1988), and palynomorphs (Stucklik, 1981; Lyra, 1986; Torres and Meon, 1990) in the South Shetland Islands; and palynomorphs in Middle Eocene ODP samples in the South Orkney region (Mohr, 1990), close to 60°S. Terrestrial dispersal corridors that linked Antarctica with the other austral land masses were severed by the Late Eocene, keeping Antarctic plant taxa from emigrating northward, and preventing active immigration to Antarctica. Oligocene-Miocene-Pliocene The Oligocene epoch, during which major tectonic and climate changes of lasting effect took place, saw a drastic reduction in vegetational diversity. For Early Oligocene Nothofagus-fern and fern bush floras on King George Island, Birkenmajer and Zastawniak (1989) suggest MAT of 11.7 to 15°C and rainfall between 1220 and 3225 mm. Also on King George Island, leaf floras of Oligocene-Miocene age suggest Nothofagus-podocarp conifer forest communities with possible MAT of 5 to 8°C and rainfall of 600 to 4300 mm (Birkenmajer and Zastawniak, 1989). In the Maud Rise area (ODP 113 material), late Early Oligocene palynomorph assemblages contain no terrestrially derived spores or pollen (Mohr, 1990), implying possible ice cover or greatly reduced vegetation on adjacent East Antarctica. In the Ross Sea area (close to 70°S paleolatitude), Late Oligocene material (from Deep-Sea Drilling Project (DSDP) site 270) indicates that a Nothofagus-dominated cold temperate forest, with Proteaceae, conifer (Podocarpus), Myrtaceae, and rare ferns still existed, despite evidence for sea-level glaciation. Also in this area, other drillholes penetrating Oligocene sediment (MSSTS-1, CIROS-1) recovered evidence of mixed Nothofagus-conifer forests, with other angiosperms also present (results summarized in Truswell, 1990). Mildenhall (1989, CIROS-1) suggested that the coastal fringes of the Ross Sea supported a Nothofagus forest with podocarps, Proteaceae, and other shrubby angiosperms. Upper Oligocene sediments in CIROS-1 contain a leaf (Hill, 1989) that resembles Nothofagus gunnii, an extant deciduous alpine Tasmanian species. Clay mineralogy, together with the fossil evidence, is consistent with a cool or cold temperate environment, and temperate rather than polar ice is indicated (Barrett et al., 1989). There is at present no good information on Miocene Antarctic vegetation; however some elements of the earlier flora must have survived in sheltered coastal refugia during major glacial advances. Wood and pollen remains, albeit reflecting a stunted, very depauperate flora, have been found near the head of the Beardmore Glacier in Pliocene glacial deposits now at 1800- to 1990-m elevations (Webb et al., 1987; Harwood, 1988). The wood is from a shrubby Nothofagus similar to extant species in Tasmania (N. gunnii) and southernmost South America (Carlquist, 1987), and the palynomorph assemblage contains only a few taxa, among them Nothofagus (fusca group), a podocarp and pollen resembling herbaceous Labiateae or Polygonaceae (Askin and Markgraf, 1986). COMPARISON OF NORTHERN AND SOUTHERN HIGH-LATITUDE VEGETATION Cretaceous Evolutionary Trends Southern Cretaceous floras are notable for their high latitude origins of major southern clades. In contrast, ancestral forms for all major northern clades (e.g., families) occur first at lower latitudes (Spicer et al., 1987). This contrast is due to differences in continental configurations and resulting differences in biotic stress. The early (Cenomanian) diversity of angiosperm, and particularly platanoid leaf forms, in the northern latitudes was at the generic or lower level. Diversity at the family level was low. These trends are a function of evolutionary novelty, coupled with hybridization and polyploidy, giving rise to morphological intergradation (Spicer, 1986). Vast land areas surrounded the North Pole through the Late Cretaceous to Cenozoic, facilitating migrations to and from the polar latitudes. Thus, new taxa were exposed to broad selection pressures, suppressing development of polar specialists. In the high southern latitudes, Antarctica remained in the polar position and was connected to lower-latitude land areas by relatively restricted dispersal corridors. The Antarctic plant communities were thus buffered to a certain extent from the competitive stress of invading taxa. Changing climatic conditions in this relatively isolated region would favor origin of specialists adapted to the changing environment, and low competitive stress would allow these specialists to become established. Major evolutionary innovation started in the Santonian and peaked in the Campanian-Maastrichtian. The increased rate of evolution coincides approximately with the end of the "Cretaceous quiet zone" (Harland et al., 1982) at the Santonian-Campanian transition and, more importantly, occurs during the Late Cretaceous cooling. Northern and Southern Floras: Deciduous Versus Evergreen In fossil floras, evergreen habit is recognized from the thick coriaceous nature of leaves and diagnostic (thick)
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Effects of Past Global Change on Life dispersed plant cuticle fragments. Deciduousness can often be inferred from fossil leaf mats representing autumnal fall, dehiscence structures on conifer short shoots, etc. It is not reliable to extrapolate past leaf retention-shedding from habits of modern analogues, because at least some plants (e.g., Sequoia, which was possibly ancestrally deciduous; Spicer, 1987) may not be obligate evergreen or deciduous taxa, but capable of either habit over time, depending on ambient light related to latitude. Microphyllous-xeromorphic (e.g., small-leaved, needle-leaved) plants can retain their leaves and survive winter cold. In extreme cold, evergreenness is not detrimental because metabolic processes are essentially shut down. In warmer greenhouse winters, however, leaf shedding was suggested as a better strategy to conserve