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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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