stored metabolites (Spicer, 1987; Spicer and Chapman, 1990). Northern Cretaceous high latitude conifers and angiosperms were almost entirely deciduous, and in the Late Cretaceous the geographic boundary between deciduous (poleward) and evergreen floras coincided approximately with the paleo-Arctic Circle (66°N—limit of 24-hr light-dark regime; Wolfe, 1985). Deciduousness still typifies present mid- to high latitude northern vegetation, along with an evergreen mixed coniferous zone. Deciduousness in the north evolved at low to midlatitudes in response to seasonally arid conditions (like modern savanna). Large leaves (for higher productivity) could be produced only if they were shed in the dry season. In conifers, inefficient vascular systems necessitated small leaves, even under relatively favorable conditions, although many conifers were also deciduous. For plant taxa that subsequently grew at high latitudes under a polar light regime, deciduousness was fortuitously advantageous. Extant southern floras are typically evergreen, but it is uncertain what habit prevailed in Cretaceous southern high latitudes. Evergreenness can be advantageous in polar latitudes because less energy is required to produce photosynthetic organs each spring, and a quick start to the short growing season is possible (Spicer and Chapman, 1990). This is true, however, only if freeze damage can be avoided in the winter or, at the other extreme, if warm winter temperatures do not lead to metabolite depletion during respiration. The southern middle to late Cretaceous floras, close to the paleo-Antarctic Circle, included some deciduous taxa. At the end of the Cretaceous, and continuing into the Paleocene, the Seymour area vegetation was evergreen. At those paleolatitudes (60 to 65°S, and this applies to parts of the Antarctic margin in the Paleogene) a prolonged dark period would not occur and an evergreen habit with mild winters would not have been a problem. Thus, warm, moist coastal areas of Antarctica could have supported an evergreen forest. (In northern latitudes, evergreens extended to 70 to 75°N during the Eocene.) There is, however, no fossil evidence to determine which habit predominated in inland Antarctica. There, deciduousness would have been beneficial and perhaps some southern plant taxa assumed a deciduous habit when growing under polar light regimes during greenhouse warmth. However, the greater continentality experienced in the south appears to have given rise to winter temperatures that were low enough to favor evergreenness as a successful strategy (sensu Spicer and Chapman, 1990). Habit of ancestral Nothofagus that evolved under these conditions remains unknown (Dettmann et al. (1990), and extant Nothofagus includes both deciduous (in Patagonia-Tierra del Fuego and Tasmania) and evergreen species. Cenozoic Vegetational Changes In both hemispheres, after a cooler earliest Paleocene, climatic warming into the Eocene was reflected by concomitant changes in high latitude vegetation. Late Paleocene and Eocene floras exhibit increased diversity and complexity and, in both northern and southern high latitudes, include typically warmer-climate (mesothermal) taxa and communities. In the Cenozoic, global climates ultimately were controlled by tectonism that occurred in southern mid- to high latitudes, culminating in the isolation of Antarctica, inception of the Antarctic Circumpolar Current, development of the modern cryospheric ocean, buildup of Antarctic ice, and deterioration into icehouse conditions (references in Kennett and Barker, 1990). Vegetational changes in the north and south correspond to tectonic events and climatic changes (Figures 9.5 and 9.6). Whereas broad dispersal corridors surrounding the North Pole allowed climatically driven northward and southward migrations, post-Eocene Antarctica was completely isolated, resulting in stepwise extinction of taxa with each climatic deterioration. Important sequential cooling steps identified in the Cenozoic southern oceans (summarized by Kennett and Barker, 1990) occurred in the Middle Eocene, near the Eocene-Oligocene boundary, in the middle Oligocene, the Middle Miocene, the early Late Miocene, the latest Miocene, and the Late Pliocene, with some intervening, short-lived warming trends. Southern high latitude floras suffered the loss of many taxa (including mesothermal types) during and at the end of the Eocene. Subsequent loss of most remaining taxa occurred through the Oligocene and Miocene, with an extremely depauperate flora surviving to the Pliocene in coastal refugia. Along ice-free margins, present-day Antarctica supports a cryptogamic flora (e.g., mosses, lichens) with only two vascular plant species found in sheltered areas of the Antarctic Peninsula. A similar diversity decline occurred in northern high latitudes during and at the end of the Eocene with the
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Effects of Past Global Change on Life southward migration of many taxa and permanent disappearance from high latitudes of mesothermal forms. Superimposed on the poleward and equatorward migrations with warming or cooling trends were significant vegetational changes such as the increase of Pinaceae and Salicaceae in high latitudes and development of an evergreen, mixed conifer forest community (including Pinus, Picea, and Tsuga). During the final cooling stages, the taiga forest community developed and, subsequently, the tundra. Possible Future Changes Geological evidence such as that presented here shows that at times of global warmth, plant productivity and carbon sequestering are high in both Arctic and Antarctic regions. Should there be a future increase in mean global temperature, tundra will be replaced by conifer forest, which in continental interiors may well retain an evergreen component. 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Representative terms from entire chapter: